ENDOVASCULAR REPAIR OF

ABDOMINAL AORTIC

ANEURYSM; A

MORPHOLOGICAL STUDY

Ravinder Singh-Ranger BSc FRCS (Gen. Surg)

A thesis submitted in partial

flalfiUment of the requirements for the

degree of

Master of Surgery

University of London

Under the supervision of Mr. M Adiseshiah MS FRCP FRCS

Department of Vascular Surgery

1998-2000

ProQuest Number: U642534

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Abstract

ENDOVASCULAR REPAIR

OF ABDOMINAL AORTIC

ANEURYSM; A

MORPHOLOGICAL STUDY

Ravinder Singh-Ranger MBBS BSc FRCS

The longer-term efficacy and durability of endovascular repair (ER) are

unknown. Preliminary, non-validated data suggest progressive shrinkage of

the aneurysm sac and length follows successful ER. These alterations are also

blamed for producing forces that disrupt device integrity.

This study uses validated methods to document the morphological and

dynamic changes affecting the aneurysm sac and neck after ER.

'Laboratory section: Six glass aneurysms with varying neck angulation and sac size

were filled with known proportions of iodinated contrast media and deionized

water. Volumes and linear dimensions were pre-determined with pyknometry

and electronic callipers. Scanning with three-dimensional spiral CT

angiography (3D SCTA) patient protocol produced near perfect correlation

and agreement between true and scanned volumes and diameters. Inter- and

intra-observer errors associated with clinical estimation of these parameters

were also within acceptable range (<5% error). Volume appeared a more

complete measure of sac morphology than diameter alone.

Satellite technology for imaging arterial motion was validated by describing a

glass aneurysm in terms of its volume. Acceptable reproduction was obtained

combining multi-camera imaging with photogrammetric analysis.

Clinical section: 88 patients treated with balloon-expanding PTFE or Talent self­

expanding endografts, were prospectively followed for 1-4 years post—ER.

SCTA performed at day 5 and at 6 monthly intervals was used to record

aneurysm neck diameters and length, aneurysm and endograft lengths,

maximal sac diameters and sac volumes.

ER produced a significant (P=0,02) increase in aneurysm volume (median

PTFE 12.4 ml; Talent 15.1 ml at day 5) followed by marked shrinkage in

Talent (median -40.8 ml by 1 year compared to preoperative) but not PTFE

patients whose volumes remained unchanged. These alterations were not

consistently followed by maximal diameter measurements and were later

shown to be due to changes in intra-luminal thrombus volume.

In PTFE patients, aneurysm length increased (median 3.2 mm; P=0.04)

concomitantly with volume at day 5 followed by a second significant increase

in both endograft and aneurysm lengths at 18 months (P<0.03). Median

increases compared to preoperative values, were 16.4 and 14.6 mm

respectively. Talent patients had no length changes.

Neck diameters increased (P=0.03) immediately after deployment (PTFE

mid-neck increase 2.5 mm; Talent 3.1 mm). This continued for 6 months

follow-up in Talent patients. No further change occurred with PTFE.

Methods for filming arterial motion were developed. Attempts to quantify

AAA neck motion are still in preliminary stages.

It is concluded that aneurysm morphology post-ER is graft-specific (balloon

vs self-expanding). Aneurysm length shrinkage does not cause endograft

distortion as previously believed. This may be the result of an interaction

between inherent graft weaknesses and repetitive differential motion with the

aneurysm neck.

TABLE OF C O N TEN TS

C h a p t e r 1: The history of aneurysm surgery.....................................................8

C h a p t e r 2: Is a new aneurysm treatment necessary?..............................................17

C h a p t e r 3: Surgery for abdominal aortic aneurysm.........................................23

C h a p t e r 4: Pathogenesis of abdominal aortic aneurysm................................ 37

C h a p t e r 5: Aims and objectives.......................................................................43

C h a p t e r 6: Imaging techniques used in this study...........................................49

C h a p t e r 7 : Experiments I: Spiral CT and MRA............................................. 67

7.1: Validation of spiral CT volume and linear measurements using

glass phantom aneurysms....................................................................................................68

7.2: Validation of a method used to determine intra-aneurysmal

flow channel and intra-luminal thrombus volum es..................................................... 79

7.3: A comparison of the intra-observer and inter-observer errors

in the measurement of AAA sac volume and maximal diameters...................... 88

7.4: Comparison of spiral CT angiography and graduated sizing

catheter in the geometric sizing of AAA for endoluminal repair........................92

7.5: The natural history of AAA after endoluminal repair. A

comparative study of balloon and self-expanding endograft systems

using Spiral CT angiography............................................................................... 100

7.6: Changes in intraluminal thrombus (ILT) after endovascular

repair of abdominal aortic aneurysm.................................................................. 114

7.7: Changes in aneurysm and graft length after endovascular

exclusion of AAA using balloon and self-expanding endograft

systems.................................................................................................................118

7.8: Natural history of the aneurysm neck after endoluminal repair

using balloon and self-expanding endograft systems.........................................125

7.9: The intra- and inter-observer differences in aneurysm neck

length and diameters found during measurement with spiral CT

angiography........................................................................................................ 132

7.10: Measurement of distance and volume with MRI: An in vitro

feasibility study.................................................................................................... 136

7.11: A comparison of gadolinium-enhanced MR angiography

versus spiral CT angiography in the evaluation of patients for

endovascular repair............................................................................................. 143

C h a p t e r 8: Photogrammetry.......................................................................... 150

C h a p t e r 9: Experiments II: Dynamic arterial imaging.................................. 161

9.1: Laboratory validation of a digital photogrammetric technique

used to measure arterial dimensions................................................................... 162

9.2: Measurement of aneurysm neck and arterial motion during

open surgical repair............................................................................................. 172

9.3: Measurement of arterial motion during open surgery using

digital photography: a feasibility study................................................................183

C h a p t e r 10: Discussion............................................................................................... 188

C h a p t e r 11: Synopsis.....................................................................................212

C h a p t e r 12: References................................................................................. 214

A p p e n d i x I: List of Figures........................................................................... 244

A p p e n d i x I I : List of Tables........................................................................... 250

A p p e n d i x I I I : Critical values for Z ................................................................ 254

ACKNOWLEDGMENTS

This thesis would not have been possible without Mr. Mohan

Adiseshiah. This work is a product of his help, encouragement and

selfless support.

The experimental work was nurtured from its embryonic beginnings

with the help of many colleagues including Leoni Blank and Stuart

Robson (Photogrammetry). Professor Tony Grass and his team were

also invaluable in helping me to understand the importance of pure

science and the techniques of precise measurement.

Professor Michael Hobsley gave up much of his time to discuss

statistical methods and theory for which I am immensely grateful.

Tony McArthur and Professor William Lees were instrumental in

teaching a novice the basics of spiral CT imaging. I am very grateful

for the trust and open policy towards research held in our radiology

department.

Finally, I would like to thank my fiancée Helen Lewis for her support

during the many anti-social experimental hours spent on this project

and her saintly patience while enduring the umpteenth practice run

before a major presentation.

PUBLICATIONS

It is a requirement of the University of London that the work

presented here is original and carried out by the named author. These

conditions have been fulfilled. Work from this thesis has been

presented in abstract form and published within the following

journals:

1. Singh-Ranger R, Adiseshiah M, Endovascular repair of

abdominal aortic aneurysms. Hosp Med. 1998; 59(10); 788-92.

Review.

2. Singh-Ranger R, McArthur T, Dellacorte M, Lees W, Adiseshiah

M. The abdominal aortic aneurysm (AAA) sac after endoluminal

exclusion: a medium-term morphological follow up based on

volumetric technology. J. Vase Surg 2000; 31(4): 490-500.

3. Singh-Ranger R, McArthur T, Lees W, Adiseshiah M. A

prospective study of changes in aneurysm and graft length after

endovascular exclusion of AAA using balloon and self-expanding

endograft systems. Eur J Vase Endovasc Surg 2000; 20(1): 90-5.

4. Singh-Ranger R, Adiseshiah M. Differing morphological changes

following endovascular AAA repair using balloon-expandable or

self-expanding endografts. J Endovasc Ther. 2000; 7: 479-85.

C h a p t e r 1

The history of aneurysm surgei

1.1 D e f i n i t i o n o f a n a b d o m i n a l a o r t i c a n e u r y s m

The first record of peripheral arterial aneurysms is attributed to Antyllus who Hved in the 2nd

century AD (fi AD 250)b He described two types caused either by spontaneous dilatation ( 'when the

artery has been ruptured’) or following trauma. Although Antyllus described a method for the

treatment of smaller aneurysms using ligation and incision, it was not until 1804, that an attempt

was made to describe the formation of aneurysms. J H Wishart’s translation of Antonio Scarpa’s

book “A Treatise on the Anatomy, Pathology and Surgical Treatment of Aneurysms.” outlined the

gradual dilatation of the arterial wall leading to eventual rupture.

The modem definition^ of an aneurysm is “A localised and irreversible dilatation of an artery.”

However, this statement ignores size, a sine qua non for clinical management of asymptomatic

aneurysms.

Although most surgeons would accept the presence of an abdominal aortic aneurysm (AAA) in an

aorta with a diameter of 30 mm or more, there is argument over the precise clinical definition. This

is because aortic diameter is a continuous variable and the exact point at which a normal vessel

becomes aneurysmal cannot be easily defined. Normal arterial diameter depends on several factors,

including age, gender and blood pressure. Standardised data show the normal external diameters of

the inffarenal abdominal aorta to be 16.6-21.6 mm in females and 19.9-23.9 mm in males / .

Some investigators define an aneurysmal segment as having an internal diameter at least 5 mm

greater than the proximal aorta, while others suggest that the ratio of the inffarenal to the

suprarenal diameter must be greater than 1.5 . AAA has also been defined by a mean aortic

diameter greater than 2 standard deviations above normal* .

The committees on reporting standards of The Society of Vascular Surgery and the North

American Chapter of the International Society for Cardiovascular Surgery^ have suggested a

consensus definition for an abdominal aortic aneurysm. This is an aortic “diameter greater than

30mm in the infira-renal portion of the aorta, or a diameter of 50% greater than the normal artery

above it.” The definition excludes arteriomegaly where there is a generalised dilatation of the aorta.

1.2 C o n v e n t i o n a l a b d o m i n a l a o r t i c a n e u r y s m s u r g e r y

Although recognised for many years, aneurysms were usually untreated apart from some ill-judged

attempts at lancing with sharp probes. In the Papyrus Ebers (Figure 1), one of the earliest medical

writings prepared circa 2000 BC, peripheral arterial aneurysms were clearly identified and the

following treatment recommended: “Treat it with a knife and bum it with a fire so that it bleeds not

too muchV' Little progress was made until the medieval era when the treatment of aneurysms

resulting from bloodletting in the antecubital fossa was described. Ambrose Pare (1510-1590),

critical of Antyllus’ technique of opening the sac, recommended using a proximal Hgature only. At

this time Andreas Vesalius (1514-1564) first recognised an aneurysm of the thoracic aorta during life

and published descriptions of “ pulsating tumours below the stomach” which were probably AAA*.

j u

Figure 1: Fragment o f the Papyrus Ebers (c2(XX) BC) containing references towards the earliest medical writings.

The first serious intervention was attributed to Giovanni Monteggia (1762-1815) in 1813, who tried

to harden aneurysm sacs by injecting chemicals into them Arterial blood flow however, was too

rapid to allow any contact with the vessel wall. Other well-described treatments consisted of

wrapping the aneurysm sac in steel wire or passing an electric current through the arterial wall in an

attempt to induce haemocoagulation. The latter was still being practised in the 1950’s some 120

years after its description in 1832. Blakemore’’ of the Columbia-Presbyterian Medical Center in

New York, reported wire insertion in 11 patients with thoracic or abdominal aortic aneurysms, most

of who eventually died of aneurysm rupture, although one patient survived for 2 years. Interestingly,

in the discussion after Blakemore’s final paper on the technique”, DeBakey reported his first two

cases of resection of the aneurysm with homograft replacement.

In 1864 Charles Moore at the Middlesex Hospital in London tried reinforcing aneurysm sacs using

steel wire fed into the abdomen via an external cannula'^. One description of this operation used up

26 yards of wire!”. Cellophane wrapping was investigated by Pearse, Harrison, and Abbott*'^* .

Other materials, including fascia lata, skin, and polyvinyl sponge, were also used, but largely to no

avail since the aneurysms grew relentlessly despite circumferential wrapping^ '^^. During the same

era, Rudolph Matas (1860-1957) described the operation of “Endoaneurysmorrhaphy^’ for a large

brachial artery aneurysm^o. This was later applied, to aortic surgery and involved clamping the aorta

proximaUy and dis tally followed by a double row of intermpted sutures along the thin sac as a

buttress. The lumbar vessels were tied off to prevent blood obscuring the operative field^h

The major landmark for the type of aneurysm surgery we recognise today came in 1908 when

Alexis Carrel (1873-1948) wrote a paper^ describing the transplantation of a vessel using an

everted suture technique to prevent clotting at the suture Hne. After winning the Nobel Prize for

Physiology and Medicine in 1912, Carrel retited from surgery. His work was continued by Robert

Gross (Boston, 1949) who described the first use of stored arterial homografts in humans for

coarctation of the aorta^ . Arterial segments were harvested from human cadavers and stored for

up to 2 months in tissue culture medium before use. Early experiments with formaldehyde and

gamma-irradiation then resulted in ffeeze-drying as the method of choice. Charles Dubost used this

technology in 1951 to perform the first successful AAA repair involving aortic resection and

vascular sutufing " . However the long-term results were poor with both this and the related

method of substitution using fresh aortic segments from road traffic victims. Grafts had to be

harvested within 5 hours of death and only from corpses free from atherosclerosis and infectious

disease. In a summary report of extensive experience with arterial homografts, Szdagyi and

colleagues observed serious deterioration of stmctural integrity over time and predicted their failure

as a vascular substitute^.

Voorhees made the first inroads into the use of synthetic material as arterial conduit. In 1948, while

performing an autopsy on an animal several months after mitral valve implantation, he noticed that

a silk suture bridging the ventricular cavity was coated with a glistening layer of what appeared to

be endocardium. He conceived “that if arterial defects were bridged by prostheses constructed of a

fine mesh cloth, leakage of blood through the walls of the prosthesis would be terminated by the

formation of fibrin plugs and would thus allow the cloth tube to conduct arterial flow^^”. In 1950,

the use of Vinyon-N was described first in dogs^ and then in hum ans^® . Other prosthetic materials

were introduced by industry and Vinyon-N soon gave way to competitive materials with more

favourable physical properties, including Orion, Teflon, nylon, and Dacron. In 1953 DeBakey and

Cooley described the use of seamless Dacron in arterial repair^ . Within less than four years, he and

his colleagues at Baylor had implanted more than 1,000 synthetic grafts, with a 90% success rate.

Since that time the actual technique of AAA repair has changed little although there have been

innovations in the form of the synthetic material. Early Dacron grafts were woven and non-porous.

10

They were rigid and difficult to handle with a tendency to fray at the edges. Modem fabrics employ

a warp or lock-knit pattern. The material is softer and suturing is easier with lesser tendency to

cause fraying. However, unsealed grafts are highly permeable and require pre-clotting with

autologous blood to prevent percolation during implantation. This serious disadvantage during

emergency repair was addressed to a large extent by the introduction of gelatin-bonded grafts.

1.3 E n d o l u m i n a l e x c l u s i o n o f a n e u r y s m s

Attempts to simplify the abdominal approach were made in the 1980’s when a vascular graft with a

rigid ring at each end was described^®. This was placed within the lumen of the aorta and held in situ

with external ligatures. The prosthesis, originally indicated for aortic dissections, was successfully

used in the management of an AAA^h However, it had no particular advantage over conventional

repair, also requiring a laparotomy and cross-clamping of the aorta.

Several groups were also beginning to explore the idea of placing a graft within the aorta using the

transfemoral approach. In 1983 Kreamer, a year medical student, devised the concept of

transfemoral endovascular repair of AAA by insertion of a prosthetic graft held to the aortic wall

with adhesives^^. When the Food and Drug Administration (FDA) ruled that adhesives could not

be used in humans, he used stents to fix his graft to the aortic wall. He later received the first patent

for a stent-graft awarded in 1987^ .

Cragg et al reported on a small series using nitinol wire prostheses in 1983^t Nitinol wire coil grafts

straightened in ice water were passed into the canine aorta via catheter, where they reformed into

their original shapes. Follow-up aortograms demonstrated long-term patency with minimal

thrombus formation. It was concluded that these coü grafts might eventually be used in the

nonsurgical treatment of several forms of vascular disease.

Balko et aP used a polyurethane and nitinol graft to treat aneurysms created in three adult sheep.

Endografts were deployed via the femoral artery. Post mortem examination later demonstrated

successful endovascular exclusion of these aneurysms.

In 1991 Parodi and Paknaz^^ described the first series of endoluminal repairs in humans. This

seminal paper described six cases: five abdominal aortic aneurysms, and one abdominal aortic

dissection.

In each case, a Pahnaz stent (Johnson and Johnson) was used to attach a Dacron graft immediately

below the renal arteries. The lower end of the prosthesis was left free and it was hoped that the

blood pressure would allow patency and simultaneous sealing against the aortic wall. This proved

untrue, and continued leakage into the sac from below required deployment of a second Palmaz

stent to fix the lower end of the graft. These tubular grafts required a proximal and distal neck of

11

normal aorta superior and inferior to the aneurysm to enable a satisfactory seal to occur between

the stent, graft and aortic wall.

The tube graft has now become unpopular due to high rates of endoleakage^^ from the distal

ftxation site. Further, only 5-10% of all AAA have the anatomical configuration required for its

deployments^. Recently, it has been argued that tube endografts may still retain a place in selected

cases. In a review of 158 patients treated with tubular (n = 57), aorto-ibac/femoral (n = 24) and

bifurcated (n = 77) grafts, tubular prostheses had the highest rate of endoleak (21/57 compared to

16/101 patients). Analysis of the cause of tube graft failure revealed a distal neck of inadequate

length (<1.5 cm) as the most common culprit. When the length of the distal neck was 2.5 cm or

greater, endoleak was much less likelyS .

Current therapeutic options for endoluminal treatment of AAA lie between aorto-uni-diac and

bifurcated endografts. The major difference between these two systems is the ability to deal with

common iliac artery anatomy.

Bifurcated endoluminal grafts

The design of bifurcated endovascular grafts for the treatment of AAA emerged directly from the

experience of conventional aorto-iliac reconstructive surgery. The first experimental use of a

bifurcated graft by Chuter et was followed in 1993 by Scott's" description of the first clinical

experience. This system delivered a preformed graft with three orifices, each of which was attached

to non-dilated artery by a Gianturco Z-stent. The Y-shaped prosthesis was introduced through one

femoral artery and manipulated into position, using a system of connected catheters. An alternative

method of bifurcated endograft insertion involves assembling several stent-graft components to

create the bifurcated prosthesis in situ (Figure 2) referred to as the "modular" approach'^^.

Aorto-uni-iliac grafts

Following Parodi’s initial description, the aorto-uni-üiac system was pioneered and utilised in many

of the early endovascular series' "' . It’s simplicity lay in the fact that it could be assembled using

easily accessible components (a length of pre-expanded PTFE, two balloon-expandable stents and a

balloon catheter) literally at the time of repair.

The graft is deployed below the renal arteries and then brought down through one of the common-

iliac arteries to be fixed either there or within the external iliac artery. The contralateral common

ihac artery is occluded either surgically by ‘T>lind-stenting” or with embolisation coils to prevent

retrograde filling of the AAA sac. A femoro-femoral bypass restores blood to the occluded side.

12

Debate still centres over which configuration represents the best option. Proponents of the

bifurcated devices argue that they are anatomically correct and so maintain the normality of blood

flow. More importantly, there is no indication to sacrifice a normal common iliac artery. However,

for a bifurcated device, bilateral access is required as well as three secure landing zones for the

endovascular graft-vessel seal. For an effective seal, the arterial wall must be undilated and free of

thrombus. Mural thrombus does not provide a reliable seal.

The aorto-uni-iliac system is technically easier to deploy than the bifurcated prostheses and this

allows a much wider applicability^. In one series, it has been used to repair AAA with stable,

contained mpture'^ . The main drawback lies over the necessity for the femoro-femoral crossover

graft which is inherent to this procedure and which sacrifices a normal common iliac system. In the

long term, there are no published data on the viability of femoro-femoral crossover grafts in

aneurysm patients. In occlusive arterial disease they have a lower patency rate compared to

aortobifemoral grafts but only when the run-off is poor. When there is good run-off, patency rates

are equivalent^. A theoretical risk for steal syndrome from the un-occluded limb has also not been

borne out. In a review^ of complications associated with crossover grafts in 136 AAA patients,

problems such as graft infection and heamatoma occurred with a frequency of 1.5% and 6.6% at a

median follow up of 7 months respectively. Only 4 patients had graft occlusion.

Figure 2: Bifurcated modular endograft system. This particular example (Talent), manufactured by World Medical was used m this study. The self-expanding, nitinol

stents are spaced discontinuously along a full-length spine that provides longitudinal stability. The metal stents, which are completely embedded m the woven Dacron

fabric, are designed to conform to variations in vessel shape and post-implant diameterchanges.

13

Both configurations of endograft may be fixed with either balloon- or self-expanding stents. Initially,

endografts were fixed within the aneurysm neck using balloon-expandable stents such as the Palmaz

stent (Figure 3). These are now being superseded by self-expanding stents, although both types of

stents typically require secondary balloon expansion after initial deployment.

Balloon-expandable stents

Balloon-expandable stents are mounted on a Gruntzig-type balloon catheter and are passively

expanded, after which they do not enlarge further. These stents require a greater force for

deployment than self-expanding stents and once in situ, have sufficient radial strength to abolish

pulsatile motion at the aortic neck“ .

With balloon-expandable stents, the operator generally sizes the stent to the minimum reference

diameter and subsequently moulds the delivered stent to conform to the vessel contour. With self­

expanding stents, the stent is typically sized to the maximum reference diameter. In both cases,

incorrect sizing or deployment of stents (for example, with protrusion of struts through to adventitia

of the vessel wall) may damage the vessel wall.

Figure 3: The Palmaz balloon-expandable stent (Johnson and Johnson). This stent is made of stainless steel: a nine-element alloy, o f which chromium is a significant contnbutor. FJectropolishing at the end o f production produces a positive surface potential, which may attract plasma proteins and render the surfece non- thrombogenic^ .

Self-expanding stents

Self-expanding stents are made of nitinol or stainless steel. Nitinol is an alloy composed of nickel

and titanium that expands and contracts owing to the effect of temperature. These properties are

based on a shape memory effect gained by heat-treatment of the raw material. Apart from its

superelastic properties, nitinol also has excellent biocompatibilityand corrosion resistance.

Corrosion has not yet been observed in clinical experiments, either macroscopically” or

histologically^.

The nitinol stent always tends to regain its preset diameter once deployed and continues to expand

slowly into the aortic wall without any loss of tensüe strength (Figure 4). Foreshortening of self­

expanding stents is commonly encountered upon withdrawal of their constraining sheaths, so that

precise positioning can be a formidable challenge. Positioning of balloon-expandable stents is more

predictable. Graft fixation relies initially on frictional forces between the stent and aortic wall

14

generated by the radial force of the stent and the constricting elastic force of the aorta, stretched by

the oversized stent. Theoretically, it is possible that the slow expansion associated with self­

expanding stents may lead to a deterioration of the elastic forces in the arterial wall with loosening of

the stent when the elastic limit is breached. This problem may not be immediately obvious as stents

are usually oversized with respect to the diameter of the aneurysm neck but has recently been

described in an animal modeP^.

Figure 4: Upper end o f talent system showing two configurations for the self-expanding nitinol stent. The uncovered stent is on the left and the open web is on the right. The former is chosen when deployment may cause obstruction o f the lumen o f an aortic side

branch, such as the renal artery.

Stent hooks and barbs have been added to improve endograft fixation. In one cadaveric study^, 137

self-expanding stents with varying lengths of hooks and barbs were deployed in human aortas.

Ivongitudinal traction was applied and the displacement force was compared with that of plain stents

held in situ by a radial force alone. It was found that barbs and hooks increased the fixation of stent-

grafts tenfold, while the radial force of stents had no impact. The length of each barb was critical.

During traction, the smaller barbs were distorted or caused intimai damage to the vessel. This type

of fixation is used by some of the commercially available devices notably the Endovascular

Technologies (EVT) Ancure (Guidant, Figure 5) and Zenith (Cook, Australia) endografts. Fixation

of the Ancure graft depends on 8 metal hooks of 3 mm in length penetrating the aortic wall just

below the renal arteries. These hooks are implanted within the aortic adventitia using 2 Al’M of

balloon pressure. A potential disadvantage of hook fixation is that heavily calcified landing zones

may not allow proper fixation. Secondly, if there is traction on the graft after deployment, significant

tearing of the aortic neck may result.

15

% W

Figure 5: The upper end o f the EVT Ancure graft showing the hooks and barbs employed for aortic fixation during deployment. A further aid to longer-term sealmg is the frayed polyester that is

said to incorporate the graft in to the aortic wall (lower figure) durmg the remodelling process.

16

C h a p t e r 2

Is a new aneurysm treatment necessa

Before a new treatment is adopted in favour of the old three requirements should be considered:

• A continuing increase in the disease load and hence demand for a less invasive and less costly

treatment may be called for. For example, if the incidence of AAA begins to markedly decrease

over the next few years then, in fiscal terms, preparations for the introduction of a new

treatment would not be economical.

• The adequacy of the current treatment, in this case open repair, has to be re-examined. If the

present treatment is safe with litde morbidity and mortality, then the introduction of a new

regimen will not have any perceived benefit.

• The efficacy of the new treatment needs to be established. Open repair, the current gold

standard, eradicates the threat of death from aneurysm rupture as the sac is physically disrupted

and then (usually) oversewn. In contrast endovascular approaches do not disturb the anatomy

of the aneurysm sac and the threat of rupture may still be present.

In this section the epidemiology of aneurysms will be reviewed to try and determine whether there

is likely to be a sufficient increase in the number of aneurysm cases in the future to merit the need

for a new treatment.

2 .1 A n e u r y s m a l d i s e a s e i n p e r s p e c t i v e

Abdominal aortic aneurysms have posed a significant challenge to the healthcare system. The

number of new aneurysms seen by vascular surgeons in 1980 was 7 times higher than that seen in

1951 * when the first inlay repair was described. In the United states, AAA is now ranked as the

13th leading cause of death and is responsible for about 15,000 deaths per year while in the United

Kingdom (UK) it is responsible for 10,000 deaths per year. The incidence of AAA has been

estimated at 52 per 100,000 per annum in men aged 55-64 years^ and 499 per 100,000 per annum

in men over 80 years ®.

However, since the first open AAA repair only took place 48 years ago " , it is possible that these

trends are simply a reflection of improvements in diagnosis and hence better case definitions^.

17

It could therefore be argued that, while the incidence of AAA continues to increase, its prevalence,

or the total number of cases in the population at any point in time, remains constant. Eventually,

there wiU come a point at which the increasing incidence will begin to level off.

Furthermore, since AAA is a degenerative disease it will be more common in an ageing population.

An average life expectancy of 85 years has been predicted in the year 2000* ®. If the increased life

expectancy of earlier generations is added to this, a substantial rise in the numbers of aneurysms

and demand for their treatment must be expected regardless of a true increase in the disease itself.

Incidence of Aneurysms

In both the United States and the United Kingdom the annual incidence of abdominal aortic

aneurysm (AAA) has shown a substantial increase fcom 1950 to 1980. 8.7 new AAA were

diagnosed per 100,000 person-years from 1950-1960 compared with 36.5 new aneurysms per

100,000 person-years from 1971-1980 in the USA^h

In England and Wales, the number of AAA deaths in men over the age of 40 increased steadily

from 202 in 1950 to 4668 in 1984. In women, during the same time period, the increase was from

201 to 259U^. Neither group had any significant increase during the 1970’s when diagnostic

ultrasound became widely available* . Further, the corresponding age-standardised mortalities were

widely different for the sexes, increasing 20-fold for men and only 11-fold for women during this

time period^ .

These observations suggest that the rising incidence of aneurysms is real and not based on

improved case ascertainment or an ageing population; if these were the main or only influences on

increasing incidence, then men and women would have been expected to have been equally

affected.

A further illustration comes from an increase in the incidence of ruptured AAA. One study in

Goteborg, Sweden found a sevenfold increase over a period of 36 years* " unaltered by any

improvements in diagnostic techniques. It has been suggested that these figures represent the tip of

a very large iceberg^^. Only 20% of patients with mptured aneurysms actually reach hospital alive

with an operative mortality of 78-94%. Free intraperitoneal mpture, which is rapidly followed by

cardiovascular collapse and death, is probably underreported in mortality statistics. The tme

incidence of ruptured AAA is hidden amongst such patients who die suddenly without medical

input and without a correct diagnosis.

18

Ptevalence of Aneurysms

The prevalence of AAA (>30 mm) increases with age. The number of AAA, estimated using

ultrasound-based screening in a sample of 12,203 Australian men^^, increased from 4.8% in men

aged 65-69 years to 10.8% in those aged 80-83 years. The overall prevalence of large (> 50 mm)

aneurysms was 0.69%. Existing estimates of point prevalence can be grouped according to whether

they have been obtained from post mortem or population screening studies.

Posf mortem studies

As sources of prevalence 02X2i, post mortem studies are flawed because definitions for aneurysms may

not always be quoted or be as strict as those used in population screening surveys. Significant

difficulty is also presented by the postmortem rate. If this is less than 100%, patients with sudden and

unexpected deaths may be over represented^^ and the frequency of ruptured AAA may then appear

falsely high.

In one series of post mortem’s carried out over a 28-year period in Makno^®, the annual prevalence

showed a significant increase in both sexes. In men, the annual increase was 4.7% (95% confidence

interval 3.6-9) compared to women where it was 3.0% (1.8-4.3). The advantages of this study were

based on the well-defined and stable population who were all served by one hospital. The post

mortem rate was also very high.

Screening studies

Recent studies, based on screening of General practice populations, may provide a more accurate

insight into the prevalence of AAA. In UK males, this varies between 1.3 and 12.7% depending on

the age group and criteria used for the definition of AAA^ . The latter is critical as differences in

prevalence data can often be explained by differences in the definition of an aneurysm. For

example, the point prevalence for AAA in males over 55 years was estimated as 4.1% in a Dutch

study ® compared to 8.2% in Norwegian study^h These would seem unusual, as both countries are

geographically comparable with similar population origins. Closer comparison however, shows that

the Dutch used a cut-off value of 35 mm for the presence of an AAA while the Norwegian series

was based on a value of 29 mm or an infira-renal diameter 50% greater than the supra-renal aorta.

Repeated comparisons of prevalence within a single location over a period of time, may provide a

better idea of trends due to consistency, but screening for AAA is new and programmes have not

been in evidence long enough for these to be visible.

In summary, there is a true increase in the incidence of patients with AAA that is independent of

changes in diagnostic technology and ageing within a population. The prevalence of AAA is still

19

unknown although the data obtained from post mortem studies seem to agree quite well with the

screening studies when adjusted for population demography^^. This suggests that there is likely to

be a continuing need for aneurysm surgery, as the number of afflicted patients will continue to

increase.

2.2. S c r e e n i n g f o r AAA

Screening for aneurysms may be an important way to eliminate a preventable cause of mortaHty^ .

AAA are asymptomatic, life threatening, treatable and amenable to detection by noninvasive tests.

In addition, the incidence increases in the presence of known risk factors such as smoking,

presence of first-degree relatives with an AAA or peripheral vascular disease, carotid artery disease

or hypertension^^’ Smoking is associated with a 5.6-fold increase in the risk of finding an

abdominal aortic aneurysm (more than 4 cm in size), but even among smokers the prevalence of

large abdominal aortic aneurysms is low in the absence of other risk factors^ . Patients with

aneurysms have a 20 percent chance of having a first-degree relative with the same condition. Male

siblings are at particular risk. The exact genetic pattern of inheritance has not been elucidated but

may involve both X chromosome-linked and autosomal dominant patterns of inheritance^^’

From this discussion, it would seem reasonable to speculate that widespread screening could

significantly increase the workload for aneurysm repaie. The majority of screen-detected AAA are

however 5 cm or less in diameter. In one report, the results from 5 large European screening

programmes found only 9 (0.18%) AAA greater than 6 cm in diameter among a total of 5005

subjects (Table 1) ®. Operative treatment of smaU aneurysms is controversial because of the

following:

• The natural history of small aneurysms is unknown. It is believed that aneurysms smaller than

5.5 cm diameter (so called “small aneurysms”) may expand at a rate of 0.18 cm per year

compared to larger aneurysms which expand at 0.28 cm/year^^. While the risk of rupture is

thought to be low, this is by no means certain, and estimates vary from 1% upwards. In Scott’s

series® , only 1 out of 8820 screened patients who did not fulfil the criteria deemed necessary for

surgery (mde injrd), mptured his aneurysm. In contrast. Darling and Brewster® showed that 18%

of 182 AAA that mptured were less than 5 cm in diameter while Szilagyi et aP- had a mpture

rate of 29.5% for small aneurysms in their series.

The risk of mpture may increase with co-morbidity. Patients with hypertension, significant

pulmonary disease and an AAA of 3 cm have a 54% risk of mpture within 3 years of

20

diagnosis® . In comparison, the annual risk of mpture ftom an AAA of 5.0-5.7cm is 6.6% and

that fcom an aneurysm of 7cm is

Open AAA repair has a significant mortality and morbidity and it would be inappropriate to

offer surgery to patients with small AAA if the risk of mpture is less than the risk of death from

conventional operation. The mortality fiom open AAA repair is currendy 3-4% in specialist

units® but may exceed 40% in patients with concomitant renal, cardiac and pulmonary

disease® . Operative complications are due mainly to the extensive dissection and aortic cross

clamping inherent to this operation. A positive risk-benefit ratio for these patients has been

demonstrated provided the mortality of surgery can be reduced below 5%®>®®.

• Patients with AAA have significandy higher risk of death from cardiovascular causes compared

to age and sex matched populations and many with small aneurysms are likely to die from

unrelated events. Only about 15% of aU AAA’s mpture. The remaining 85% of patients die

from causes unrelated to the aneurysm® . This suggests that most small aneurysms could safely

be observed until they reach a size where the risk of death from open surgery is less than the

risk of death from mpture.

The interval between observations is unclear, as aneurysm expansion may not be linear and does

not depend upon starting diameters. One smdy^° demonstrated that exponential growth was

more common: larger aneurysms exhibited faster rates of enlargement implying that more

frequent observation of these patients was necessary. This was not confirmed in a smdy of 302

screen-detected aneurysms^^, where it was found that aU sizes of aneurysm showed similar rates

of growth.

The uncertainty is increased by smdies that show no significant differences in mortality between

emergency repair of mptured small and larger aneurysms^^. It has been suggested that these small

aneurysms should be treated early while the patient is fit. Proponents of this philosophy argue that

as enlargement continues during a patients lifetime, there should be no need to expose an

individual to the continued risk of mpture while waiting for the AAA to reach a size acceptable for

open repair. Further, although not proven, it is argued that as aneurysms continue to enlarge,

patients will be older and less fit at the time of surgery^ .

In view of this, there have been moves by some to offer open surgery to all individuals with

asymptomatic AAA. Ramo et aP operated on aU small AAA detected within a ten year period in

Finland. Their results showed that while small AAA’s could be electively repaired with low

21

mortality and morbidity, such a strategy still did not affect the ratio of ruptured and elective

aneurysms that presented to their establishment.

Number (%) of AAANumber of patients 3.0-4.5

cm4.5-S.9

cm6 cm + Total

426 18 (4.2) 5 (0.12) 0(0) 23 (5.4)1312 62 (4.7) 10 (0.08) 4 (0.03) 76 (5.8)906 20 (2.2) 9 (0.99) 0(0) 29 (3.2)1800 67 (3.7) 12 (0.67) 3 (0.17) 82 (4.6)561 42 (7.5) 9 (1.60) 2 (0.36) 53 (9.4)

T ab le 1: Yield and size o f abdominal aortic aneurysms from 5 European screening programmes. See text for explanation.

Results from the UK Small Aneurysm Study were presented recendy " . This randomised trial was

set up for patients aged 60-76 years with small asymptomatic aneurysms. 1090 patients either had

early elective surgery or surveillance at regular intervals with ultrasound. There was a 30 day

mortality of 5.8% in the operated group: these were fit patients with favourable aneurysms. At 2, 4

and 6 years follow-up, no difference in mortality between the two groups was seen.

These results do not support a policy for early open surgery for small aneurysms. Before this can be

recommended, safer treatments are required. Interestingly, the small aneurysm study was

performed during a time when endovascular repair was in its infancy and its feasibility was stiU

being established. This situation has now changed. The lower mortality rates now being

reported^^’* suggest that endovascular repair is a safer alternative to open surgery {mde injrd).

Further, as small aneurysms grow, their necks shorten thus making an endovascular approach

difficult^ . Whether or not the results of the small aneurysm study should be extrapolated to this

method of treatment is still a matter for debate. In the meantime the therapeutic strategy for such

aneurysms must lie somewhere between the extremes of watchful waiting and “supervised

neglect”.

22

C h a p t e r 3

Surgery fot abdominal aortic aneurysm

3 .1 I n d i c a t i o n s f o r s u r g i c a l r e p a i r o f a n e u r y s m s

The current indications^^ for elective repair of AAA are:

• a symptomatic or rapidly expanding aneurysm (annual increase in diameter of more than 5 mm)

• a large aneurysm (diameter greater than 5.5 cm)

• a complicated aneurysm (i.e. one with thrombosis, embolism, or hstulization).

3 .2 H o w SAFE IS CONVENTIONAL ANEURYSM SURGERY?

In major centres the elective mortality of aneurysm repair is quoted as 3-4%. This understates the

tme risk. Community-based studies reveal a much higher mortality. In New York, a survey of 3570

patients undergoing conventional aneurysm surgery, found that the mortality of open AAA repair

was as high as 10-14%® . These differences are closely allied to operator experience: there was a

striking difference in the mortality obtained by surgeons treating more than 25 aneurysms per year

compared to those treating fewer. In a second study from the UK, the results of aneurysmal

surgery over a 13 year period^^, produced an overall mortality of 6.7% for elective surgery, 16% for

urgent cases and 53% for emergency repair.

Cross clamping of the aorta produces a major insult to the heart that has to pump against a

significandy greater resistance. There may also be a profound reduction in renal and splanchnic

blood flow associated with an increase in the base deficit after clamp removal^*^. This, allied with

the presence of cardiac morbidity in up to 60% of patients presenting for AAA repair ® ’ ®, is

responsible for most of the mortality and morbidity of open repair. Between 60 and 70% of in-

hospital and late mortality is due to cardiac causes This increases even further in the presence

of poor left ventricular function. When the ejection fraction is less than 35%, up to 80% of patients

may suffer from myocardial infarction with 20% eventually dying^^ .

26% of patients may experience complications after conventional repair^^ . A detailed analysis of

reports (Table 2) from the USA, dating from 198U®® showed early complications after elective

operations were common. Late complications, such as aortoenteric fistula, occurred 3-5 years after

surgery and were responsible for an additional 2% mortality.

23

3.3 S u i t a b i l i t y f o r e n d o v a s c u l a r r e p a i r

Not all aneurysms are suitable for endovascular exclusion. Those that can be treated must fulfil

certain anatomical criteria that apply to the proximal neck (Table 3) and common ihac artery, so-

called “landing-zones” where the endograft is fixed to the vessel wall. The diameter and quahty of

the femoral arteries is also critical as these form the access sites for insertion of the endovascular

devices. Based on such criteria, about 68% of patients will be suitable for either a straight tube,

bifurcated or aorto-uni-iliac graft systems ® .

The aneurysm neck

The aneurysm neck is defined as the area of normal undilated aorta between the renal arteries and

the beginning of the aneurysm sac. For consideration of endovascular repair, it should be at least

15 mm in length and not more than 30 mm in diameter with no lining thrombus (Table 3). When

one renal artery is placed at a lower level than its counterpart, there may be a danger of excluding

the kidney with the stent-graft although it is possible to use a bare stent over the renal origin so that

perfusion continues uninterrupted.

Complication Frequency (%)

Myocardial events 15

Pulmonary insufficiency 8

Renal insufficiency 6

Bleeding 4

Distal thromboembolism 3

Wound infection 2

T ab le 2: Early specific complications after conventional surgery for abdominal aortic aneurysm. Adapted from Ernst (1993)^°®.

Myocardial events include varying degrees o f ischaemia, dysrhythmias and congestive cardiac failure.

It has become apparent that the aneurysm neck is a changing structure closely related to the growth

of the aneurysm sac. Armon et found a strong statistical relationship between sac size and neck

length. Significantly more short (<15 mm), wide (> 30 mm), and hence, unsuitable proximal necks

were found in patients with aneurysms larger than 7 cm in diameter. Shortening and widening of

the aneurysm neck with increasing sac size was also noted by Parodi^® who analysed the dynamic

geometry of small AAA in relation to growth: in the first stage of aortic expansion, there were well

demarcated proximal and distal AAA necks. Continued growth resulted in a gradual loss of the

24

distal neck while the proximal neck was retained. In AAA larger than 7.0 cm, only rarely was a

distal neck found. The proximal neck was now much wider and much shorter than in smaller

aneurysms.

These observations suggest that smaller aneurysms may lend themselves more easily to

endoluminal exclusion although this is a controversial point. In their study, comparing endoluminal

repair versus no treatment for aneurysms less than 5 cm in diameter. May et al achieved a 4.4%

mortality^

Length < 15 mm Diameter > 30 mm Thrombus LiningConical Profile (> 4 mm increase in diameter) Angulation of Neck to Aneurysm > 70 degrees CalcificationIrregular Lumen_________________________

Table 3: Characteristics of the neck that might be considered unsuitable for endograft implantation.

The common iliac artery

The distal end of the endograft is usually fixed within a common iliac artery. This should ideally be

at least 25-30 mm in length and not more than 15 mm in diameter. An aneurysmal common iliac

artery requires exclusion from the circulation by implantation of the prosthesis within the external

iliac artery.

By its very nature, insertion of the endovascular device requires that the iHac and femoral arteries,

through which the prosthesis wiU be introduced, are not occluded by atheroma or rigid due to

intramural calcification. Occasionally it may be necessary to perform percutaneous transluminal

angioplasty to a stenosis before operation while tortuous common iliac arteries can be straightened

by digital manipulation.

3.4 H ow SAFE IS ENDOVASCULAR REPAIR OF AAA?

The benefits of the endovascular approach He in the physiological and clinical outcomes compared

to open repair.

Physiological benefits

The reduced physiological stress of endovascular repair was highlighted by Baxendale et who

compared the surgical stress response in 20 patients during conventional and endovascular AAA

25

surgery. Parameters used to record stress consisted of mean arterial pressure, cardiac output, total

peripheral resistance and serum lactic acid levels. It was found that endovascular repair imposed

significandy less perioperative haemodynamic and metabolic stress on the patient compared with

conventional open surgery. This was supported in a second larger study, where the morbidity and

mortality rates following conventional and endovascular AAA repair were compared in 153

individuals using physiological s c o r i n g ^ T h e operative severity score P-POSSUM, was

significandy greater in the conventional group (26.3 versus 19.7; P < 0.001).

Endovascular repair thus has the potential to be a safer procedure than open surgery, especially in a

population of patients characterised by a high prevalence of cardiovascular pathology.

Clinical benefits

Clinical comparisons of the outcomes between open and endovascular repair are complex.

Conventional aneurysm surgery is the established gold standard and has been in existence for the

past 50 years. In contrast, endovascular techniques and technology are continuously improving.

The longest reported follow-up of endovascular aneurysm repair would now be just over 5 years "

whereas follow-up for conventional repair extends well beyond 10 years ®> ^ .

Comparisons are further complicated by the inclusion of early endovascular series containing

patients deemed unfit for conventional repair. If emergency conversion was necessary, the

mortality and morbidity were high. In their study of 113 patients undergoing endoluminal repair.

May et al found that conversion to open repair entailed a 43% mortality for patients who had

previously been rejected for open repair. In this series, which also included patients fit for open

repair, all deaths occurred in the unfit patients^

Results from endovascular series have been presented in dual format. Firstly, there are individual

reports that concentrate on the outcomes from a single unit. These suffer from the drawbacks of

smaller patient numbers and the heterogeneity of endoprostheses implanted. In addition, early

results may be little more than a reflection of the surgeons’ learning curve making comparisons

between units difficult. A further problem is the “intention to treat” principle. To avoid later

selection bias, this entails the registration of all patients selected for endovascular repair before the

operation takes place. Subsequent details of the operative procedure, the early outcome and follow-

up information are then compared for everyone. Within individual series, this precaution is rarely

followed.

Such difficulties may be avoided by collecting endovascular data within a single registry. This would

allow larger numbers of patients to be included as well as the possibility of comparisons between

the different graft designs. Analysis of aggregated data has the potential to identify problems at an

26

early stage and to establish whether any adverse outcomes observed are a result of sporadic

random events or due to recurring systematic failure.

A further advantage of registration is the greater patient heterogeneity than exits within a single

centre. For example although the anatomical criteria involved in the selection of suitable aneurysms

for endovascular repair are similar in most units, patient co-morbidities may differ. Some units may

bias their endovascular practice to individuals who are rejected for open repaie, whilst others may

offer this as a first line treatment to all. Results from the former may include a higher mortality due

to the higher risks involved with conversion and will therefore provide a prejudiced conclusion.

The strength of a registry lies in its ability to provide better case definition and individual patient

information. In this respect it is limited by the quality and completeness of the data return.

Prerequisite for a successful registry is primarily the willingness to participate in a project that has

no immediate benefit for each unit. In addition, a common basis of definitions and classifications

of complications is essential. Incomplete reporting and incorrect information can significandy

restrict the usefulness of the exercise. As submission is voluntary, there is still a risk of bias in the

data collected by each individual centre. This can be overcome to some degree by selecting centres

at random for auditing purposes. This would involve verifying patient details on the registration

forms and numbers treated by cross-checking radiology, theatre and endograft company records.

Individual series

Results from some of the larger single series are presented in Table 4. The study by Blum et is

the most homogenous containing the largest number of patients (154 recruited firom 3 different

centres) treated with the same endograft (Stentor). In this report the primary success rate (defined

as aneurysm exclusion without conversion or endoleak), was 86% with a conversion rate of 1.9%.

During a mean follow-up period of 13 months, 10-14% of patients developed endoleaks. Similar

figures are obtained from the other reports with primary success rates varying from 73% (Chuter et

to 83% (Mialhe et Early conversions to open repair, which vary from 0 to 10%, were

inevitably due to technical failures such as iHac artery stenoses which could not be crossed, diiac

ruptures and malpositioninig of the endograft causing uncontrollable Type I endoleakage.

Mortality rates are separated into procedure-related deaths and those relative to pre-existing co­

morbidity. Procedure-related mortality was commonly due to arterial ruptures, conversion to open

repair, arterial thromboemboHsm^^® and colonic ischaemia due to exclusion of the common iHac

arteries^This varied from 0% to 8%. MortaHty related to co-morbidity varied from 0.6% to 8%.

The Chuter study ^® is of particular interest because it demonstrates the learning curve effect over

two time intervals between 1993-1995. A total of 40 patients spHt into two equal groups were

followed for a mean of 18.8 and 10.9 months respectively. Patients in the second half of the study

27

had a lower overall combined morbidity/mortality rate (15% vs 50%) and higher overall success

rate (85% versus 65%). The mortality rate for the series as a whole was 7.5%.

Late follow up to 1 year was available for the Parodi series. This demonstrated 4 further Type I

endoleaks and a further procedure-unrelated death ^®.

Outcomes in endovascular registries

Registries for endovascular patients have been set up on both sides of the Atlantic. These relate

either to single graft types such as the EVT study, or to collections of patients from a particular

country or continent.

Reference Number of patients

Mean follow up

(months)

Endograftused

Primary success rate

Conversionrate Endoleak

rate

Procediue-related

mortality

Mortality due to

comorbidity

Blum 1997 (117)

154 12.5 Stentor 86% 1.9% 10-14% 0% 0.6%

Chuter 1996 (118)

Gianturco41 10.9-18.8 and

Dacron65%-85% 5% -l0% 20%-25% 5% 7%

Mialhe 1997 (119) 79 6 Stentor 83% 0% 17% 2.4% 2.4%

Parodi 1996 (120)

50 17Palmaz

andDacron

80% 6% 12.5% 8% 8%

T ab le 4: Clinical outcomes after endovascular repair in some o f the major series reported within the last 2 years

The United Kingdom Registry for Endovascular Treatment of Aneurysms (RETA),^^! recorded

473 endovascular procedures between January 1996 and January 1999. 88% of these patients were

American Society of Anaesthesiology (ASA) grade I-III and 12% were classed as grades IV-V.

Primary success was seen in 75.7% (358 patients) with a conversion rate of 5.5% (26 patients).

There was a 30-day mortality of 6.3% (30 patients) with a variation of 6.6% (17 out of 409

individuals) in fit patients and 23.2% (13 out of 56) in unfit patients. The mortality in cases that

underwent conversion to open repair was even higher at 30.8%. Persistent primary endoleaks

remained in 6.6% of patients at discharge.

The RETA data has now progressed to include 1 year follow-up data for 173 of the original group.

This demonstrates a further mortality of 10.4% (16 patients); although most deaths were related to

pre-existing cardiac morbidity in patients classed as ASA IV-V, two cases of delayed AAA rupture

were seen. One of these could be directly attributed to persisting endoleakage. Data for persisting

28

endoleaks were available in 5 other patients, only one of whom did not require any form of

intervention. The complication rate was relatively high, with 12.5% of patients having a problem

with the endograft or aneurysm requiring treatment or close monitoring.

The Endovascular Technologies (EVT) phase 1 trial conducted under the Food and Drug

Administration (FDA) in the USA^^, had much stricter physiological and anatomical criteria for

selection. This was reflected in the improved survival for patients requiring conversion. From a

total of 46 treated patients, there were 39 (85%) successful deployments with only 7 conversions.

There were no complications in the latter group. The 30 day mortality was 0%. Patients with

complications had mainly technical reasons for their morbidity such as iliofemoral injury (8

patients) or postoperative wound infection (7 patients). Hospital stay ranged from 1 to 13 days.

There were 8 (21%) persistent cases of endoleak and of great concern was implant failure due to

attachment system fractures seen in 9 patients (23%). This halted the further progress until graft

modifications had taken place.

The phase 2 trial results were recently presented^^3 recruited patients had to be acceptable

candidates for open surgery and results were analysed at an independent radiology centre. A total

of 526 patients underwent treatment with tube, bifurcation or aorto-uni-ihac grafts. I l l patients,

unfit for open repair, were chosen as controls. Despite the rigorous selection however, the

mortahty (5.6%) in converted patients was still double that in the endovascular group. Patients with

aorto-aortic tube endografts had the lowest mortality (0%) followed by the bifurcated (2.6%) and

then the aorto-uni-ihac (4.6%) groups. The mortahty in untreated controls was 2.7%.

The Eurostar Registry was estabhshed in February 1996 to gather data on the results of

endovascular repair from 43 European centres. Registered patients are followed up at regular

intervals for 5 to 10 years.

Of the 1138 patients consecutively registered on the Eurostar database^ " , the procedure was

completed successfully in 98%. Minor technical problems were encountered during deployment in

17%, but with few exceptions these were resolved by appropriate additional endovascular

manipulations undertaken at the time. Only one patient died during the course of the operative

procedure, although another 25 patients died within 30 days, giving an in-hospital mortahty rate of

2.3%. Nearly ah of the patients who died had severe co-morbidity. The Eurostar data was also

useful because it clearly showed that the majority of registered patients had a very satisfactory

outcome from their operation. The cumulative survival rate at 18 months was 85%, with the

majority of patients dying from causes unrelated to their aneurysm.

29

specific complications associated with endovascular repair

Specific complications with the endovascular approach have been well descnbed ^> ^* . A major

drawback is the risk of distal embolism of thrombus and dislodged particles from the irregular

friable intraluminal surface. Continued aneurysmal expansion firom an incomplete graft-vessel wall

seal may result in rupture. Occlusion of the renal arteries may occur as a result of improper

placement of the proximal end of the graft, and failure to restore flow to a patent mesenteric artery

may lead to colonic ischaemia. Other obstacles have been tortuosity of the access arteries and

dislodgement with migration of the graft.

Distal thromhoemholism

Aneurysms are usually lined with incompletely organised thrombus. Manipulation of catheters and

wires may result in distal embolism. Peripheral embolization is relatively rare after open repair and

may be avoided by distal iliac clamping before placing the proximal aortic clamp. In contrast,

clinical experience of endovascular deployment suggests that it may be a significant complication of

this technique with a high mortality and an incidence that varies between 4-17% in some reports^^ .

In Parodi’s series^^ of 87 patients, distal thromboembolism resulted in 3 deaths (75%) out of the

four occurrences recorded. Large, tortuous aneurysms with a substantial amount of intraluminal

thrombus may pose an increased potential for peripheral embolization.

Endoleak

Endoleak is a term that describes "The inability to obtain or maintain a secure seal between the

aortic wall and a transluminaUy implanted intra-aneurysmal graft ^®". Endoleaks can be classified in

the following ways:

• Chronologically endoleaks can be described according to the time of onset after implantation

of the endovascular device: endoleaks are immediate or primary when detected within 30 days

of repair and delayed or secondary when discovered afterwards. They are recurrent when they

reappear following apparently successful initial sealing.

• Physiological criteria describe endoleaks according to their source of inflow: directly from the

aorta or from collateral channels.

• Anatomical criteria describe the site of origin of the endoleak. Proposals for an anatomical

classification of endoleak have been made by White et al (Table 5) 128-130

30

Most commonly there has been incomplete sealing of the prosthesis against the aortic neck or

common iliac artery (Figure 6). This type of endoleak (Type I) causes continued filling of the AAA

leading to increasing pressure and eventual rupture^^ ' Patent lumbar vessels or retrograde filling

from the intemal iliac artery (Type II) may also cause continued AAA expansion (Figure 7). Two

other categories are endoleak due to fabric tears, graft disconnection, or disintegration (Type III),

and flow through the graft material due to wall "porosity" (Type IV).

Figure 6: CT scan o f a patient w ith an upper stent endoleak. C ontrast can be seen collecting betw een the stent and the aortic

neck (arrowed).

Type I: from aorta-endograft seal failure

• Proximal end• Distal end

Type II: non graft-related• Inferior mesenteric artery• Lumbar arteries• Intemal iliac artery

Type III: fabric tear• Graft fabric tear• Graft body disintegration• Modular disconnection

Type IV: graft porosity

• Graft wall porosity• Small holes due to sutures/stents

Table 5: Anatomical classification o f endoleak

Type IV endoleak is usually seen in the first few days after repair with certain grafts such as the

AneuRx and is evident as a blush within the aneurysm sac on completion angiography.

In the Eurostar Registry, analysis of 899 patients^" indicated that endoleak had been identified on

discharge in 18“ o. A further 13% of patients developed new endoleaks during the first year of

follow-up although most resolved within 6 months. In contrast. Type II endoleaks were noted up to

18 months postoperatively.

The importance of endoleak lies in the observation that failure to exclude an aneurysm sac from the

circulation may lead to rupture^^\ Not all endoleaks produce this complication. During the course of

31

follow-up examination, up to 50% of early leaks may seal spontaneously^^^’ Consequently, there

is debate about the management of these patients. While it is agreed that Type I endoleaks should

be corrected, usually with additional endovascular manipulations, the timing of this intervention is

not always certain. This may relate to difficulty in the grading of endoleaks. Small or minor

endoleaks may conceivably seal spontaneously by thrombosis whereas larger leaks could necessitate

earlier intervention. One way of distinguishing objectively between endoleaks is to measure thek

pressure effects within the sac.

The importance of Type II endoleaks is as yet undetermined. In contrast to Type I endoleak, it has

been demonstrated that aneurysms with collateral patency do not show an increase in diameter at

least up to 18 months follow up ^ . However, some have taken the view that a large calibre patent

inferior mesenteric artery or large lumbar arteries (particularly the 4* lumbar pait) should be

embolized as soon as they are detected. Some evidence for this approach has been obtained from

studies of aneurysms treated by surgical ligation and extra-anatomic bypass^^ . Shah^^ reported two

patients who had persistent perfusion and enlargement of aortic aneurysms after open exclusion

and bypass. Lumbar artery collateral vessels and coagulopathy led to aneurysm rupture in one case,

and the collateral iliac flow was ligated with subsequent diminution of the aneurysm in the other. A

large patent channel may maintain aneurysm pressurization analogous to a pseudoaneurysm.

In animal models, spontaneous thrombosis of patent lumbar and other branches has been

demonstrated after deployments^®. In humans, however, Matsumara and Moore found no

difference in spontaneous closure between proximal and other endoleaks^^^ Large type II

endoleaks have been managed successfully by radiological embolization of the patent vessels using

steel coilsS 9 ot gelfoam sponges^^ despite reservations that this may increase the risk of paraplegia.

Finally, some workers have suggested that even if an endoleak seals spontaneously, the patient

remains at risk for continued enlargement of the aneurysm and subsequent rupture^^®. Others

however, argue that if a leak seals, the clinical behaviour of the aneurysm sac is similar to that after

endovascular repair in the absence of a leak ^ k

In a recent presentation comparing open (n = 253 patients) and endoluminal repair (n = 181), it

was found that when patients with endoleak (n=4) had rupture, major haemodynamic changes and

a high mortality rate were not always evident as rupture occurring de novo in an untreated AAA

(P<0.05). It was concluded that although failed endoluminal repair did not prevent rupture it

enhanced survival following open surgery for rupture, possibly by ameliorating the haemodynamic

changes associated with the rupture process " .

32

Postoperative pyrexia

A significant majority of patients have a sterile pyrexia commencing 24-36 h post-stenting and

lasting up to 3 days " . This is associated with a leucocytosis and elevation of non-specific serum

inflammatory markers such as C-reactive protein^"^. The aetiology of these observations is still

unknown although treatment is not necessarily indicated or provided.

It has been suggested that the cause may be related to bowel ischaemia caused by exclusion of the

inferior mesenteric and one or both intemal iHac arteries during graft deployment. In agreement

with this, colonic mucosal pH has been shown to decrease both during endovascular deployment

and afterwards " .

It is now known that the mural thrombus of an aortic aneurysm contains high amounts of

Interleukin 6 (IL-6)^" . Manipulation with introducers and catheters inside the mural thrombus

might cause direct release of this cytokine to stimulate leucocyte activation and the production of

tumour Necrosis Factor alpha (TNF-OC), which is able to mediate a systemic inflammatory

response. Levels of TNF-OC have been shown to increase after endovascular repair and can be

correlated to decreases in blood pressure " . These inflammatory responses could also be related to

endothelial cell damage by similar graft-related manipulations.

Another possibiHty is that manipulations inside the aortic aneurysm may cause release of various

toxic products from colonising micro-organisms. It is known that bacteria, mainly Staphylococcus

epidermidis, contaminate a proportion of aortic aneurysm thrombi, reported at 5% to 25%. It is still

argued whether this represents contamination or true colonisation '^ '^^ . There is no good evidence

to impHcate this as the mechanism causing endovascular pyrexia.

Vertebral body infarction has been recognised on MRI scans following successful exclusion of the

aneurysm sac ^ . This could also be responsible for a rise in body temperature although at present,

there are no studies examining this hypothesis.

Renal failure

Endovascular repair potentially carries its own renal morbidity compared to open repair. This

relates to radiographic contrast media and their reno-toxic effects, intravascular catheter and graft

manipulations and the risk of obstmcting the renal arteries with the prosthesis.

The adverse effect of iodonated contrast on renal function is recognised although the pathogenesis

of this compHcation is unclear^^ . Patients with pre-existing renal deficit and dehydration are at

particular risk^^t In addition to more general causes of renal disease, patients with aneurysms are at

greater risk of significant renal artery atheroma and nephropathy secondary to hypertension. It is

33

not uncommon to use 300-500 ml of contrast in difficult manipulations despite a conscious effort

to limit the volume injected.

Embolic occlusion of renal vessels is possible but usually where there is thrombus lining the aorta

above the renal arteries or where there is distal aortic occlusion just below the neck so that the

dislodged material travels preferentially into the kidneys.

Stent-grafts may be placed over the renal artery origins accidentally or deliberately where the

anatomy does not allow other options. When bare stents cross the renal ostia, intravascular

haemolysis and platelet microemboHsation into the renal vessels may occur leading to a long-term

decline in function. Antiplatelet agents may reduce the risk of this complication.

Superior s ten t leak

IMA leak

Inferior s ten t lea Lumbar artery leak

Figure 7: Sites o f endoleak after endovascular repair o f abdom inal aortic aneurysms

Graft distortion

Recent reports^ - ^ have identified a new cause of concern. Following implantation, it is apparent

that all endografts are subject to considerable physical stresses over time. These can result m

distortion of the prosthesis that may in turn be associated with new endoleaks and other clinical

complications such as graft limb thrombosis. There have also been reports of tears occurring in the

fabric of some grafts^^ , but there is as yet insufficient data available to draw any conclusions as to

whether this is a sporadic or regularly occurring phenomenon.

34

Delayed aneurysm rupture

Rupture of the aneurysm following endovascular repair has been described in individual reports

as well as in the larger registries. 6 of the patients registered with Eurostar had delayed rupture of

their aneurysms post-exclusion, three of which died^^ . These ruptures occurred in patients

exhibiting no signs of obvious endoleak, graft migration or suspected graft failure. It is possible that

minor fabric tears or holes (Type III/IV endoleaks) may be responsible for this problem if

undetected over a period of time thus causing a slow and delayed rise in sac pressure.

3.5 F U T U R E T R E A T M E N T F O R AAA

While endoluminal repair has obvious benefits for treatment of AAA, it is a procedure with a

number of unanswered questions:

Firsdy, it is difficult to predict, for individual patients, when the risk-benefit analysis favours a

choice of endovascular treatment over conventional treatment or no treatment at all. Different

kinds of patients present different problems. Patients with large aneurysms and poor health are

clearly at high risk of mortality if they under go either observation or conventional treatment, but

they are also at high risk if they undergo endovascular treatment as failed repair and conversion to

open operation may well be fatal.

Conversely, patients who are good risk for conventional operations will derive little benefit (in

terms of mortality rate) ftom the endovascular alternative, but they are also at a relatively low risk,

since a failed endovascular repair can always be rescued by open surgery.

Two prospective randomized trials of open versus endoluminal surgery in patients with large

aneurysms (>55 mm) have been initiated in the United Kingdom. The EVAR (Endovascular

aneurysm repair) 1 trial compares the outcome for fit patients undergoing endovascular versus

open repair. The EVAR 2 trial examines the outcome of endoluminal repair versus best medical

treatment in patients with large aneurysms rejected ftom open surgery on the grounds of high risk.

It is expected that these studies will provide answers to the above questions that can presently only

be partly answered with case control studies ^®.

Emergency repair of AAA

Endovascular graft repair of ruptured aortic aneurysms has been sporadically reported^^ ' '^h There

are inherent limitations in this type of procedure, including the need for preoperative

measurements of the aneurysmal and adjacent arterial anatomy to determine the appropriate size

and type of graft and the delay required to obtain proximal occlusion. The endograft system with

broadest versatility in this respect consists of the generation PTFE graft fixed with Palmaz

35

stents. The value of this graft lies in the ability to construct it at the time of surgery using readily

available components.

Emergency endovascular repair of leaking AAA has produced mixed results^^ '^^ , probably

because many of these patients have already sustained life-threatening hemodynamic insults by the

time they are treated. In theory, endovascular repair has the advantage of being able to exclude the

aneurysm from the circulation without destabilising the retroperitoneal heamatoma. However, the

main problem lies in rapid and accurate sizing of the neck, which has to be perfect to prevent

endoleak or later graft migration. Many of these patients can become rapidly shocked without

warning and the decision to await the results of a CT scan may mean the difference between life

and death.

Because of these constraints, it must be accepted that endovascular repair is not yet ready for

adoption as the standard treatment for the majority of patients with abdominal aortic aneurysms.

The most important message that can be drawn at this time is that the technology of endovascular

repair is still evolving. Newer endografts are constantly appearing and inevitably there is a

significant time lag before their clinical effectiveness of can be properly evaluated. Current data

relates mainly to the first three generations of device. It is impossible to state whether or not the

newer devices have addressed effectively the problems identified with previous generations of

endograft. This means that patients having endovascular repair of their aneurysm must be followed

up carefully according to a properly structured surveillance programme.

36

C h a p t e r 4

Pathogenesis of abdominal aortic aneurysm

As the biology of the arterial wall has unfolded with time, it becomes obvious that the aorta is not

an inert and passive conduit but a complex and metabolically active organ. In concert, a change in

the simplistic mechanical views that govern the behaviour and pathology of this region should

follow. In particular, aneurysms are not simple bulges in attenuated areas of aorta, but are the

summation of a complicated process of remodelling within that vessel. Even cursory histological

examination will show marked and obvious changes in tissue architecture during aneurysm

formation.

4 .1 T h e a o r t i c w a l l

In the 18th Century, Robert Hook (1635-1703) observed that the deformation an elastic body

underwent in response to an applied force was directly proportional to that force within a certain

point called the elastic limit. Once this was breached, the body retained a permanent deformation.

This law cannot be easily applied to aortic tissue as the human aorta is not a homogenous tube of

constant proportions. There are macroscopic and microscopic constraints that limit the

applicability of physical laws to living tissue.

Macroscopic constraints

Blood vessels in vivo are at a different length to their in vitro preparations. For example, when the

upper thoracic aorta is removed at post mortem from subjects less than 35 years of age, it undergoes

an immediate retraction of at least 22% of its original length. This retraction lessens with age but

may still lie within 10-15% of the in vivo length^^ . The main reason for this phenomenon is

vascular tethering. It is believed that the aorta grows at a much slower rate than the vertebral

column to which it is tethered, so that a considerable stretch is set up during growth. As the

individual begins to age and lose the elastin and collagen in this vessel, the tethering effect is

reduced manifest by the reduction in its retraction.

It has been well documented that the majority of aneurysms occur in the inffarenal segment of the

abdominal aorta * . The aorta becomes stiffer and thinner as it progresses distaUy. This may be

because the constituents of the aorta are not uniformly distributed along its length. The loss of

elasticity is due to an increase in collagen and decrease in elastin fibers in this segment " . This has

important implications for the transmission of a pulse wave and for the change in diameter of this

segment during the passage of the arterial pulse.

37

Microscopic constraints

Histologically the aortic wall is composed of three main layers incorporating three main

components (Figures 8 and 9).

The three layers described ftom the lumen outward consist of the tunica intima, the tunica media

and the adventitia. They are separated by condensations of elastin, which form the intemal and

external elastic laminae respectively.

The intima is formed by a layer of endothelial cells supported by a thin rim of connective tissue.

The media is the functional layer in the context of aortic biomechanics and consists of collagen,

elastin and smooth muscle cells. The smooth muscle cells are called myofibroblasts because their

synthetic properties closely resemble those of true fibroblasts. The constituent elements are held in

a semiliquid matrix composed of mucopeptides and water which makes up more than 70% of the

aortic wall.

The outermost adventitia is formed by connective tissue that surrounds the vasa vasorum and

merges in with the loose areolar tissue of the retroperitoneum. The abdominal aorta has a sparse

vasa vasorum and it is believed that this segment of artery obtains its nutrition mainly from the

luminal blood flow.

Elastin

Elastin is a 70,000 dalton protein which is unusually rich in proline and glycine. The details of

elastin synthesis and secretion are not fully understood but we do know that extracellular elastin

molecules are arranged in a folded mesh-like structure within the aortic wall (Figure 8), believed to

unravel as the aorta pulsates^^ .

Elastin is synthesised early in life and has a long biological half-life of at least 70 years. The amount

of functional elastin within the aorta is finite. Experiments have shown that elastin may be

synthesised by myofibroblasts in later life but is non-functional and does not contribute to vessel

strength^^ . Elastic fibres are not composed solely of elastin, but also contain a surface glycoprotein

that forms microfibrils within the meshwork. These microfibrils appear before elastin in

development and may serve to organise the secreted elastin molecules into the mesh arrangement.

Mutations in fibrillin, one of these fibrillar proteins, have recently been described in Mar fan's

syndrome where AAA forms part of the clinical picture^^ .

Individual elastin fibres are 5-10 times more easily stretched than rubber and closely follow the

principle of Hooke’s Law up to the yield point. This point is only reached when an elastin fibre has

approached more than(60470% of its original length ^®.

Changes in elastin

Destruction of elastin in the infrarenal aorta precedes the formation of an aneurysm. Histological

examination of AAA walls reveals a deficiency of elastin fibres with a reduction from the normal

elastin concentration of 12% to

The pathogenesis of this elastin degradation is unknown. Since elastin is synthesized and deposited

during early life with a long half-Hfe, it is unlikely that aneurysms develop as a result of inadequate

synthesis. If this were the case AAA would be seen at a much earlier age.

One promising lead has been in the direction of elastase activation. Elastin is a protein that is

extremely stable and highly specific elastases are required for proteolysis. Two types of elastase

have been isolated from the walls of AAA^^ . Although these are also found in normal aorta, the

quantities are significantly less. These elastase enzymes can be derived from macrophages,

endothelium, myofibroblasts and platelets. This is interesting as this group of cells is found in large

numbers within intra-aneurysmal thrombus and has also been implicated in the genesis of

atheromatous plaques.

Excessive protease activity may not be the only cause for elastin degradation. Decreased levels of

protease inhibitors within the aneurysm wall have also been identified^Dim inished levels of

alpha-1-antiprotease have been found in the walls of AAA, especially in ruptured aortas^^b There is

also an association between Chronic Obstructive Lung Disease, a risk factor for aneurysm rupture,

and alpha-1-antiprotease deficiency^^ . In a small study, the MZ variant of alpha-1-antiprotease has

been linked with an increased frequency of ruptured AAA^^ .

Decrease in elastin may also be due to dilution, as discordant synthesis of other matrix proteins

during dilatation, causes a relative decline in elastin levels " .

Collagen

Collagen is not a single protein but a family of characteristic molecules that constitute 25% of the

total mammalian protein. It is synthesised throughout Hfe. The central feature of all collagen

molecules is their stiff, triple-stranded helical structure. Three (%-helical peptide chains are wound

around each other to create a rope-like structure about 300 nm long and 1.5 nm in diameter. There

are at least seven different forms of collagen tt-helical chain and theoretically this could give rise to

more than 100 different forms of triple helix. In practice, fewer than 12 forms are known, the

major types described by the numerals I-V. Types I and III are the main arterial form and

molecules assemble to form long collagen fibrils within the tunica media (Figure 9). The structure

is strengthened by the formation of unique covalent cross-links within and between the constituent

39

collagen molecules. The extent and type of cross-linking varies between tissues and where tensile

strength is crucial, as in the Achilles tendon, the linkage is especially dense.

Collagen has significantly lower extensibility compared with elastin and does not obey Hooke’s

Law. The yield point is reached at a maximum extension of only 10-50% of the original length.

Collagen however is very strong^^ with a tensile strength reported as 5 x 10^ to 5 x 10^ dynes/cm^

compared with elastin which has a tensile strength of 3.6 x 10^ to 4.4 x 10^ dynes/cm^

Changes in collagen

Although elastin bears most of the tensile load within the vessel wall at physiological pressures,

collagen fibres are increasingly recruited as the pressure rises^^ . For an aneurysm to form therefore,

collagen must fail.

However, no qualitative changes in collagen or gene mutations have been demonstrated in patients’

with AAA. In fact, the total protein content of aneurysms is actually 8.4 fold greater than normal

aorta^^ . Whether this can be attributed to increased synthesis of collagen is arguable. Studies of

collagen messenger RNA (mRNA) in myofibroblasts, have shown raised levels of both type I and

IIT^ . The increased synthesis may be balanced by increased degradation. Increased coUagenase

activity in tissue from AAA has been reported although this may only occur in ruptured aneurysms.

The source may be the smooth muscle cells in the media as shown by immunoreactive studies ^®.

In the Ehlers-Danlos syndrome, a deficiency of type III collagen causes spontaneous aortic rupture

with little preceding dilatation^This reinforces its important role in the pathology of the aortic

wall.

Myofibroblasts

The myofibroblast is the sole cell type normally found in the arterial media. It is a terminally

differentiated cell that expresses cytoskeletal marker proteins Hke OC-actin and myosin heavy chains.

These enable it to contract in response to chemical and mechanical stimuli. Uniquely it is able to

revert to a proliferative and secretory state equivalent to that seen in vasculogenesis^®®. These two

phenotypes are influenced by a variety of growth factors, cytokines and other molecules. Smooth

muscle cells in culture wiU produce extracellular matrix proteins such as collagen, elastin and

proteoglycans.

The role of smooth muscle in arterial mechanics is controversial^ This is pardy due to the

difficulty in elucidating a mechanical relationship for a structure capable of existing in two different

phenotypes. Smooth muscle contraction results from the formation and breaking of protein links

involving actin and myosin chains. Thus shortening and lengthening is not a simple elastic

40

phenomenon. This may not be appreciated in vitro where there has been rigor mortis or in vivo where

general anaesthesia is known to cause paralysis of vascular smooth muscle.

Further, smooth muscle in the aorta is actually orientated in a circumferential manner and it is now

clear that its contractile tension is capable of decreasing the diameter of even large arteries although

individual cells do not appear to shorten more than 25-30% of their resting lengths^® .

41

Figure 8: Cross section o f aortic wall showing laminar arrangement o f elastin (high power usmg elastin stain).

Figure 9: Haematoxylin and eosin stained section of aortic wall showing thick tunica media containing large amounts o f collagen and

myofibroblasts (high power).

42

C h a p t e r 5

Aims and objectives

This thesis will investigate 2 areas of endovascular repair. These are the morphological changes

affecting aneurysms after treatment and the dynamic nature of the interaction between the stent-

graft and aortic wall.

5 .1 M o r p h o l o g i c a l c h a n g e s

Endovascular repair involves the placement of an endograft within an aneurysm with the aim of

completely excluding the sac from the circulation. It differs from conventional repair because the

anatomy of the AAA remains intact. Currentiy, much uncertainty surrounds exacdy how the

natural history of the aneurysmal process is modified after endovascular repair.

Charting sac morphology accurately and reliably provides a valuable method of establishing the

efficacy and durability of the repair. It is particularly of value in the detection of changes in the

neck and sac of the aneurysm post-exclusion.

The aneurysm sac

It has been shown in animal m o d e l s a n d in humans^® that aneurysmal sac pressure and wall

stress are sharply reduced on completion of successful deployment thus reducing the risk of

rupture. Analysis of aneurysm size ® and pulsatile wall motion using indirect measures, such as

echo-tracking ultrasound, also confirm reduced sac pressure on deployment^ ®.

Unfortunately, it is not currendy possible to continue pressure measurement in vivo much beyond

the point of deployment. Further, there is no evidence that the pressure reduction is maintained

long-term. It has been argued that patent lumbar arteries may continue to perfuse the aneurysm

sac thus delaying the risk of rupture rather than preventing it permanendy^® . In one series of 1103

patients with AAA, after open proximal and distal ligation of the sac combined with an aorto-

aortic bypass, 17 (2%) continued to have enlarging sacs despite complete anatomical discontinuity

from the abdominal aorta and common ihac arteries ®®.

Shrinkage of aneurysm sacs after exclusion is a second end-point measure of successful

endovascular repair ' » 189-192 diameters of successfully excluded aneurysms are said to

decrease, except in cases of endoleakage or retro-perfusion. Mahna and colleagues described a

decrease of up to 6 mm in the maximal sac diameter of successfully excluded aneurysms in 35

patients. Patients with endoleak also had a decrease in diameter but only after further

endovascular intervention was performed!® .

43

Shfinkage however, is by no means absolute. In one group of 7 patients followed for 15 months

with spiral CT, there was continuing shrinkage of the sac in only 2 (29%) patients. In the

remainder the AAA either remained stable (43%) or increased in size due to endoleaks (29%)^^ .

One reason for such discrepancies may be related to the use of imaging software that has not

been validated for aneurysm measurements. This was originally recognised for measurements of

aneurysm diameter made with ultrasound that were shown to be highly operator dependent: the

coefficients of repeatability vary as much as 3.0-7.5 for AP diameter and 10.2-15.4 for transverse

diameter^ " . Similar studies have rarely been performed with any of the common imaging

modalities presently used for endovascular repair.

The use of maximal diameter measurements as indicators of postoperative aneurysm morphology

suffers from a number of disadvantages. Firstly, axial slices from conventional CT are not always

perpendicular to the vessel lumen. Secondly, one cannot be certain that the same maximal

diameter is measured on every occasion that a patient is scanned. An aneurysm sac may change its

shape but still retain the same value for its maximal diameter (Figure 68). Measuring more than

one diameter within the sac is likely to increase the accuracy of measurement. If all these

diameters could be described with a single numerator, this would then increase the convenience

of this measure still further. It is proposed here that aneurysm volume may be a better indicator of

aneurysm sac morphology. The volume of a sphere is proportional to the cube of its radius and so

only a small change in the latter will lead to a much larger change in volume. The small change in

diameter may not always be reliably measurable.

The fate of the aneurysm sac will be recorded in a prospective study using 3D volumetric spiral

CT angiography after the completion of adequate validation studies. An introduction to the basic

principles of the imaging techniques used in this study is provided as a prelude to these

experiments.

Intra-luminal thrombus

Related to the anatomy of the sac post-exclusion, is the fate of intra-luminal thrombus (ILT).

Thrombus is a dual-edged sword. It may have an important role in the pathogenesis of AAA. A

potential interaction may be to cause wall anoxia by acting as a diffusion barrier to oxygen passage

into the tunicae intima and media^^ .

Histological studies have demonstrated that a complex and self-sustaining “ecosystem” exists

within the structure of ILT * . Immunocompetent cells that are capable of antigen presentation

and cytokine production are found in greater numbers within AAA thrombus compared to non-

aneurysmal atherosclerotic tissue^^ . It has been suggested that these cells contribute to the

44

pathogenesis of aneurysm formation by cytokine secretion altering the biological turnover of the

constituents of the aortic waU ®.

Thrombus may also have a protective role to delay aneurysm rupture. Segments of ILT have a

unique elastic modulus and follow simple elastic theory in vitrxP' . This has been confirmed

mathematically with finite-element models of aneurysms. Finite element analysis allows the

computation of mechanical stress distribution on a body subjected to known forces or

deformations. Using this technique it can be shown that thrombus reduces the stress on the

aneurysm wall in proportion to its thickness^oo. When the thrombus is placed eccentrically within

the AAA lumen, the greatest points of stress are found in the thinnest regions of wall.

More recently, it has been suggested that ILT may protect against Type II endoleaks by plugging

the orifices of patent arteries. Broeders et al demonstrated that the incidence of persistent

backflow from sac branches was dependent on both the number of patent lumbar vessels as well

as the presence of thrombus and its localisation: the presence of posterior wall thrombus was

associated with a lower incidence of back-flow^^h

During conventional AAA repair thrombus is removed but endovascular repair leaves it intact.

To date, there has been no attempted study to elucidate the fate of ILT after endoluminal

exclusion of AAA. This study will set out to prospectively record the fate of ILT, using

volumetric technology, after endoluminal exclusion.

The aneurysm neck

The long-term success of endovascular repair is dependent on the secure fixation of the stent-

graft at the proximal and distal attachment sites. Progressive dilatation of the inffarenal neck may

jeopardize this success.

Conflicting views exist in relation to the fate of the neck and data on changes after endovascular

exclusion are sparse. On the one hand, it is thought that this part of the aneurysm is fairly stable

and unlikely to dilate over time. Histological studies demonstrate crossing fibres in the adventitia

that enter the neck from the renal arteries. These may make it more resistant to dilatation. The

pressure required to balloon dilate this region of the aorta is also much higher than the pressure

needed to dilate the lower inffarenal aorta^^. This is in direct contrast to experimental results

suggesting, both in vitro and in vivo, that a generalised metabolic dilatory diathesis may affect all

arteries in individuals with aneurysms. Sonesson showed a significant increase in the stiffness of

the aorta in patients with AAA compared to unaffected controls, confirrning the altered

mechanical properties of the aneurysmal wall. When the same index was calculated for the

common carotid arteries of both groups, similar differences were found suggesting a generalized

process of the vasculature with focal manifestation in the abdominal aorta.^^

45

The long-term fate of the infrarenal neck has been studied after open aneurysm surgery. Hallet^^

et al demonstrated, in a population-based study, that neck enlargement occurred in 13% of

patients after open repair. Illig and colleagues^os found an average growth of nearly 10 mm over

10 years in necks initially measuring more than 27 mm in diameter. Similar results were obtained

using trans-lumbar aortography as the imaging method in a retrospective study of 800 patients.

There was a mean dilatation of the aortic neck of up to 5 mm and an elongation of up to 10 mm

at 42 months^^^. This type of change would be more than sufficient to cause loss of the endograft-

vessel wall seal despite the routine graft oversizing of 2-4 mm. However, it could equally be

argued that conventional repair, which results in surgical disruption of the aorta, alters arterial

biology and that neck dilatation is simply a reflection of this interference.

Reviews of neck diameter post-endovascular repair also suggest that this region may continue to

dilate. Malina ® reported on proximal neck dilatation of 2 mm in the first postoperative year using

self-expanding stents for fixation. Matsumura and Chaikof, using the self-expanding EVT graft,

also observed continuing dilatation of up to 3 mm for at least 2 years in their 59 subjects * . In

contrast. Walker et al found no significant change in the proximal neck diameters in their 4 year

follow-up of 112 patients after endovascular repair. Tlie.^thors^did-not unfortunately specifyihe'*

nature of stent used in their grafts ®®.

Aneurysm neck dilatation can be difficult to separate ftom the physical effect of the endo­

prosthesis. It is as yet unclear whether neck enlargement is limited to the size of the prosthesis or

whether it continues on unabated with time. Some authors have suggested that dilatation is more

likely to occur with balloon-expandable rather than self-expanding endografts^o^. The sequel to

this, late perigraft flow or endoleakage, has recently been reported in older patients^^®.

One aspect of this study was to record the changes in aneurysm neck diameter occurring after the

deployment of endografts fixed with balloon- and self-expanding stents.

5.2 D y n a m i c s t u d i e s o f t h e a n e u r y s m n e c k

Endovascular repair involves the insertion of a semi-rigid graft into a dynamic arterial system. The

aneurysm is never static and there is always movement between the neck, sac and the üiac arteries.

These graft-vessel wall interactions may be important in long-term graft stability. Attaining a

blood-tight seal between the graft and underlying aortic neck has as much to do with the nature

and quality of the aortic tissue as it does with the construction and function of the device itself.

The individual components that form the aortic wall and their mechanics will influence the extent

of arterial motion and hence the amount of stress experienced by the prosthesis over its lifetime.

It is proposed here that the effects of aU these different motions may summate over a period of

g )

time to result in a physical alteration of the endograft. The dilemma lies in defining and

quantifying the extent of differential movement.

Filming and measurement of instantaneous changes in arterial diameter presents the first hurdle.

Arterial walls change their radius by less than 10% in response to a pressure change of almost

50% during the cardiac cycle ^ .

Further complexity is added by the changes occurring during diseased states. The changes from

normal to aneurysmal aorta involve considerable remodelling of the arterial wall accompanied by

an increase in the elastic modulus or stiffness index " . This in turn is likely to alter the dynamic

behaviour of the aortic neck. This area has never been imaged before in its normal let alone

pathological state.

Previous investigators have focused on the dynamic measurement of arterial diameter to calculate

its elasticity in health and disease. The major categories for the instruments that have been used

are electrical, radiological and optical.

Electrical methods usually incorporate some form of strain gauge calliper and produce results

dependent on the constraint introduced by the device^^ - 216. xhe technology can however, be

rniniaturised to such an extent, that it can be used on blood vessels as small as 10mm indiameter^^^

A major problem is the invasive nature of the electrical methods precluding safe use in humans.

Furthermore, the study of diameter changes over a long period of time requires implantation of

the device, which was difficult even in animal subjects.

Ultrasound has been the most popular method used to date for measuring arterial diameters.

MacSweeney et aP- used M-mode ultrasonography to calculate the elastic modulus of the

aneurysmal aorta which was correlated with a decreasing elastin content in the tunica media. The

regional elastic properties of the aorta have also been examined using transoesophageal

echocardiography220 and by an echo-tracking and phase-locking device^^i. 214 hig method, two

electronic markers are placed on the signals corresponding to the vessel wall on a real-time

display. An echo-tracking phase loop then restores the position of an electronic gate relative to the

moving echo-signal returning from the vessel wall. The output signal from the echo-tracking

circuits represents the distance between the vessel walls. This method produces very accurate

measurements of aortic diameter and can detect a change as small as 8 m m ^. However it is

difficult to position the electronic markers accurately as distinguishing where the vessel wall ends

and the lumen starts is a problem. Furthermore, images can only be obtained in 2 dimensions due

to the constraints of the display. Although this does not hinder the measurement of diameter, it

prevents visualisation of the whole artery and 3 dimensional motion studies are not possible.

47

optical methods to record diameter changes have been used on isolated arterial segments, passing

a coUimated light beam across the vessel onto a photoelectric light tube ^ - Changes in vessel

diameter cause variations in the amount of light falling on the tube and diameter changes of up to

6 mm may be detected. This method requires the arterial specimens to soak in an oxygenated

solution and cannot be applied in vivo. Values for the elastic modulus obtained using this method

are very high and it is now clear that this is due to an absence of vasa vasorum around the

specimens^.

Filrning of the pulsatile change in arterial diameter was attempted by Hiramatsu et al who

described a needle-probe videomicroscope used to film subendocardial vessels in a beating

heart^^. They found in anaesthetised pigs, that there was a total ratio of change in diameter of

24% of these vessels during the cardiac cycle. The same experiment was repeated a year later in

canine subjects to measure diameter changes during prolonged diastole^^ .

The aim of this study was to identify and measure arterial motion during open repair of aneurysms

and other vascular procedures using conventional and digital cameras to record changes. These

data were analysed using photogrammetric principles novel to this field. A working knowledge of

these principles is presented in Chapter 8 so that the reader may follow the reasoning behind each

experiment.

48

C h a p t e r 6

Imaging techniques used in tliis study

The fundamental difference in requirements for execution of repait between conventional and

endovascular grafting for aortic aneurysm is the role of imaging. Endovascular repair is heavily

reliant on comprehensive imaging techniques. Preoperatively this must be capable of providing

accurate and reliable measurements of the aorta, iliac vessels and demonstrate anatomical features

that may complicate the procedure.

For a typical bifurcated graft, measurements of the length and diameter of the proximal aortic

neck, the length of the vascular channel and the üiac arteries are essential (Figure 10).

Undersizing the endograft diameter can cause an inadequate seal between the graft and the

aneurysm wall resulting in endoleak or perigraft flow that has been associated with AAA

mpture^^h Similarly, oversizing of the graft and crimping of the excess material also produce

endoleak or rupture of the aortic wall. Insecure attachment also results in graft migration and

occlusion of major visceral arteries.

C u r r e n t i m a g i n g t e c h n i q u e s

There are several available methods for imaging the abdominal aorta prior to endoluminal surgery.

Most centres use a combination of arteriography and conventional or spiral CT as part of the

endovascular-imaging algorithm. Magnetic resonance imaging (MRI) has recently been advocated

in view of the relatively lower toxicity of the contrast agents utilised and the ability to image in all

planes^®. However, it is an evolving technology with many ongoing improvements and changes

still occurring at the time of writing.

6.1 Contrast angiography

Preoperative angiography offers an excellent representation of the anatomy of the contrast filled

aortic lumen and its branches, but it fails to give an adequate measurement of the dilated sac in

cases where the aneurysm is partially filled with an adherent mass of thrombus (Figure 11). It is

performed using either conventional x-ray film or digital subtraction technology^^. The

advantages of digital subtraction include the fact that it uses less contrast media, multiple views

can be rapidly obtained and images can be observed in rapid sequence so that flow can be

assessed. Since the degree of magnification cannot be ascertained when using digital subtraction

angiography (DSA), a catheter with radio-opaque markers must be used when providing

49

measurements (Figure 30). Diagnostic studies and interventions may be instituted via the

transfemoral route using the Seldinger technique-^^\ Care must be taken to make the puncture

extra-peritoneally to avoid undetectable retroperitoneal bleeding. Arterial damage such as intimai

tears may be avoided by ensuring that the guidewire exits the needle intraluminaUy and travels

freely with minimal resistance. Brachial artery puncture may also be performed. We prefer the

latter during endograft deployment as it provides an excellent route for assessing the deployment

process and supplies an access port for further endovascular interventions.

D3

D4

F ig u re 10: M easurements required for the construction o f an endovascular graft. B oth the upper (D l) and lower neck (D2) diam eters are needed together w ith the neck length (LI) to docum ent the shape o f the upper fixation point. T he maxim um diam eter o f the sac and the iliacs are D3,

D 4 and D5 respectively. The length o f the lA FC is L 1+L 2+L 3 for the right limb and L 1+L 2+L 4for the left.

Large doses of contrast can have a deleterious effect on renal function that may be more likely if

contrast is injected preferentially into the renal arteries. Patients with elevated creatinine or

contrast allergies may still undergo angiography using digital subtraction and carbon dioxide (CO2)

contrast^^k These images are of poorer quality and useful only for diagnostic or therapeutic

50

manoeuvres such as positioning of the stent-graft before deployment. The quality does not

support the accurate measurements detailed above.

In addition to assessing preoperative aneurysm dimensions, DSA can also be used as a therapeutic

aid to manage co-existing problems prior to endografting. Access vessel stenoses can be treated

by angioplasty at the time of the diagnostic arteriogram, facilitating the introduction of a large

delivery device. If an aorto-uni-ihac device is being considered, coil embolization of the

contralateral common or internal iliac artery can be carried out. Similar embolization techniques

may also be used intra-operatively to prevent retrograde sac perfusion via small pelvic and lumbar

collaterals^^®’

Intra-procedure angiography during endograft deployment is almost a sine qua non. Angiography

provides excellent views of the renal vessels and aneurysm neck during deployment and gives

early warning of renal arterial occlusion.

A completion angiogram should be performed to search for proximal/distal endoleak, graft kinks

or significant graft compression and altered vascular flow. Additional endovascular techniques can

then be instituted during the same session and their success or failure confirmed with further

injections.

Post-procedure follow-up angiography is only really useful in confirming the presence of an

endoleak or detecting endograft disruption. It cannot be used to demonstrate the continuing

enlargement or shrinkage of the aneurysm sac, measurements that are important in determining

the success of the procedure.

6.2 Principles of conventional and spiral CT (SCT) scanning

A plain radiograph is simply a map of the attenuation coefficients of the various tissues being

imaged for the energy spectrum of a typical x-ray beam. Radiographs usually allow contrasts of

the order of 2% to be seen easily so that a 1 cm thick rib or a 1 cm air-filled trachea can be readily

distinguished. However, blood vessels and other soft tissue details, such as the cardiac anatomy,

cannot be conventionally visualised. A second problem is the loss of depth information caused by

the projection of a 3-dimensional structure onto 2-dimensional film. This can be overcome to

some extent with conventional tomography or viewing stereo-pairs of film (Chapter 8).

A CT scan differs from a radiograph because it provides two-dimensional information concerning

a three-dimensional slice of the patient without any superimposition from other regions (Figure

12). The x-ray beam does not interrogate parts of the body outside the sHce, thus eliminating the

problem of "depth scrambling". The resulting sequence of images (axial scans) show the sectional

51

anatomy with a spatial resolution of about 1 mm and linear attenuation coefficient (density)

of better than 1%.

Figure 11: A ortogram o f the abdom inal aorta in a patien t w ith an abdom inal aortic aneurysm sac m easuring 6.5 cm. N either the

an teroposte rio r (left) n o r lateral (right) view o f the abdom inal aorta dem onstrates the size o f the aneurysm because o f the lam inated throm bus filling the vascular channel o f the aneurysm , allowing

b lood (and contrast) to flow through a m uch reduced lum en size.

Conventional C T

Conventional (slice-by-slice) C l’ is performed by sequential scanning at multiple pre­

determined levels. Ih e incremental data are collected with the patient stationary and

suspending respiration at each level. This often results in stepping or misregistration related

to the difference in the position of the diaphragm and other organs during multiple breath-

holds. These artefacts produce poor quality images particularly if intravenous contrast agents

are given as with aortic images. The scan times are lengthy (1-3 seconds per slice) and

because of the interscan delay (typically 1-5 seconds between slices), the number of slices

with optimal contrast enhancement are limited. Consequently, large amounts of contrast

medium may be necessary with resulting renal complications.

Spiral C T

Spiral CT is based on the continuous acquisition of data from an entire anatomic volume

and differs from conventional CT because a whole volume can be scanned within a short

time interval. An entire CT examination of the head, chest or abdomen may be performed

within 90 seconds, often with only a single breath hold required by the patient. With

52

shortened acquisition, only brief contrast injection times are needed for optimal vascular

opacification. Moreover, a bolus of injected contrast can be captured with the peak aortic

concentration, thus simulating angiography^^.

Spiral scanners are based on a slip-ring gantry system, which does not have any electrical wire

contact, and the X-ray tube can therefore rotate a full 360 degrees around the patient without

entanglement.

The spiral CT scanner table also moves relative to the tube at predetermined speeds: in this way a

helix of raw data can be acquired. Axial scans within the scanned volume can then be

reconstructed at any level giving the possibility of overlap and hence improving anatomical

accuracy.

It is essential to recognise that spiral CT data does not represent any single section of tissue but

the entire 3-dimensional scanned volume. This data is reformatted into axial planar images, similar

to those of conventional scanners, using special mathematical algorithms'^. The initial scan data,

stored in the form of small units of volume known as voxels, is reformatted on 2-dimensional

scans as pixels. Each voxel has its own specific density, proportional to the linear attenuation

coefficient of the tissue. This is rescaled in terms of a CT or Hounsfield (HU) number defined as:

CT îlUtllbcr jdtissue" jdwater / jd-water X 1000

where |I refers to the linear attenuation coefficient. By convention, water is given a Hounsfield

value of 0 while air is -1024 HU. The HU of different tissues are relatively close to each other and

relatively close to 0. However, provided the projection data are recorded with sufficient accuracy,

differences between soft tissues can be displayed with a high degree of statistical confidence.

Changing the contrast of the display can further enhance visualisation of small differences. This is

related to the HU number by means of a level and width control allowing the small range of

numbers corresponding to the soft tissues in question to drive the screen from black to white.

Visually, HU are represented by a different shade of grey for each pixel on the 2D film. Since the

human retina does not have the ability to distinguish between all the different grey-scale

resolutions, the amount of available data within scans is not always appreciated.

Reconstruction's of |4 are made on rectangular arrays that define the resolution of the CT scanner.

The scanner used in this project was a Siemens Somatom plus 4 with a rectangular array

resolution of 512 x 512 pixels.

53

Partial volume artefact

CT slices do not usually intersect anatomical structures at right angles. A long, thin voxel could

well have one end in soft tissue and the other end in bone. The value of |X for these longitudinally

arranged voxels spanning between the two ends of a segment of anatomy (Figure 13) are simply

averaged to give a single grey-scale value for one pixel. For tissues where there is not much

change in density, this is not a significant problem. However, where there are sharp changes in

density, averaging can produce values that do not correspond with any real tissue at all.

The partial volume artefact wiH exist for aU situations where the resolution of the scanner is

insufficient. For example, CT resolution is about 1 mm x 1 mm x 1 mm whereas a typical cell is of

the order 10 |lm x 10 jim x 10 |lm. This results in a blurting of the intensity distinction between

tissue classes at the border of two tissues.

54

Monitor: X-Ray T|

Detectors

Computer

Figure 12: Principles o f spinil CT scanning. An x-ray tube and detectors for measuring the x-rays are placed opposite inside the gantry housing. Dunng data acquisition (scan procedure) the patient, lying on the table is moved through the gantry opening either step by step or continuously (Spiral CT technique). As the tube and detector rotate

around the patient. X-rays are collimated to a fan beam with selectable thickness that controls the width of the slice imaged. Passing through the patient, the x-rays are

attenuated according to the patients’ size and type o f tissue scanned. For every 360° tube and detector rotation, data profiles are taken from about 1000 viewing directions. A profile is measured approximately every 0.33°. The resulting "attenuation profile" is then digitized by the image computer, which reconstructs images by transferring the

attenuation values from all the views to a pixel matrix.

VI V2 V3

Voxel 1

Voxel 2

Voxel 3

Figure 13: Demonstration o f partial volume artefact. The 3 voxels (VI, V2 and V3) span across the lumen and thrombus o f the aneurysm. Because V2 lies across the boundary

zone, its Hounsfield value is intermediate to VI and V2, a situation which is not representative o f the real object V2 therefore has a partial volume between thrombus and

lumen.V' C V:

3'

Data presentation

Volumetric acquisition of data allows display of images as conventional transverse slices,

multiplanar reformations (MPR) or three-dimensional reconstructions (3DR). MPR and 3DR

require post processing of the raw data at a separate workstation.

Axial Reformatting

The standard display technique for CT images is the generation of axial sHces. These sHces suffer

from a number of disadvantages.

First and foremost, they are not orientated perpendicularly along the true anatomical centre of the

aorta. This is due to the inherent tortuosity of this vessel causing deviation from the longitudinal

axis of the body. The resulting elliptical cross-sections make diameter measurements unreliable.

Although it may be possible that the narrowest diameter of the ellipse is the true diameter, this is

not always the case as the aneurysm is not usually a perfect cylinder or cone. Measurements of the

AAA vascular channel are extremely difficult using conventional CT and the current standard is to

perform angiography with the disadvantage of further invasion of the patient and additional doses

of contrast and radioactivity.

Post processing

MPR and 3DR are steps that depend on post-processing carried out on a workstation separate

from the original scanner. All the data sets used in this study were processed on the Medical

Graphics Interactive (MGI, University College London imaging group) workstation. The

procedure that was used for 3D modelling is presented in Figure 15.

Multiplanar reformation (MPR)

The first 3-dimensional use of a volumetric data set was made by the introduction of multiplanar

reformatting, which allowed for visualization of a cross-sectional plane other than the one in

which the scans were made^^>

In this technique, axial slices are stacked in the positions they would occupy in vivo allowing

visualisation of any of the 3 orthogonal planes (sagittal, transverse and coronal) simultaneously. A

further improvement allows electronic markers to be placed either within or on the surface of the

object (Figure 19). Distances between markers can then be calculated.

Most vascular structures are not orientated in a single plane and perpendicular reformation makes

full anatomical visualisation of curved vessels such as the diac arteries impossible. The workstation

used in this study was configured to overcome this problem by linking the multiplanar data to the

56

orientation of the 3-dimensional image. When the 3D image was manipulated through 360

degrees the MPR revolved by exactly similar amounts in all 3 planes.

One disadvantage of MPR is that the longitudinal resolution is generally inferior to the transverse

resolution. This is due to the table speed, which may generate a streak artefact. The image quality

of vessels located perpendicular to the scanning axis such as the aorta, is hence inferior to the

quality of vessels parallel to the axis. For large structures such as aneurysms however, this is of

little consequence during measurement.

3-dimensional reconstruction

Early 3-dimensional images were simply wire-riame images (Figure 15: stage 3) of high-contrast

tissue interface surfaces such as bone,^^» 36 skin^^ and lung^®. Connecting tissue contours with

perpendicular line segments^^ or subdividing the strips between contours into triangles^'^ is called

"tiling" and creates a covering composed of elements that can subsequently be used for shading of

that surface. Modem techniques for surface rendering require that the structure to be imaged is

first isolated or segmented from the total data volume. This takes place in two stages.

For highly contrasting tissues, such as bone versus air, the segmentation process makes use of the

Hounsfield number or magnetic resonance-number threshold. Volumes of interest are simply

extracted from the CT images by setting windows based on Hounsfield units. For example, a

value of 0 HU will display everything with a density greater than water while 100 HU or higher

will only display voxels with a density greater than that of a mixture of blood and iodinated

contrast.

For interfaces with little contrast (eg, aortic wall versus surrounding soft tissues) segmentation

may become impossible without manual interaction with the image. The area of interest is picked

out manually with the aid of a painting/editing tool (Figure 14). The post-processing computer

then extracts this new data set from each slice, keeping images in the correct sequence and

orientation by aligning individual pixels to a reference grid. A superficial wire-mesh constructed by

joining corresponding pixels. Generating a shaded surface on the selected object creates the

illusion of depth. This is referred to as the surface shaded display (SSD).

57

Figure 14: Demonstration o f manual editing o f abdominal aortic aneurysm from surrounding bone and

soft tissues. I he aneurysm sac has been selected manually with the editing tool for segmentation. This

process has to be repeated for every slice in the data set.

Both types of segmentation were used in this study. AAA lumen, wall and thrombus were

manually edited out from each axial spiral CT slice and then further separated by setting

windows to exclude the thrombus and aneurysm wall to leave only the vascular channel on

display. The differing threshold values for the aneurysm and its vascular channel were

calculated in a separate experiment (Chapter 7.2).

Data from conventional CT can also be reformatted to give three-dimensional images. This

type of post-processing is much less accurate because, in anatomic volumes affected by

respiratory movements, there is a chance that some structures may not be visualised due to

misrepresentation.

Surface Shading

Surface shading (Figure 18) or "depth encoding" displays the distance between observer and

the surface element in a grey level (the shorter the distance, the whiter the surface). Further

shading detail makes use of the orientation of the object in combination with a virtual light

source to calculate the amount of light reflected by the surface. Sophisticated algorithms '**

are used to calculate the light gradients to yield high-resolution images. Imaging of a surface

58

using illumination derived with virtual light source(s) is referred to as "surface rendering^^^ "

Software tools have been developed to make this process less labour intensive '^ ,

6.3 Principles of Magnetic Resonance Imaging (MRI)

This section describes the physics of MRI and the related phenomena of relaxation times. These

basic methods are used to derive images that are a function of proton density, the freedom of

hydrogen ions to rotate and the amount of water contained within the scanned object.

Components of an MRI imaging system

The main components of the MRI hardware system are far more complex than spiral CT and

incorporate:

• The main magnet

• Gradient systems

• Radiofrequency (RF) Cods

• Transmitters

• Receivers

• Computer

The main magnetic field, within which the patient lies, is kept constant. Its strength is measured in

units called Tesla (1) and typically ranges from 0.08 T to 2 T.

The gradient system consists of three electromagnets that can be turned on and off rapidly. They

are arranged along the x, j and directions in three-dimensional space. These electromagnets

generate a weaker and spatially dependant magnetic field that, when applied appropriately, wiU

encode spatial information in to the MRI signal.

RF coils are antennae that transmit and receive radio waves. These consist of large volume cods

that surround the body part being imaged or smaller surface cods, which are placed directly under

the area of interest and provide finer anatomical detad.

Ad of the hardware except for the main magnet is under the control of a highly sophisticated

computer and associated software. This is used to construct the relevant image sequence and

process the signals obtained from the imaging protocol.

Proton spin

The nucleus of an atom must contain an uneven number of protons in order to be affected by a

magnetic field. The hydrogen nucleus contains a single proton, which has a positive charge. This

charge is distributed and rotates about a central axis to create a magnetic field (Mp).

59

The multiple protons located within human tissue all generate magnetic fluxes. Under normal

conditions these cancel each other out.

However, when a patient is placed in a magnetic field, the proton becomes orientated either

parallel or anti-parallel to the magnetic field. Parallel alignment creates a low energy state whereas

an anti-parallel array results in a high-energy state. At room temperature and equilibrium in a

magnetic field of flux density IT, protons are usually aligned in the lower energy state by an excess

of approximately 3 x 10'* .

The measurement of Mp requires it to be physically tilted away fcom its original direction (Bo or

to produce a measurable signal.

The Latmor frequency

When energy in the form of radio waves is added to the patient, the low energy parallel direction

of the protons can change to high-energy anti-parallel arrangement. The angle of displacement is a

function related to the amount of energy added to the system.

A radio frequency (RF) pulse of an amount that displaces the net magnetic field 90 degrees from

its original position (Bo or 0 into the X)i plane is called a "90 degree pulse".

The radio frequency used here is very specific and is called the Larmor Frequency. It gives rise to

a typical signal known as a Free Induction Decay.

Precession and resonance

Protons spin about their own axis, but in addition, they also rotate around the axis of the MRI

system. This second type of spin is called precession. An analogy would be to visualise the

spinning of a child's top.

After revolving about its own axis, it wiU begin to wobble as the speed slows. This second type of

spin or wobble is similar to precession. The frequency of precession is related to the Larmor

Frequency and the strength of the external magnetic field.

Resonance occurs when the patient is exposed to a RF pulse identical to the Larmor Frequency. A

specific resonance then occurs as the protons absorb that quantum of energy. This is released as

the system returns to equilibrium.

Gradient fields

In order to form an image, the source and strength of the signal must be determined. This is

achieved by the use of a gradient field, which is a weaker magnetic field superimposed on the

main magnetic field. The gradient adds to the main magnetic field on one side and subtracts from

the other. Therefore, each distant point in the patient is in a slightly different magnetic field from

60

all of the others. Since protons will precess at a rate proportional to their particular magnetic field,

those at different points along the gradient wiU precess at different frequencies depending on the

site. The resulting signals, when encoded, wiU determine the point of origin.

Frequency encoding or read gradient

A great advantage of MRI over CT is that imaging in any plane is possible. The planes used are

defined by the three-dimensional Cartesian co-ordinate system.

The axis lies parallel to the long axis of the MRI magnet. The x axis goes from side to side and

the j/ axis is vertical. The x and Y axis define the transaxial plane, the ^ and x axis define the

coronal plane.

The x , j and ^ axes also correspond with the three sets of magnetic field gradients. When switched

off and on, protons at different points in the body experience their own specific combination of

gradients and these emit a unique signal.

A fixed gradient is applied along the plane that is to be imaged. This is called the frequency

encoding or read gradient.

Phase encoding gradient

Once the fixed or read gradient is selected, the other two gradient fields can be manipulated iu

order to separate the signal from different points along the slice. Phase differences are created

along one dimension of the selected slice by increasing the strength of the gradient perpendicular

to the read gradient during the scan.

This gradient is termed the phase encoding gradient.

Two dimensional Fourier transformation

The result of these various manipulations with the gradient result in a highly complex signal which

can be simplified using a mathematical manipulation known as two-dimensional Fourier

Transformation. The MRI computer changes the signal parameters from amplitude versus time to

signal strength versus position.

This now indicates the signal intensity associated with each different point along the selected axis

allowing transformation into the corresponding image.

Basic Image Interpretation

In order for pathology or tissue to be visible in a magnetic resonance image there must be

contrast or a difference in signal intensity between it and the adjacent tissue.

61

Depending on which pulse sequence is used, tissues will show up as either dark, bright or a shade

of grey. For example, collection of water such as CSF or urine will appear dark on a T1 image and

bright on a T2 image. T1 and T2 contrast, and proton density and flow are the basic MR image

variables and will be briefly considered here.

T1 weighting

A radio ftequency pulse is used to displace the longitudinally aligned protons by 90 degrees in the

transverse plane. When the RF pulse is turned off, the protons will begin to realign themselves in

their original longitudinal direction. The velocity of this return to the original position is direcdy

related to the brighmess of the visualised structure.

It is cmcial to measure the emitted signal early so as to differentiate the structure of interest from

signals sent out by surrounding tissues as eventually aU the protons will realign and show no

visible image differentiation.

This sampling time is known as TR and to maximise T1 contrast one must use a short TR

sampling time.

T2 weighting

T2 contrast relates to the transverse magnetisation. During T1 contrast imaging, a 90 degree RF

signal is used to realign protons in the transverse or plane. Turning off the RF signal then

results not only in longitudinal realignment, but also in what is called a dephasing in the transverse

plane.

To explain this further, while in the transverse plane when the RF pulse was on, the protons were

aU precessing in phase. When the RF pulse was turned off they started to dephase at different

rates. At this moment, a foUow-up 180 degree RF pulse is given which puts the dephasing protons

back in phase. This signal is now measured in the transverse plane. As time passes, these protons

again become out-of-phase and the signal decreases.

Tissues with a long T2 remain in phase for long periods of time and emit a stronger signal. Since

all tissues are initially in phase, maximum T2 contrast can be obtained by delaying the sampling

time in the transverse plane. This time is referred to as TE. Therefore for maximum T2 contrast, a

long TE is desirable.

T1 contrast is thus essentially controlled by TR, and a short TR is necessary to achieve maximum

T1 contrast. Conversely, T2 contrast is controlled by TE and for maximum results, a long TE is

desirable. If we negate the T1 contrast by using a long TR and then use a short TE to negate the

62

T2 contrast, we will be left with an image that derives its contrast only from a difference in proton

density. This is called the proton, or spin, density image.

Magnetic resonance imaging has two typical advantages in vascular imaging:

• The flow-void phenomenon provides excellent contrast between the vessel lumen and the

vessel waU "*

• It is possible to obtain information regarding flow " ^

Magnetic resonance angiography has been used as a basis for the three-dimensional

reconstruction of vascular trees^" , but the small vessel branches still present segmentation

problems and require both noise reduction^^^ as well as contrast restoration^^s.

Segmentation of small vessels can be avoided by the use of a Maximal Intensity Projection (MIP)

technique. Mathematical rays are cast through the data volume and the MIP technique is then

used to select the maximal intensity encountered along each ray path to construct an image. When

surrounding tissues are removed, this can result in detailed images of the vasculature. However, as

these vessels tend to be lost in three-dimensional images (partial volume effect) and low contrast

vessels tend to be lost in MlP’s, these techniques are of complementary value.

MRI contrast agents

Magnetic resonance contrast agents are unique in radiology because it is not the chemical that is

detected, but rather the effect that the chemical has on surrounding molecules. MR imaging relies

on signal originating from hydrogen nuclei in water and fat. The reason that different anatomic

structures can be seen on an MRI image is largely due to differences in T1 and T2 between

different tissues. Gadolinium is an intravenously injected heavy metal MR contrast agent that

improves image quality by facilitating Ti relaxation wherever it is concentrated. It shortens the T1

of surrounding protons, making them appear brighter. Because gadolinium normally stays in

blood vessels it has the effect of making vessels, highly vascular tissues, and areas of blood leakage

appear brighter.

Gadolinium is manufactured chelated to diethylenetriamine penta-acetic acid, as are other heavy

metals used for imaging, such as technetium. In its chelated form, gadolinium is excreted by the

kidneys with a serum half-Hfe of approximately 1.5 hours.

63

Gadolinium has no known significant side effects and no nephrotoxicity has been recorded to

date. For these reasons it is safe for use in patients with renal insufficiency '* ' The Magnevist

injection used in this project was the N-methylglucamine salt of the gadolinium complex of

diethylenetriamine penta-acetic acid.

64

Figure 15: Three-dimensional modelling from spiral CT volume data

Stage 1: The area o f interest is highlighted on each slice using a special painting tool. This is the most time consuming part o f the whole procedure. In this cross-section of

the human arm, the extensor digitorum is highlighted.

Stage 2: The individual areas are stacked in the correct alignment to form the musclebelly

65

Stage 3: A wire mesh is created over the stacked CT slices

Stage 4: An artificial surface is added to the wire mesh using a virtual light source to form the surface shaded display (SSD).

66

CHAPTER 7

EXPERIMENTS I

SPIRAL CT ANGIOGRAPHY AND MAGNETIC RESONANCE ANGIOGRAPHY

7.1: Validation of spiral CT volume and linear measurements using glass phantom aneurysms

7.2: Validation of a method used to determine intra-aneurysmal flow channel and intra-luminal thrombus volumes

7.3: A comparison of the intra-observer and inter-observer errors in the measurement of AAA sac volume and maximal diameters

7.4: Comparison of spiral CT angiography and graduated sizing catheter in the geometric sizing of AAA for endoluminal repair

7.5: The natural history of AAA after endoluminal repair. A comparative study of balloon and self-expanding endograft systems using Spiral CT angiography.

7.6: Changes in intraluminal thrombus (ILl) after endovascular repair of abdominal aortic aneurysm.

7.7: Changes in aneurysm and graft length after endovascular exclusion of AAA using balloon and self-expanding endograft systems.

7.8: Natural history of the aneurysm neck after endoluminal repair using balloon and self­expanding endograft systems.

7.9: The intra- and inter-observer differences in aneurysm neck length and diameters found during measurement with spiral CT angiography

7.10: Measurement of distance and volume with MRI: An in vitro feasibility study

7.11: A comparison of gadolinium-enhanced MR angiography versus spiral CT angiography in the evaluation of patients for endovascular repair

67

7.1

Validation of spiral CT volume and linear measurements

A i m

The aims of this study were twofold. Firstly, to validate the linear measurements obtained from

the spiral CT scanner used in this study and secondly, to assess the reliability and accuracy of

describing abdominal aortic aneurysms using volumetric data.

B a c k g r o u n d

Spiral CT angiography is becoming an accepted technique for the assessment and measurement of

aneurysms preoperatively and during follow-up. Although, important decisions concerning the

suitability for endovascular repair and procedural success are based on these measurements their

reliability and accuracy have not been assessed previously.

One area of concern is postoperative sac morphology. Conventional repair has the advantage that

the aneurysm sac is operatively disrupted and (usually) oversewn. In contrast, endovascular repair

does not disturb the sac. The fate of the aneurysm sac is hence inextricably linked with the

efficacy of this technique.

Equivocal results have been obtained for the fate of the aneurysm sac post-exclusion^^®'^^ . While

it has been argued that aneurysmal shrinkage represents the successful clinical end point of

endovascular repair, not all sacs exhibit this. In some cases the aneurysm does not change its

dimensions'^ while in others, it may enlarge even in the absence of complications such as

endoleak^^ .

While CT angiography is commonly used for fbllow-up^\ this method has never been properly

validated for the measurement of aneurysm sac dimensions. Measurement of diameters from CT

scans may be of poor quality, often consisting of little more than a ruler or callipers used to

measure distances on axial slices^^ 53 most obvious difficulty with using maximal diameter

in follow-up is that there is no guarantee that exactly the same diameter will be measured in the

same position within the sac on every occasion. After endovascular repair, the aneurysm may

change shape, for example from spherical to pear-like, with no alteration in the maximal diameters

but a shift in the position of the diameters within the sac (Figure 68).

Based on these arguments, it is proposed here that volume is a more complete measure of the

overall morphology of an aneurysm. A single volume measurement incorporates all the diameters

of the sac in its calculation. If a shape change does occur, a volume measurement in conjunction

68

with the 3D surface-shaded display is more likely to indicate this than a single maximal diameter

measurement. Finally, since the volume of a sphere is proportional to the cube of its maximal

diameter, only a small (and sometimes un-measurable) change in diameter can cause a much larger

(and measurable) simultaneous change volume.

M a t e r i a l s a n d m e t h o d

Phantom aneurysms were hand blown using glass tubing of similar dimensions to the human

adult aorta: 25 mm in outer diameter and 2 mm wall thickness. To produce tortuosity, aneurysms

were allowed to bend randomly during heating. Eccentric rather than a concentric sac shapes were

fashioned (Figure 16). Only one aneurysm (F) was blown as a perfect spherical shape with no

angulation between the stems and the “sac.” The phantom aneurysms were shut off with leak

proof graduated (19/26) glass corks.

All the phantom aneurysms were measured with electronic callipers (Mitutoyo absolute digimatic,

Japan). The maximal AP and transverse diameters were averaged over eight observations to

obtain the mean and standard deviation.

Phantoms were filled with a mixture of de-ionised water and iodinated contrast media

(Omnipaque 350 mg Iodine/ml) in proportions similar to those found in vivo, after bolus

tracking, during SCTA of an adult with an average intravascular volume of 5 L. In this situation,

the administration of 150 ml of intravenous contrast would result in an increase of the

intravascular volume to 5150 ml. Assuming negligible homeostatic volume correction, the ratio of

contrast medium to blood would then be 1:34.33 ml.

The density of this solution was calculated using a pyknometer (Figure 17), applying Archimedes

principle, in the laboratory at 21.9°C.

The volumes of the phantom aneurysms were then measured in the laboratory in three stages:

1. The total volume of the contrast-water mix was calculated by weighing the empty and then

the full phantom after scanning. The difference in weight divided by the density of the

mixture was the volume of the solution used.

2. The volume of the glass comprising the aneurysm was calculated separately by dividing the

density of the glass used by the dry weight of each phantom. The density of the

69

f %

Figure 16: Glass aneurysms used to validate spiral CT angiography data.

« *

I

Figure 17: Pyknometer used to calculate fluid density. The vessel takes a maximum o f 50 ml o f fluid when full. If the dry

weight IS then subtracted from this, the density is derived from the formula: Density = (weight o f pyknometer and solution —

dry weight o f pyknometer)/ volume (50 ml).

70

glass was calculated by the displacement method in water using a piece of the same glass tubing.

3. The total Yohmie. of the aneurysm was calculated as the sum of the volume of solution and the

volume of glass (Table 8).

These volume and linear measurements are referred to here as the measurements.

Imaging

Phantoms were imaged flat in a Siemens Somatom Plus 4 spiral CT scanner using the clinical

protocol in Table 6. A medium field of view (350 mm centred on 90 mm) was used without

postprocessing zoom, with kVp of 120 and a tube current of 300 mA. Serial images were acquired

and transferred to the MGI workstation for 3D reconstruction via Ethernet cable.

Measurements

Image data was reformatted into multiplanar (MPR) and three-dimensional surface shaded display

(SSD) form for geometric and volume analysis on the MGI workstation (Figures 18 and 19). The

workstation displays three reference orthogonal MPR's (coronal, transverse and sagittal) linked to

the orientation of the three-dimensional SSD (Figure 19). Using the measuring module, up to 20

markers could be placed on the surface of the subject or within its lumen. Distances and grid-

references of the markers were then computed in mm.

The maximal AP and transverse diameters were measured as the mean of eight observations.

These are referred to as the scanned diameters.” Volumes were calculated using a voxel-counting

technique at a window of 0 Hounsfield. These are referred to as the ''''scanned volumes.”

S t a t i s t i c a l a n a l y s i s

All data are expressed as the actual values in mm or ml. Percentage errors were calculated using

the following equation:

Percentage error = (true value-scanned value) / true value

Linear regression was utilised to examine the relationship between the tme and scanned

measurements obtained from the phantom aneurysms. Additional calculations were undertaken

for the 95% confidence or prediction intervals of each regression line. The data assume that there

71

is 95% chance that the two confidence bands enclose the tme best-fit linear regression line,

leaving a 5% chance that the tme line is outside those boundaries (Figures 20 to 22).

l a . e . t . . n e t - M u d io m l 4 rm (k n jic a

Figure 18: A surface shaded display o f an abdominal aortic aneurysm produced using the MGI workstation

(UCL imaging group).

L . M C I -

Figure 19: A multiplanar reformation (MPR) display linked to the 3 dimensional orientation o f the abdominal aortic aneurysm

at the top left. The coronal display is at the top right. The transverse and sagittal displays are bottom left and right

respectively.

72

The slope of the regression line may be used to assess the degree of agreement between individual

values. The closer this approximates to unity, the better the agreement between the scanned and

true values ' .

Collimation 5 mmTable speed 7.5 mm/secondPitch (scanner rotation 1 per 1.5second)Contrast Omnipaque 350 (120-150 ml)Contrast injection rate (pump injection) 5 ml/second

Delay to start of scan 21 secondsStart level coeliac axisFinish level external iliac arteriesReconstruction interval 2.5 mm

T able 6: Spiral CT angiography protocol used for the imaging of patients and glass phantom aneurysms in this study

R e s u l t s

Values obtained for the density of the glass tubing used and the iodinated contrast-deionized water to mixture were 2.24 g/cm^ and 1.01 g/cm^.

1. Geometric measurements

Maximum transverse and anteroposterior (AP) diameters of phantom aneurysms were averaged

over 8 observations using electronic callipers and the measuring tool on the post-processing

workstation. The average of the true maximal diameters (± standard deviation) compared to their

scanned counterparts are shown in Table 7.

For both dimensions, near perfect correlation and agreement was obtained between the true and

scanned AP (Figure 20) and transverse diameters (Figure 21). The slope of the regression line for

AP diameter was 0.97 ± 0.03 and for transverse diameter was 1.0 ± 0.01.

Table 7 also shows that the standard deviations for measurements made using SCTA, with only

three exceptions, are an order of magnitude larger than those obtained with electronic calipers.

This illustrates that the variability of diameter measurement with spiral CT, while clinically

acceptable, is sldll almost 10 fold greater than measuring the object directly.

2. Volume Measurements

Scanned volumes were compared with the true values as shown in Table 8. The median

percentage error was 1.1 (range 0.36-2.72).

73

Tm e diameters (mm) Scanned diameters (mm)Aneuiysm

anteroposterior transverse anteroposterior transverse

A 43.33±0.1 48.83+0.07 44.50+0.46 49.18±0.64

B 55.44±0.06 54.81+0.05 56.75+0.05 55.34±0.46

C 60.51±0.07 60.47±0.1 61.19±0.71 60.69±0.63

D 69.81+0.07 69.84±0.06 70.08±0.50 70.08±0.50

E 76.54±0.05 78.92±0.03 75.83±0.47 79.41±0.48

F 79.74±0.03 79.80±0.03 80.70±0 80.01+0.66

T ab le 7: A Comparison o f true and scanned diameters o f glass phantom aneurysms. The regression plots and equations for this

comparison are shown in Figures 20 and 21.

ANEURYSM

TR UEVOLUMfmlat21.9°q

[ES SCANNEDVOLUMES

PERCENTERROR

volume of glass

volume of solution

totalvolume 0 Hounsfield

A 39.78 133.96 173.74 178.06 2.72

B 40.13 180.59 220.72 224.86 1.88

C 39.06 216.44 255.50 256.43 0.36

D 47.28 233.66 280.94 284.92 1.42

E 46.83 274.36 321.19 323.06 0.58

F 49.78 308.71 358.49 360.99 0.7

Table 8; Comparison of true and scanned volumes of glass phantom aneurysms

74

The accuracy of the volume data obtained using the spiral CT scanner was reflected by near

perfect correlation between the scanned and true volumes (Figure 22). The gradient of the

regression line (0.99) did not deviate significantly from the ideal line at 45° demonstrating near

perfect agreement also between true and scanned volumes.

At a true volume of 0, the calculated intercept (5.9 ml) was not significant at the 5% level

(P=0.08), suggesting random as opposed to systematic error (Chapter 10). This was further

reflected by a wide confidence interval that included zero: -0.92 to 13 ml (95% confidence limits).

C o n c l u s i o n

Both linear and volume data obtained with spiral CT show near-perfect correlation and agreement

with the true dimensions of the phantom aneurysms scanned. linear dimensions had greater

variability expressed by larger standard deviations compared with the true measurements. This

may have been due to the subjectivity involved in placing electronic markers over what was

perceived to be the edge of the maximum diameter of an aneurysm on screen. In contrast, within

the laboratory, electronic calipers could easily be slipped up and down the phantom in 3

dimensions to locate this parameter.

Volume analysis involves a mathematical calculation of the number of volume units (voxels)

within the object. This was done entirely by the post-processing software using edge detection and

voxel-counting algorithms. Because of the reduced subjectivity involved with this measurement it

may be a more accurate method of describing tubular structures with complex three-dimensional

morphology such as aneurysms. The volumetric approach may be particularly valuable in the

presence of a changing sac shape. This is because volume provides a mathematical snapshot of

the overall morphology of an aneurysm not available from two-dimensional parameters such as

the maximal sac diameters.

75

Correlation between scanned and trueAP diameters

100-1

GG

75-

-d 50-

<

ÏG 25 -S(/J

750 25 50 100

True AP diameter (mm)

Variables 95% Confidence IntervalsSlope: 0.97 ± 0.03

Y-intetcept: 2.70 ± 1.42 ,2=1

Slope: 0.91 to 1.03 Y-intercept: -1.24 to 6.64

F igu re 20: Linear regression analysis. Anteroposterior diameters measured from SCTA scans versus true anteroposterior

diameters for six phantom aneurysms. The dotted lines are the 95% confidence intervals for the slope o f the regression hne.

Statistical analyses are shown in the table.

76

Correlation betweenscanned and truetransverse diameters

s

I9uCO

uC / D

lOOn

7 5 -

i 5 0 -

2 5 -

O-f0 100755025

True transverse diameter (mm)

Variables 95% Confidence IntervalsSlope: 1.0 ± 0.01

Y-intercept: 0.49 ± 0.36r2=l

Slope: 0.98 to 1.0 Y-intercept: -0.51 to 1.5

Figure 21: Linear regression analysis. Transverse diameters derived from derived from SCTA and postprocessing versus true transverse diameters for six phantom aneurysms. The dotted lines are the 95% confidence intervals for the slope of the regression line. Statistical

analyses are shown in the table.

77

Cottelation between scanned and true volumes

400n

g 300-

200 -

^ 100 -

. 200 300 4000 100

True volumes (ml)

Variables 95% Confidence IntervalsSlope: 0.99 ± 0.01

Y-intercept: 5.9 ± 2.5r2=i

Slope: +0.96 to 1.0 Y-intercept: -0.92 to 13

Figure 22: Linear regression analysis. Volumes derived from SCTA and post-processing versus the true volumes of six phantom aneurysms. The

dotted lines are the 95% confidence intervals for the slope o f the regression line. Statistical analyses are shown in the table.

78

7.2

Validation of a method used to determine intra-aneurysmal flow channel and intra-luminal thrombus volumes

A i m

The aim s of this study were to define objective criteria to calculate a tissue segmentation threshold

for shaded surface display (SSD) rendering of the intra-aneurysmal vascular channel with SCTA,

and to validate a subtraction method used to derive intraluminal thrombus volume.

B a c k g r o u n d

For this study, aneurysms were divided into 2 anatomical volumes (Figure 23). These were the

intra-aneurysmal flow channel (lAFC) and the total aneurysm volume. In patients, both volumes

were calculated between the level of the lowest renal artery to the aortic bifurcation. The lAFC is

surrounded by the intraluminal thrombus (ILT). This tissue relationship was simulated with

phantoms as described below.

Calculation of total aneurysm volume at segmentation 0 HU has been previously validated using

glass phantom aneurysms. In contrast, the lAFC cannot be scanned separately from the whole

aneurysm. Its volume can only be displayed by editing it out on each axial slice, by hand, using the

painting tool on the workstation (Figure 14). This is a time consuming and laborious method

especially when the lAFC is anatomically tortuous.

There is a sharp change in density from the mixture of iodinated contrast and blood in the lAFC

to the surrounding ILT. If the Hounsfield value for this interface was known, it could theoretically

be used to segment out the lAFC from the rest of the aneurysm. An important assumption of this

method, is that for the short period of time over which scan data is acquired, the density of the

thrombus-blood interface must not alter significantly due to permeability of the ILT.

As a separate but related step, subtraction of the flow channel volume from the total aneurysm

volume will give the volume of intraluminal thrombus (Figure 23).

M e t h o d a n d m a t e r i a l s

A single glass phantom aneurysm (Aneurysm F) was prepared for scanning. This phantom was

constructed as a perfect sphere with no sac eccentricity or neck-to-sac angulation. A latex

79

balloon with negligible wall thickness was placed within the centre of this aneurysm. The

aneurysm outside the balloon was then filled with the same mixture of contrast-deionized

water as previously described in Chapter 7.1. The central balloon was filled with a known

quantity of deionized water. Water was used to provide a sharp change in boundary and

hence HU values with the contrast-water mix outside. This model was intended to act as a

simulation for the lAFC-ILT interface (Figure 24). The amount of water in the balloon was

varied in increments from 20 ml up to 70 ml before each scan. The scanning protocol used

is in Table 6.

The images obtained from the spiral CT scans had Hounsfield profiles calculated for

representative CT slices across the phantom and its contained balloon using software

available on Displmage version 4.8 (UCL imaging group). The HU value obtained for the

contrast-water and water interface (in effect, the edge of the balloon) was used to set a

window to segment the balloon and its content away from the phantom

ITe remaining volume (volume of aneurysm excluding balloon) was then mathematically

subtracted from the initial volume (volume of aneurysm including balloon at 0 HU) to

provide the volume of the water-filled balloon in the centre of the phantom (Figure 24).

Individual spiral C l’ slices were examined qualitatively to document that an acceptable

subtraction had taken place (Figure 26).

i n ’

Figure 23: 3D reconstruction o f an abdom inal aortic aneurysm treated with an aorto-uni-iliac P T F E graft. I h e

intralum inal th rom bus is show n m blue. Subtraction o f the volum e o f the flow channel (yellow) from the total

volum e o f the aneurysm will p rovide the volum e o f throm bus.

80

Figure 24: Spiral CT multiplanar reformation of glass aneurysm F filled with contrast-deionized water and containing a central water-filled balloon. Note the areas of infolding o f the

balloon material at the top o f the neck. The HU profile from this experiment is illustrated in Figure 25. Two individual CT

slices from this phantom are illustrated in Figure 26.

S t a t i s t i c a l a n a l y s i s

Data were analysed with linear regression to calculate the correlation between the true and

scanned volumes. The gradient of the regression line was used to estimate the level of

agreement between volumes. A slope of 1 represents the line of equality with perfect

agreement between variables. Results are presented in Table 9.

R e s u l t s

The Hounsfield value for the edge of the balloon was selected by viewing HU profiles of

slices taken from individual scans. The value corresponding to the change in density at the

balloon edge was measured from a line graph (Figure 25). Normally one would expect a

steep line with such a sharp change in density at this region. Instead, there is a gentle slope.

This is due to the partial-volume effect (Figure 13). The HU value selected for this interface

was the value half way along the slope. All the scans examined agreed on the same value of

165 HU for the edge of the balloon. This was confirmed in two further experiments:

1. Individual spiral CT slices were viewed before and after segmentation on the

workstation. A qualitative assessment (Figure 26) showed that only areas of water and

81

balloon were removed from the centre of the phantom by the segmentation process when

165 HU was used as the threshold.

2. Aneurysm F was re-scanned on a 7* occasion but this time with a reversal of the contrast

compartments: de-ionised water was used to fill the glass aneurysm while contrast was added

inside the balloon. The interface value of 165 HU remained unchanged (Figure 27).

Amount of water added to

balloon (ml)

Scanned total volume of

phantom at 0 HU (a)

Scanned total volume of

phantom at 165 H U (b)

Calculated amount of water in

balloon in ml (a-b)

Percentageerror

20 361.65 340.29 21.37 +6.8540 361.60 320.57 41.03 +2.5850 361.77 311.65 50.13 +0.2660 361.74 302.11 59.63 -0.6165 361.74 296.38 65.37 +0.5770 361.60 291.90 69.71 -0.41

T ab le 9: Volumes obtained after SCT scanning o f glass aneurysm F containing different volumes of water in a central

balloon.

A graph of the results in Table 9 is shown in Figure 28. The regression line shows near-perfect

correlation between the two sets of volumes with narrow confidence intervals. Since the

gradient of the regression line (0.97) does not significantly deviate from the line of equality, this

implies near perfect agreement between the scanned volume of the balloon (calculated at 165

HU) and its true volume.

The Y-intercept for the graph gives the volume of the empty balloon (1 ml). Confidence intervals

for this intercept are wide suggesting that the volume of the empty balloon can vary from as little

as 0.71 ml to as much as 3.4 ml. Clearly this cannot be accurate due to the negligible wall

thickness of the balloon. The P value for the intercept (P=0.5) however, does not reach

significance at the 5% level, suggesting that this observation is most likely due to random as

opposed to a systematic error.

Figure 29 is a graph of the percentage error plotted against the volume in the central balloon. The

percentage error decreases proportionately as volume in the central balloon increases. In the

clinical studies (Chapter 7.5) patients’ LAFC volumes had a median value 62.94 ml (interquartile

range 50.31 to 92. 11), in the upper range of this graph where the percentage error is low.

82

Graph showing the Hounsfield values across a CT slice takenfirom a glass phantom aneurysm with central water-filled balloon

500 T

300 -

100 -

3 -100

w -300 + IQ

-500 --

-700 -

-900 --

1100 ^

Distance (mm)

Figure 25: Hounsfield profile o f a spiral CT slice taken from the glass aneurysm in Figure 24. N ote the symmetry. The density increases to 400

HU while the glass is being scanned but then drops to 300 when the water- contrast mix is reached. There is a gradual decline in HU to zero as the water-filled balloon is analysed. The gradual slope in values seen at the

balloon edges is due to a partial volume effect. The value for the interface is taken exactly halfway along this slope. A segmentation value o f 165 HU is

obtained. This displayed the aneurysm minus its central balloon on the workstation. The graph should be compared with the axial slices in Figure

26.

83

oO

F ig u re 26: Representative SCTA slices taken from the p h an to m illustrated in Figure 24. T he upper images are taken from slice num ber 62 and the low er from slice 65. ITie images on the left o f each p icture are a t 0 H U while those on the right are at 165 H U . T he w ater and central

balloon have been com pletely edited o u t by the p o s t­processing software.

F ig u re 27a: C ross section o f glass aneurysm F filled with deionized w ater and contain ing a central contrast-

deionized water-filled balloon. T his m irro r experim ent was set up to confirm that the H U value fo r the edge o f

the balloon was linked to the change in density across the balloon interface. T he profile is show n in Figure 27b.

84

Ptofile of the Hounsfield values across a CT slice taken from aglass phantom aneurysm with central contrast-filled balloon

800 T

600 --

400 -

200 - -

3>

2 40 100W -200 -

IOX

-600 --

-800 --

-1000

-1200 J-

Distance (mm)

Figute 27b: Graph of the HU profile across a spiral CT slice taken from a glass phantom filled with water and containing contrast in it's central balloon. This profile is the mirror image of the profile in Figure 25. As

expected, the value for the edge of the contrast-containing balloon is still165 HU.

85

Graph of scanned versus actual balloon volumes in a phantom aneurysm

75-1

II

I 2 5 -

us10 20 30 40 50 60 70 800

Actual volume of de-ionised water in balloon (ml)

Variables 95% Confidence IntervalsSlope: 0.97 ± 0.01

Y-intercept: 1 ± 0.49r2=l

Slope: 0.94 to 0.99 Y-intercept: 0.71 to 3.4

F igu re 28: G raph show ing the actual volum es w ithin the central balloon against the scanned volum es using 165 H U for

segm entation o f volum es. T he do tted lines represen t the 95% confidence intervals for the slope. T he table contains num erical

param eters pertaining to this analysis.

86

Percentage errors versus balloon volume

7.5-1

I ” ■I ».I

0 .0 -

-2.550 60 70 8030 40200 10

Volume of de-ionised water in balloon (ml)

Figure 29: In this graph, the percentage error obtained from scanning different balloon volumes is plotted against the actual

volumes. The percentage error appears to decrease as the volume scanned increases.

C o n c l u s i o n

The experiment demonstrates that an internal volume, such as that of a water-filled balloon, can

be quite accurately segmented from the total volume of an object, such as a glass aneurysm. The

volume of water contained in the balloon can then be calculated with low percentage error.

In vivo, it follows that the interface between thrombus and blood/contrast should also have a HU

value that could be used to set a similar segmentation window. The correlate of the water-filled

central balloon volume would of course be the volume of the lAFC and that of the glass

phantom, the total aneurysm volume.

Once the volume of the lAFC is known, the volume of thrombus can also be calculated by simple

subtraction (volume of AAA-volume of lAFC). These principles were used in calculating

aneurysm and LAFC volumes to investigate the natural history of the aneurysm sac and thrombus

after endoluminal repair. One disadvantage of this method is that edge detectability is likely to be

easier for the observer when dealing with regular interfaces as in the experimental set up. The

tissue irregularity seen at the thrombus-IAFC boundary in some patients may make automated

edge detection less accurate.

87

7.3

A comparison of the intra-observer and inter-observer errors in the measurement of AAA sac volume and maximal diameters

A i m

This study was designed to determine intra-observer and inter-observer errors in the assessment

of abdominal aortic aneurysm maximal diameters and sac volume using SCTA.

B a c k g r o u n d

For traditional reasons measurements of aneurysm sac diameter have been chosen as the method

of follow-up post-endovascular repair. However, their accuracy is questionable. This becomes

particularly important as they are often used to describe the efficacy and durability of this new

technique.

One disadvantage is that a single diameter may not be recorded at the same level on each occasion

of follow-up, so an aneurysm shape change may be missed. It is hence possible for an aneurysm

to shrink post-exclusion in volume terms, but because of a concurrent change in shape, with no

alteration in its maximal diameters. Furthermore, it is impossible to guarantee that linear

measurements are always taken at the same angle with respect to the long-axis of the aneurysm

even if the level is correct. This introduces a second variable that can confound the results from

this source even further.

The measurement of aneurysm volume may be a more accurate tool to follow the natural history

after endovascular repair. Mathematically a volume measurement is no more than the summation

of an infinite series of cross-sectional slices across the aneurysm sac. This would suggest that it

might describe the morphology of an aneurysm more completely than a measurement taken from

one cross-section.

For a volume measurement to be clinically useful, its reliability and repeatability need to be

known. The British Standards Institution (BSI) definition of reproducibility^^, states that 95% of

the differences obtained with repeated use of the test should lie within 2 standard deviations (BS

5497,1979). This is the working definition that wiU be adopted for this study.

In this report, linear and volume measurements taken from randomly chosen spiral CT data sets

were used to calculate the coefficient of repeatability.

P a t i e n t s a n d m e t h o d

From a total of 120 spiral CT scans initially analysed at least 6 months prior to this study, 10% (12

scans) were randomly chosen for repeat analysis. This was carried out by numbering the scans in

chronological order and then selecting them out by using a series of 12 random numbers

generated by a Geiger-MuUer tube measuring background radioactive decay and interfaced to a

computer.

Spiral CT scans, originally stored on optical disk (3M, Imation Enterprises Corp. Oakdale, Minn)

in compressed format, were re-loaded on to the MGI workstation for repeat analysis. For the

intra-observer study, the spiral CT scans were re-examined after the delay of at least 6 months

following the first analysis. A different investigator re-analysed these 12 scans for the inter­

observer part of the study. Both investigators were judged to be of equivalent experience in using

the post-processing software for measurement of aneurysm parameters and were blinded to each

other's results. Only the patients original data file number with no measurement data, stored on

optical disk, was made available to the investigators. Thus both observers were blinded to the first

set of data in each analysis.

The following parameters were recorded:

1. Total aneurysm volume between the lowest renal artery and the aortic bifurcation

2. Volume of the intra-aneurysmal flow channel (lAFC)

3. Maximum aneurysm anteroposterior (AP) diameter

4. Maximum aneurysm transverse diameter

In addition both investigators were asked to comment on the level of difficulty encountered in

reading each scan.

Total aneurysm volume was calculated at a segmentation window of 0 Hounsfield (HU). The

window used to calculate the volume of the intra-aneurysmal flow channel was set up separately

for every patient from the HU profiles of representative SCTA slices taken from each scan

(Figure 32).

S t a t i s t i c a l a n a l y s i s

AH readings were expressed as the actual values in mm (linear measurements) or ml (volumes).

Intra-observer and inter-observer differences were assessed using the method described by Bland

and Altmann254. The observer errors were calculated using the coefficient of reproducibility as

defined by the British Standards Institution. This figure was derived by first calculating the

differences in readings between the observers. These differences were squared and their sum was

divided by the number of observations. The square root of this value was used as the standard

89

deviation (SD) of the differences between observers. The coefficient of repeatability is twice this

figure.

Outliers

Data values far removed from their pairs in the calculation of the repeatability coefficient were

classed as "outliers". For the purpose of the initial statistical analysis, such data were excluded but

the reason as to why this difference could exist was examined by noting the comments made by

each observer in the study. The outliers were tested for their exclusion by using the extreme

studentised deviate (BSD) method.

In this method, the first step is to quantify how far the outlier lies from the other data values. The

Z ratio is calculated as the difference between the outlier and the mean divided by the SD. If Z is

large, the value is far from the others:

Imean - value I Z = -

SD

Critical values for Z (appendix 1) can be used to derive a probability value. The critical value

increases with sample size, as expected.

R e s u l t s

The results obtained using the BSI definition are presented in Table 10. Suspect readings or

oudier's were rejected from the analysis and were not repeated in order to avoid the bias which is

inherent in trying to fit results into a predetermined pattern. The total number of patients that

were used in each analysis is noted in parentheses in Table 10.

C o n c l u s i o n s

Both the intra-observer and inter-observer analysis of volume and linear changes have shown that

the spiral CT technique with post-processing of data into 3D and multiplanar formats has

acceptable reproducibility. Table 10 shows that the coefficients between observers are mostly

comparable except in two cases, namely lAFC volume and AP diameter estimation. In the

former case this may be due to the calculation of the HU value for the edge of the blood-

thrombus interface which is quite subjective. The observer has to plot a HU value profile across a

representative slice of the aneurysm and from this has to calculate a figure to use in segmenting

out the flow channel volume.

90

In the case of the AP diameter, estimation of the posterior limit of the aneurysm was found to be

quite difficult due to the impingement of the vertebral column that sometimes obscured the

aneurysm sac greatly. In some patients the aneurysm had actually eroded backwards into the

intervertébral disc space. In addition, soft tissues such as pancreas and renal veins sometimes were

difficult to distinguish from the boundaries of the aneurysm sac. Both observers noted these

points.

Parameter measuredIntra-observer coefficient of

reproducibility

Inter-observer coefficient of

reproducibilityTotal AAA volume

LAFC volume Maximum AP diameter

Maximum transverse diameter

5.70 ml (11) 4.38 ml (11)

3.40 mm (11)

2.31 mm (11)

4.39 ml (10) 6.49 ml (10)

4.42 mm (11)

2.46 mm (11)

Table 10: Intra-observer and inter-observer coefficient o f repeatability in the measurement o f aneurysm parameters. The figures in parentheses indicate the number o f patients used in each analysis after the

omission o f outiying points. The range o f total aneurysm volumes surveyed was 99.75-402.66 ml; The range o f lAFC volumes was 44.10-319.63 and maximal diameters ranged 33.9-82.4 mm (AP); 39.2-85.9

(transverse).

A further detail to note are the units used in Table 10. Adthough the magnitude of the volumes

and linear measurements appear to be similar, it should be remembered that volume is actually

proportional to the cube of the diameter. Hence an intra-observer coefficient of 2.31 mm for

difference in transverse diameter translates to a volume change of at least 12.33 ml. In this respect,

the actual coefficient of 5.7 ml represents excellent volume reproducibility.

Based on these results, it would be reasonable to conclude that 3D spiral CT linear and volumetric

analysis of aneurysms post-exclusion is a reproducible technique with clinically acceptable

variability between different observer readings. The variability for total aneurysm volume is

mathematically much lower than that for maximal diameters suggesting that volume may be a

more useful clinical measure for following changes in aneurysm morphology.

91

7.4

Comparison of spiral CT angiography and graduated sizing catheter in the geometric sizing of AAA for endoluminal repair

A i m

To compare the agreement between SCTA and sizing catheter angiography in the assessment of

patients for endoluminal repair.

B a c k g r o u n d

Measurements of aneurysm morphology for endovascular assessment are commonly made using

intra-operative angiography with a graduated catheter. This method suffers from a number of

disadvantages. It requires a femoral puncture with recognised complications such as haemorrhage,

intimai dissection and distal embolisation. Angiography can only provide a two-dimensional

projection of the aorta and often only two orthogonal views are available, making interpretation

of the true anatomical dimensions difficult. There is also a magnification error, which may be

reduced but not eliminated, by imaging as close to the patients’ abdomen as possible. Narrowing

the field of view can similarly reduce parallax. Further errors due to radial distortion and camera

tilt (described in Chapter 8) also apply to imaging using fluoroscopy.

In an effort to improve the accuracy of these measurements, graduated marker catheters

(radiodense markers at 1 cm intervals) may be employed (Figure 30) but are stiU not ideal. Such

markers facilitate only longitudinal measurements parallel to the angiocatheter. Measurements

made perpendicular to the graduations still necessitate a degree of observer estimation (“eye-

baUing”). This method does nothing to reduce the danger from repeated and cumulative exposure

to higher levels of background radiation.

Spiral CT is a minimally invasive imaging technique that can also be used to provide geometric

distance measurements from 3D reconstructions. It combines all the advantages of conventional

CT scanners together with the benefits of angiography. This study compares measurements of

AAA made with SCTA and then graduated angiocatheter in the same group of patients.

92

P a t i e n t s a n d m e t h o d

26 patients underwent spiral CT assessment for endoluminal repair of AAA. Spiral CT was

carried out using the protocol displayed in Table 6 and post-processing of data was

performed on the MGI workstation. The SSD of the aneurysm was displayed at a threshold

of 0 HU and its orientation was linked to the MPR reformation.

ITie first 7 of these individuals, found to have suitable AAA anatomy for endovascular

repair, underwent intraoperative angiography using an aortic sizing catheter (Cook,

Australia). Ethical committee permission was obtained for this study from the joint

UCL/UCLH Joint Ethics Committee. The catheter, graduated at 1-cm intervals, was

introduced along the length of the aneurysm via a femoral arteriotomy.

Measurements were taken by both techniques of AAA neck length and diameter and the

length of the intra-aneurysmal flow channel (IAFC). The proximal neck was defined as the

distance between the lower border of the most distal renal artery and the beginning of the

aneurysm sac. Where a distal cuff was present, the length and diameter of this was also

recorded although no patients were treated with aorto-aortic tube prostheses. The latter was

defined as distance between the end of the sac and the aortic bifurcation

Figure 30: Fluoroscopic image o f a graduated angiocatheter (Cook, Australia) used to measure aneurysm dimensions.

93

S t a t i s t i c a l a n a l y s i s

All data were expressed in mm either as the mean or median with the range in brackets. Linear

regression was performed to evaluate strength of the relationship between measurements

obtained with two methods. Agreement between values was assessed using the method described

by Bland and Altman^. The limits of agreement were defined as mean ± 1.96 times SD of

differences. The precision of the agreement was calculated by using the approximation V(3s2/n)

for the standard error of the limits of agreement: s was defined as the standard deviation of the

differences. The number of patients (n=7) was used to work out the confidence intervals (Cl) at 6

degrees of freedom using standard tables^^. The value of t at P=0.05 was 2.45. The equation used

for this calculation (Table 14) was mean difference ± (2.45 x standard error).

R e s u l t s

Patients assessed for endoluminal repair with SCTA had a mean age of 72 years (range 54-88).

There were 16 males. The median neck diameter was 22 mm (16-30), median neck length was

26.5 mm (7.9-55) and maximum AP diameter was 55.4 cm (40-70). 11 patients had distal aortic

cuffs free of thrombus and less than 30 mm in diameter. The median cuff diameter was 24 mm

(15-29), and median cuff length was 14.5 mm (4-60). 5 patients (19.23%) were regarded as having

unsuitable anatomical features for endovascular exclusion of AAA. The remaining 21 patients all

had favourable aneurysmal anatomy for repair.

The first 7 patients suitable for endoluminal repair were all male with median age 69 years (range

57-80). The results obtained with SCTA and sizing angiography are displayed in Table 11.

MethodNecklength(mm)

N eckdiameter

(mm)

Distal cuff length (mm)

Distal cuff diameter

(mm)

Flow channel length (mm)

SCTA 21(22.75-38.25)

23(22.0-26.0)

12(5.75-17.25)

21.5(16.75-24.25)

140(116.25-178.75)

Sizingcatheter

22(22.25-32.75)

21(21.5-26.5)

13(6.25-18.75)

20.5(15.75-23.25)

130(109.25-159.75)

Table 11: Comparison o f aneurysm measurements between SCTA and sizing catheter angiography. The table shows the

median value with the range in parentheses.

Spearman’s correlation was used to study the relationship between the values obtained by the two

methods. Near perfect correlation was found for all measurements between the two methods

(Table 12).

94

Good correlation between SCTA and sizing catheter is expected since both techniques are in

effect estimating the same variables in a single group of patients. This is the only assumption that

can be made from these data. Correlation coefficients do not influence the level of agreement

between 2 variables^" . Agreement between SCTA and sizing catheter can be assessed with a plot

of the mean difference against the actual measurement.

Spearman’scorrelation

results

Necklength(mm)

Neckdiameter

(mm)

Distal cuff length (mm)

Distal cuff diameter

(mm)

Flowchannellength(mm)

rho (r) 0.99 0.85 0.99 0.99 0.96P value 0.02 0.04 0.02 0.03 0.02

Table 12: Values fo t ran d P after Spearman’s correlation comparing SCTA and sizing catheter angiography.

Both SCTA and sizing catheter provide data that are indirect. The actual dimensions of the

aneurysm can never be absolutely known. Even direct measurement cannot provide the true

values due to biological changes within exposed tissues during surgery and post mortem. A close

approximation can however be reached by using the mean of the two values obtained by the two

methods. Graphs of the mean difference against the mean (Figure 31) known as Bland-Altmann

plots are used to display the level of agreement.

The graphs show that there is no consistent relationship between the difference and the mean for

aneurysm measurements relating to the proximal neck and intra-aneurysmal flow channel.

Interestingly, there seems to be a fixed error in the measurement of the distal cuff length and

diameter. Sizing angiography overestimates diameter of the distal cuff by 1 mm and

underestimates its length by 2 mm in most patients compared to SCTA. This is discussed further

below.

The mean and the standard deviation of the differences can also be used to calculate the limits of

agreement between SCTA and sizing angiography (Table 13).

95

mean

F igure 31a: AAA proximal neck length

mean

F igu re 31b: AAA proximal neck diameter

0.8 - -

%c 0 . 6 - - 0)

0.4 --

0.2 - -

OCX0 10 20 30

mean

F igu re 31c: Distal cuff length

96

-0.5 - -

mean

Figure 31d: Distal cuff diameter

mean

Figure 31e: Length o f intra-aneurysmal vascular channel

Figure 31: Bland-Altmann graphs showing the degree of agreement between SCTA and sizing angiocatheter

97

Region of aneurysm Mean difference(d)

Standard deviation (SD)

Limits of agreement

(d±1.96 SD)neck length 0.86 3.72 -6.43 to 8.15

neck diameter 0.57 1.62 -2.67 to 3.81cuff length -1.29 0.95 -3.19 to 0.61

cuff diameter 0.71 0.49 -0.27 to 1.69flow channel -1.14 7.34 -15.82 to 13.54

T ab le 13; Summary o f the lack o f agreement (bias) between SCTA and sizing angiography.

Table 13 demonstrates that the use of angiography to measure aortic aneurysm parameters is

subject to clinically unacceptable measurement errors. For example, estimation of aneurysm neck

length using an angiocatheter may undersize by up to 7 mm or oversize by up to 8 mm. These

values could mean the difference between the selection and rejection of patients for endovascular

repair. Similarly the errors inherent in the estimation of aneurysm neck diameter could increase the

risk of postoperative complications such as endoleakage.

Precision of estimated limits of agreement and bias

So far estimates of the lack of agreement between SCTA and sizing catheter have been obtained

only for this particular series of patients. It could be argued that a different sample might give

different values^^ possibly showing better agreement. The confidence intervals (Cl) for the limits

of agreement in Table 13 have been calculated in Table 14 to investigate the precision of these

estimates.

region Cl lower limit of agreement Cl upper limit of agreementneck length -12.41 to -0.45 2.17 to 14.13

neck diameter -5.27 to -0.07 1.21 to 6.41cuff length -4.71 to -1.67 -0.91 to 2.13

cuff diameter -1.06 to 0.52 0.90 to 2.48flow chaimel -27.58 to -4.06 1.78 to 25.30

T ab le 14: Precision o f lack o f agreement between SCTA and sizing catheter angiography. This table shows the confidence intervals for the bias (which refers to the overall lack o f

agreement between the two methods) and the limits o f agreement. The intervals are calculated for each limit using the equation in the text.

98

The confidence intervals in Table 14 show a wide range of values about those seen in Table 13.

This confirms the considerable discrepancies between the 2 methods of measurement particularly

when measuring the length of the vascular channel.

C o n c l u s i o n

The study shows that the agreement between SCTA and calibrated angiocatheter is poor. There are

large disparities between the two techniques with a wide scattering of differences about the mean.

The discrepancies are much larger for neck and flow channel lengths than for diameters. For

example in the case of flow channel length, angiocatheter may undersize by 16 mm or oversize by

14 mm compared to the gold standard of SCTA. This effect is emphasized further by the results in

Table 14. Here it can be seen that the confidence intervals for these disagreements are even wider

and in the very worst situation angiocatheter may under- or oversize SCTA measurements by as

much as 28 mm and 25 mm respectively. One reason for this may be related to the physical nature

of the angiocatheter (Figure 67a).

Angiocatheter diameter measurements differed from SCTA by —3 to +4 mm in this experiment.

The sequelae of poor sizing in this region are significant. Graft migration due to undersizing is just

as important as endoleakage due to the crimping of excess graft material in oversizing.

The best agreement was obtained for sizing of the distal cuff parameters. These limits were narrow,

compared to the other regions and their confidence intervals were relatively tight. Unfortunately,

this observation has almost no clinical relevance as aorto-aortic tube grafts are now seldom

deployed.

The use of sizing angiography to measure aortic aneurysm parameters is therefore subject to

clinically unacceptable limits of under or over-estimation. In the worst scenario, this could result in

life-threatening complications.

99

7.5

The natural history of AAA after endoluminal repair. A comparative study of balloon and self-expanding endograft systems using Spiral CT angiography

A i m

The aim of this study was to document the natural history of AAA after endovascular repair using

spiral CT angiography (SCTA). This method was used to follow changes occurring after

endovascular repair utilising two separate endografting systems.

B a c k g r o u n d

It is now recognised that changes in sac morphology post-endovascular repair of AAA, are not

only indicators of the efficacy of this technique, but are also inextricably linked with endograft

durability " .

Efficacy of endovascular repair is presently judged using sac maximal diameters. It is assumed that

successful repair results initially in a reduction in sac pressure followed by later sac shrinkage. The

latter part of this statement may be incorrect for the following reasons:

Firstly, as has previously been suggested and confirmed by other investigators^^®, maximal diameter

measurements may be an inaccurate method for following sac change. They have poor

reproducibility and may miss changes in sac shape occurring in parts of the aneurysm above and

below the region of interest. In various series ^ ^ ^ , the sac as judged by AP diameter, shrinks,

remains unaltered, or increases in size. Although an increase in sac size is most commonly due to

endoleakage this cannot always be demonstrated using currently available imaging technology.

Volume measurement may provide more accurate information concerning the natural history after

endoluminal exclusion^^k Volume is a composite measure of all the cross-sectional diameters of an

aneurysm and is relatively easily estimated either commercially or using the post-processing

software available with aU spiral CT scanning equipment.

P a t i e n t s a n d m e t h o d s

During the interval April 1995-November 1999,194 patients were assessed for endovascular repair

of their AAA using spiral CT angiography (SCTA). O f this group 88 patients were selected for

endovascular repair based on standard anatomical and physiological criteria. 27 patients were

treated with home-made, 1® generation PTFE devices fixed with Palmaz stents Johnson and

Johnson), while the remainder had custom tailored Talent (World Medical, Florida) endografts. 12

100

PTFE and 18 Talent patients, who had reached follow up for 2 years or more, were chosen for

further study. There were 24 males and 6 females.

Techniques used to manufacture and deploy our PTFE system have been previously described"^ . A

length of PTFE (Impra, USA) was selected according to dimensions obtained from SCTA, and

pre-expanded to 3 times its original diameter. A Palmaz stent (Johnson and Johnson) was sutured

to the upper end of the graft (Figure 41). The stent-graft was then mounted on a Gruntzig balloon

catheter (15-30mm diameter according to the AAA neck diameter). This was introduced into a

covering sheath of 20-24 French. Once the upper stent had been deployed within the aneurysm

neck, the graft was held within the chosen common iliac artery by a nylon suture^^ whilst a second

Palmaz stent was placed to anchor its lower end. The contralateral common iliac artery was

occluded with Tungsten coils and an 8 or 10mm Dacron femoro-femoral crossover graft was

performed.

Talent grafts were custom-tailored, also according to specifications obtained from SCTA, using a

thin-walled Dacron fabric supported by a series of interlinked nitinol stents. Two configurations

were used: aorto-uni-üiac (4 patients) and bifurcated modular (57 patients).

Patients were foUowed-up with SCTA at day 5 post-exclusion and at 6-monthly intervals thereafter.

Spiral CT angiography was performed with a Siemens Somatom plus 4 scanner using the protocol

described in Table 6 for aU subjects. Patients were asked to remain still, without breathing,

throughout the scan process. Scout images were obtained, and the table was then positioned so

that scanning would begin at the coeliac axis and end a short distance below the bifurcation of the

common iliac arteries (sacroiliac joints). lodinated contrast material (Omnipaque 350 mg

Iodine/ml) was injected via a peripheral vein at a rate of 5 ml/s with bolus tracking (the total

volume administered was approximately 120 to 150 ml). After a 21 second delay, spiral scans were

performed using a pitch of 1.5 mm/s and a 5 mm beam collimation. An index of 2.5 mm was used

for the reconstructed images with a pixel resolution grid of 512x512. The entire scan time lasted

30 to 40 seconds, and approximately 100 axial images were obtained from each patient. Data were

then transferred to a workstation, where the pixel grid size was reduced to 128 x 128. Bone and

soft tissues other than the AAA were manually removed from axial images using the programme-

editing tool (Figure 14).

Data obtained fcom SCTA were transferred to a post-processing workstation (UGL-imaging group)

via Ethernet cable for reformatting into 3-dimensional and multi-planar display.

Patients with complications requiring conversion to open repair, or those with a follow-up of less

than 2 years were excluded from the study.

Using the SCTA data sets, the following linear dimensions and volumes were recorded (Table 15):

101

1. Sac maximal AP and transverse diameters

Maximal AP and transverse diameters were recorded at the widest point of the sac as judged using

the 3D SSD and multiplanar reformation. These were measured using the electronic markers and

calliper system built into the software.

2. Shortest longitudinal distance from the lowermost renal artery to the maximal AP or

transverse diameters

The distance from the beginning of the proximal neck of the aneurysm to each maximal sac

diameter was also measured. The proximal neck of the aneurysm was defined as starting just below

the level of the lowest renal artery. These measurements were made to investigate the possibility

that the maximal diameters may have altered in position within the sac after endovascular repair.

Measurements of aneurysm sac1. Maximal AP diameter

2. Distance from lowest renal artery to maximal AP diameter

1. Maximal transverse diameter2. Distance from lowest renal artery to maximal

transverse diameter

T ab le 15: Measurements carried out on the aneurysm sac (mm) after endovascular repair

In addition to the linear measurements shown in Table 15, all patients had an aneurysm volume

calculated on each occasion, from the level of the lowest renal artery (or beginning of the proximal

neck) to the aortic bifurcation. The method used to calculate these volumes was based on a voxel

density calculation and summation (Chapter 6).

Two anatomical volumes were calculated:

1. The total aneurysm volume

2. The volume of the intra-aneurysmal flow channel (lAFC).

A window of 0 HU was used to calculate the total aneurysm volume. The Hounsfield value used to

calculate lAFC volume was derived from HU profiles obtained from separate CT slices taken from

individual patients after scanning. A typical plot of this type is shown in Figure 32 and the method

used to calculate the segmentation window is also explained.

102

S t a t i s t i c a l a n a l y s i s

Measurements are displayed as the median (mm or ml) followed by the 25th and 75* percentiles

respectively.

Comparisons of results between the two groups were performed with a Mann-Whitney U test.

Comparison of patient outcomes within the two groups was performed using the Wilcoxon sum of

ranks test. In both cases P less than or equal to 0.05 was regarded as attaining significance.

R e s u l t s

4 patients in the PTFE group developed complications. One patient, in atrial fibrillation, had a

saddle embolus with contiguous thrombosis of his aortic graft to the level of the renal arteries.

After failed thrombolysis, this was treated with an axülo-bi-femoral graft. A second patient

developed graft infection at 8 months requiring open repair (Figure 33). Two patients with Type I

endoleak detected by SCTA and confirmed by DSA, were excluded from further analysis at 8 and

15 months respectively. The former patient had successful endovascular therapy to stem an upper

stent leak and was re-entered into the study at 12 months from the original repair. In all the

remaining patients we were unable to demonstrate endoleakage or slippage or migration of the

stent graft. Lumbar and/or mesenteric artery filling was observed in 7 patients but this was absent

after 6 months.

30 patients selected for this study had reached a minimum of 2 or more years follow up. 12 of

these patients had PTFE balloon-expandable, aorto-uni-iliac grafts and 18 had Talent, self­

expanding, fuUy supported Dacron grafts. There were 24 males and 6 females with a mean age of

72.9 years (range 55-93).

Pre-treatment linear dimensions and volumes for the whole group are illustrated in Table 16 below.

There were no significant differences (Mann-Whitney U test) in these starting values between the

two patient groups.

AP diameter (mm)

Transverse diameter (mm)

Total aneurysm Volume (ml)

Flow Channel volume (ml)

Median 52.40 53.35 157.47 96.61

25% percentile 45.20 48.45 122.32 72.90

75% percentile 59.90 62.15 209.32 113.28

Table 16: Starting sac diameters and aneurysm volumes for patients in this study.

103

IiI

350

300

250

200

150

100

50

020

-50

-100

distance A-B (mm)

'ITirombus-blood interface (150HU)

Vascular flow channel

Intra-luminalthrombus

Figure 32: Calculation o f a Hounsfield profile using a spiral CT slice. On the x-axis is marked the distance A-B. The increasing values on thej-axis represent'the edges and then lumen of the lAFC, on either side o f which lies intra­luminal thrombus. The density/HU within the lumen increases to a maximum caused by the mixture of blood and iodinated contrast medium. Because o f the partial volume effect, the increase and decrease in HU towards and then

away from the lAFC appears as a gentle slope rather than a sharp cut-off. The value used to segment the lAFC volume is chosen half way along the slope of this profile, in this case 150 HU. This value represents the edge o f the

thrombus/blood-contrast interface.

104

The different numbers of patients followed up at each time interval are presented in Table 17.

Length of follow-up Preoperative 5

days6

months1

year1.5

years 2 years 2.5years

3years

PTFE 12 12 10 9 7 5 4 2Talent 18 18 11 10 7 3 - -

T ab le 17: N um ber o f patients foUowed-up at each time interval

within the 2 groups selected for this smdy

1. Changes in total aneurysm volume

For the whole group, 5 days after endoluminal exclusion, there was a significant (P=0.02) increase

in total aneurysm volume (25 of the original 30 patients: 10 PTFE and 15 Talent). For the PTFE

group, the median volume increased from 128.54 ml to 140.93 ml.

Corresponding volumes for the Talent group increased from 179.27 ml to 194.37 during the same

period. The actual volumes for both groups are recorded in Table 18 and illustrated in Figure 34.

For the PTFE group, only patients with complications had any further significant changes in total

aneurysm volume (Figure 35). The rest of the group showed no further significant change in

volume to the conclusion of the study.

The patient with a PTFE graft blocked by saddle embolus and contiguous thrombosis had sac

shrinkage, from a preoperative volume of 109.31 ml to 53.17 ml at 1 year. The 2 patients with

endoleakage continued to have an increase in their total aneurysm volume until further treatment.

Only one of these patients had any demonstrable concomitant increase in AP or transverse

diameter.

In the Talent group there was a significant (P=0.01) decrease in total aneurysm volume to a median

143.84 ml over the next 6 months of follow up. At 1 year a further significant (P=0.03) decrease in

volume was observed to a new median of 138.8 ml. Further significant changes in volume could

not be demonstrated to the conclusion of the study (Table 18).

2. Changes in the volume of the intra-aneurysmal flow channel

For both groups, immediately after treatment, there was a significant (P<0.005) decrease in the

volume of the lAFC which then remained stable for the duration of follow up (Table 18). The

initial decrease in flow channel volume was demonstrated for 22 out of the 30 patients.

3. Changes in maximal diameters and their levels

The maximal AP and transverse diameters and their shortest distances from the lowest renal artery

are recorded in Table 19a and 19b.

105

At day 5 post-endovascular repair, when there was an increase in aneurysm volume, analysis of AP

and transverse diameters did not show any significant changes for either group.

For the PTFE group these diameters then continued to remain stable throughout the rest of the

study period. For the Talent group, both AP and Transverse diameters showed a significant

(P<0.05) decrease up to 6 months. Subsequently, no further change could be demonstrated in

either diameter despite a further significant decrease in volume to 1 year.

Aneurysm and flow channel volumes (ml)

Median PTFE volume (interquartile range)

M edian Talent volume (interquartile range)

Total preoperative volume 128.5(115.1 to 163.7) 179.3 (136.9 to 266.7)Total volume at day 5 140.9 (119.5 to 175.8) 194.4 (153.3 to 273.8)

Total volume at 6 months 138.7 (125.7 to 178.4) 143.8 (91.9 to 188.5)Total volume at 1 year 137.3 (101.4-373.1) 138.8 (66.1 to 186.6)

Total volume at 1.5 years 138.1 (100.6-234.8) 138.5 (66.4 toi 60.3)

Preoperative LAFC volume 96.6 (80.7 to 110.0) 94.7 (66.1 to 121.1)lAFC volume at day 5 53.1 (43.3 to 72.2) 73.2 (57.0 to 90.9)

lAFC volume at 6 months 48.7 (45.5 to 58.9) 69.8 (56.1 to 83.8)lAFC volume at 1 year 52.3 (42.1 to 52.9) 74.8 (60.6 to 85.8)

lAFC volume at 1.5 years 49.9 (44.1 to 56.4) 60.6 (52.5 to 80.8)

T able 18: Changes in aneurysm or flow channel volumes for patients with

PTFE or Talent endovascular grafts.

Throughout this study, the location of the maximal AP and transverse diameters in the PTFE

group tended to shift caudad although there was no significant change in the total volumes. This

agreed with observations made on individual 3D reconstructions: sacs gradually changed from

spherical to pear-shaped in this group (Figure 36). Talent-treated patients did not demonstrate this

shift. The position of both maximal diameters varied irregularly within the sac whilst total

aneurysm volume decreased.

106

Figure 33: PTFE graft infection requiring removal and open repair. This patient presented with a 4-month history o f backache,

low-grade pyrexia and leucocytosis.

107

Change in total aneurysm volume after endograftingwith PTFE

4 0 0 -1

BB

I'o

200 -

1□□

oH 100 -

183 365 548 730 91350Time (days)

Figure 34a: Column scatter graph showing the change in aneurysm volume after endografting with PTFE. The median volume has been

indicated by the line at each interval.

Change in flow channel volume after endografting with PTFE

4001

2 0 0 -

100 -

Time (days)

F igu re 34b: Coltiinn scatter graph showing the change in intra- aneurysmal flow channel volume after endografting with PTFE.

The median volume has been indicated by the line at each interval.

108

Change in total aneurysm volume afterendografting with Talent

750 1îI 500- §

250-

H

730183 365Time (days)

548

F ig u re 34c: Column scatter graph showing the change in total aneurysm volume after endografting with Talent. The median

volume has been indicated by the line at each interval.

Change in flow channel volume after endografting with Talent

300 1

&

3 200-O

I*0

100 - ♦ ♦

183 365Time (days)

548 730

Figure 34d: Column scatter graph showing the change in intra- aneurysmal flow channel volume after endografting with Talent. The

median volume has been indicated by the line at each interval.

109

500-1

1 endoleakage at 1 year400-II 300-

endoleakage at 6 months200 -

normal median volume(9

100-

postoperative saddle embolism

7505002500

Time (days)

F ig u re 35: G raph show ing changes in total aneurysm volum e following the onset o f com plications in patients w ith P T F E endografts. T h e m edian volum e curve for patients w ithou t

com plications is included for reference.

Maximal diameters (mm) Median PTFE diameters ( in te rq u a rtile ran g e )

Median Talent diameters (in te rq u a rtile ran g e)

Preoperative AP diameter 52.2 (49.1 to 56.1) 55.3 (44.6 to 61.0)AP diameter at day 5 51.3 (48.5 to 57.0) 54.6 (46.1 to 64.2)

AP diameter at 6 months 53.5 (50.6 to 56.4) 44.1 (40.9 to 53.5)AP diameter at 1 year 55.0 (50.1 to 55.2) 47.6 (41.9 to 50.1)

AP diameter at 1.5 years 53.7 (47.9 to 60.1) 30.1 (28.7 to 45.7)

Preoperative transverse diameter 50.7 (46.1 to 58.9) 53.9 (49.4 to 66.3)Transverse diameter at day 5 52.3 (45.5 to 60.6) 55.5 (53.2 to 67.6)

Transverse diameter at 6 months 53.9 (48.2 to 59.6) 50.3 (40.7 to 59.1)Transverse diameter at 1 year 53.2 (48.4 to 60.6) 52.5 (46.6 to 64.0)

Transverse diameter at 1.5 years 57.1 (49.2 to 61.8) 32.0 (26.4 to 50.1)

T a b le 19a: Changes in aneurysm m axim al diam eters fo r patients w ith P T F E o r T alen t endovascular grafts

110

Level of maximal diameter (mm) Median PTFE levels (interquartile range)

Median Talent levels (interquartile range)

Preoperative AP level 70.1 (65.2 to 85.0) 72.5 (60.1 to 88.0)AP level at day 5 70.0 (66.3 to 88.2) 78.8 (66.9 to 97.3)

AP level at 6 months 80.6 (66.2 to 86.2) 76.7 (57.6 to 90.2)AP level at 1 year 81.1 (71.4 to 88.4) 74.6 (64.3 to 85.0)

AP level at 1.5 years 87.8 (67.6 to 93.2) 57.7 (44.6 to 76.6)

Preoperative transverse level 76.5 (62.9 to 91.9) 75.3 (64.1 to 86.4)Transverse level at day 5 78.0 (66.3 to 87.2) 81.4 (67.7 to 93.1)

Transverse level at 6 months 77.9 (72.3 to 88.1) 75.6 (59.9 to 93.3)Transverse level at 1 year 82.7 (69.6 to 90.5) 74.0 (69.2 to 80.4)

Transverse level at 1.5 years 78.6 (72.9 to 91.9) 73.7(55.7 to 91.0)

T a b le 19b: This table shows the distance from the low est renal artery (a fixed anatom ical point) to the level, w ithin the aneurysm , a t w hich each m axim al diam eter was m easured. A ll m easurem ents

represen t the shortest distance along a straight line betw een the tw o points.

C o n c l u s i o n

The study demonstrates differing effects of balloon and self-expanding endograft systems on

aneurysm morphology. These changes, described using volume measurements, are incompletely

followed by changes in the maximal diameters.

Both endografts caused an initial significant increase in total aneurysm volume and a decrease in

lAFC volume. The decrease in flow channel volume resulted from the exclusion of part of the

blood-flow channel in the aneurysm sac (Figure 36) and its replacement with a narrower

endovascular graft. The increase in aneurysm volume was not accompanied by any significant

change in the maximal diameters.

Follow up based on measurement of maximal sac diameters may also miss a change in sac shape

after endovascular repair. Although, a change in sac shape could lead to an alteration in sac linear

dimensions, maximal diameters may not necessarily change (Figure 68). One way of recording an

alteration in the location of the maximal diameters within the aneurysm sac is by measuring the

level at which each maximal diameter is recorded with respect to a fixed point such as the lowest

renal artery. In this study, the level did not vary consistently for the Talent group, confirmed by an

analysis of the individual 3D reconstructions. The changes in position of maximal diameter were

irregular reflecting asymmetrical sac shrinkage.

In the PTFE group, although total aneurysm volume remained stable, sac shape and position of

maximal diameters did change. There was a tendency for the sac to become pear-shaped with time

as the maximal diameters moved in a caudad direction. A change in sac shape seems to have taken

111

place without any change in magnitude of maximal diameters. Although the clinical significance of

this observation is as yet unclear, it does demonstrate that the value of diameter measurements in

follow-up may be of limited use thus providing further support to the idea that aneurysm volume is

a more complete measure of sac morphology.

112

F ig u re 36: T his sequence o f 3D reconstructions show the effects o f endografting w ith P T F E (top) and Talent (below) o n aneurysm sac m orphology. T he first reconstruction is preoperative followed by the sac at day 5.

Subsequent reconstructions at 6-m onthly intervals clearly show the PT F E -trea ted sac becom ing m ore pear-shaped w ith time while the T alent sac show s m arked and irregular shrinkage over its length. T hese pictures are reproduced

for qualitative exam ination only and the images betw een the two patients are n o t draw n to the sam e scale.

?/ V

t

1*'—à %

113

7.6

Changes in intraluminal thrombus (ILT) after endovascular repair of abdominal aortic aneurysm.

A i m

To investigate changes in thrombus volume after endovascular repair using balloon and self­

expanding endograft systems.

B a c k g r o u n d

The role of intraluminal thrombus in relation to the risk of aneurysm sac rupture is controversial.

On the one hand, the presence of thrombus is felt to indicate weakness and impending rupture

with no importance in the overall mechanical properties of the aortic waU ®» On the other hand,

there is good evidence to suggest that thrombus is not a passive entity and its presence may be

intimately linked with the pathogenesis of the aneurysm^^^»

Although, thrombus has been described as “ A stagnant red cell-suspension entangled within a

platelet and protein mesh ^ ®,” histological studies demonstrate a more complex architecture^^< .

Intraluminal thrombus consists of a collection of highly active immunocompetent cells and

heterogenous proteins all housed in a system consisting of multiple canaliculi and blind-ending

ducts. It has been postulated that thrombus may have an important role during the pathogenesis of

the aneurysm: cytokines diffusing into the aortic wall influence the normal remodelling process

resulting in weakening and expansion^^®.

The endovascular era has engendered a further possible role for thrombus. Protection against Type

II endoleak from patent lumbar and mesenteric vessels may be achieved by a simple plugging

effect. Patients with thick circumferential or posteriorly placed thrombus appear less likely to

develop Type II endoleaks compared with individuals who have minimal or anteriorly placed

thrombus^‘2. It follows from these arguments that a decrease in the amount of thrombus after

deployment, may predispose to the development of endoleaks from patent lumbar arteries.

Equally, the endovascular graft may lose its circumferential support and hence become a weaker

mechanical structure with time.

In this study, the effect of endografting on the thrombus volume was followed using 3D-spiral CT

angiography.

114

P a t i e n t s a n d m e t h o d

From a total experience of 88 patients undergoing endovascular repair of their abdominal aortic

aneurysms, 30 individuals, who had reached follow up for 2 or more years, were chosen for further

study. 12 patients were treated with pre-expanded PTFE aorto-uni-iliac grafts and 18 had Talent,

supported Dacron grafts (World Medical). There were 24 males and 6 females with a mean age of

72.9 years (range 55-93).

Spiral CT angiography was performed with a Siemens Somatom plus 4 scanner using the protocol

described in Table 6 for all subjects. Volumetric data was transferred to a workstation, where bone

and soft tissues other than the AAA were manually removed from axial images using the

programme-editing tool (Figure 14). Volumes were calculated, using a voxel-summing technique,

for the whole aneurysm and for the intra-aneurysmal flow channel. These volumes were then

subtracted, using a previously validated technique (Chapter 7.2), to provide the volume of intra­

luminal thrombus (ILT).

S t a t i s t i c a l a n a l y s i s

Individual values are presented as the median volume (ml) with the interquartile range in brackets.

Results were analysed within groups using the Wilcoxon matched paies test. Between groups

analysis was performed with a Mann-Whimey U test. A P value less than or equal to 0.05 was

regarded as attaining significance.

R e s u l t s

Before endovascular repair, the median volume of ILT for PTFE patients was 33.47 ml (percentiles

22.41 to 55.98 ml). For Talent patients’ equivalent volumes were 79.85 ml (38.41 to 152.30 ml).

There was a significant difference between these two groups at the 5% level (Mann-Whitney U test;

P=0.01) suggesting that the Talent patients did contain more aneurysm thrombus preoperatively

than the PTFE group (Figure 37).

115

Preoperative intra-luminal throm bus volumes

500 -1

400

Ii

300 -

200 -

100 -

PTFE TalentPatient group

F igure 37: This graph shows the differences in preoperative intraluminal thrombus volume for the two populations used in this study. The box-whisker plot illustrates the 25*'’ (upper line o f box) and 75* (lower line o f box) percentiles. The median value for each

group is the centre line. The bars above and below each box indicate the maximum and minimum values respectively.

1. Outcome for PTFE-treated patients

After endovascular exclusion, the median PTFE ILT volume significantly (P=0.002) increased to

87.67 ml (67.76 to 120.80 ml). No further significant changes in volume were seen to the

conclusion of the study at 2.5 years (Table 20).

7 Type II endoleaks, noted immediately after endografting with a combination of spiral CT and

contrast angiography, had aU disappeared without any intervention by 6 months.

Time (days) Median thrombus volume (ml) Interquartile range P value*0 33.47 22.41 - 55.98 -

5 87.67 67.76 -120.8 0.002183 92.90 80.49 - 122.8 0.85365 97.98 77.95 - 99.22 0.91548 91.63 76.54 -120.7 0.12730 87.42 74.65 -104.4 1.00913 107.0 78.00 -174.60 0.50

*Wilcoxon signed rank test

T ab le 20: The effects o f endografting with PTFE on volume o f intraluminal thrombus. After an initial increase in thrombus volume, there is no further significant change up to 2.5 years (913 days) follow-up.

116

2. Outcome for Talent-treated patients

For Talent-treated patients there was also an initial significant (P=0.004) increase in thrombus

volume at day 5 after endovascular repair. The median thrombus volume increased by 34.15 ml

during this interval (Table 21).

At 6 months, there was a drop in thrombus volume (P=0.01) of 41.63 ml. The new median volume

(72.37 ml) was now less than the preoperative median thrombus volume. No further significant

change in ILT volume was observed to the conclusion of the smdy. 5 cases of Type II

endoleakage, observed at day 5, had all resolved by 6 months without further intervention.

Time (days) Median thrombus volume (ml) Interquartile rangée (ml) P value*

0 79.85 38.41 -152.3 -

5 114.0 87.09 -186.5 0.004183 72.37 41.56-105.9 0.01365 84.40 51.90 -109.5 0.19548 24.25 9.39-79.55 0.25

*Wilcoxon signed rank test

T ab le 21: The effects o f endografting with Talent on volume o f intra-aneurysmal thrombus. After an initial increase in thrombus volume, there is a continuing loss to

1-year follow-up.

C o n c l u s i o n

The significance of the larger amount of ILT in the Talent group preoperatively, is unclear (Figure

37) but is probably a reflection of the slightly larger aneurysms treated later on in our learning

curve. The median preoperative AP diameter for the PTFE group was 52.2 mm, transverse was

50.7 mm and preoperative volume was 128.5 ml. For the Talent group, the equivalent figures were

55.3 mm, 53.9 mm and 179.3 ml respectively.

Post endovascular repair, there is an increase in the amount of ILT which then alters depending on

the endografting system used. In patients treated with PTFE balloon-expandable endografts, the

ILT remains constant in volume.

In contrast, the amount of ILT with the Talent self-expanding system decreases almost to its

preoperative level after which no further significant change is noted.

Although patients in both groups experienced endoleaks, these resolved without further

intervention. Talent patients had no particular propensity to suffer from further type II leaks as a

result of the loss of thrombus. These results suggest that apart from the possible plugging of

lumbar artery ostia by thrombus preoperatively, there appears to be little relationship between the

amount of sac thrombus and type II endoleakage.

117

7.7

Changes in aneurysm and graft length after endovascular exclusion of AAA

A i m

The aim of this study was to investigate changes in aortic and endograft lengths in patients treated

either with balloon-expandable PTFE or self-expanding Talent endografts.

B a c k g r o u n d

Post-deployment disruption of endografts has been attributed to longitudinal shortening of the

aneurysmal aorta, lengthening of the prosthesis following deployment or migration of the

endograft from its proximal or distal anchoring sites. Most weight has thus far been given to the

theory that aneurysms shorten longitudinally after endovascular repair, a process akin to the

contracture of a scar*^ .

The main problems associated with these observations are that the measurement techniques used

are seldom validated and secondly, there is an incomplete understanding of the pathogenesis of the

aneurysmal process. It is difficult to understand how the continuing loss of elastin and collagen that

is fundamental to aneurysm formation could lead to any type of contracture.

In a prospective observational study, changes in aortic and endograft lengths for two populations

of patients are described using validated spiral CT angiography. One group was treated with a 1®

generation pre-expanded PTFE graft held in situ with two balloon—expandable Palmaz stents. The

second group received the fiilly supported, self-expanding Talent (World Medical) endograft

system.

P a t i e n t s a n d m e t h o d

68 patients were treated for AAA with 2 separate endografting systems. There were 59 males and 9

women with mean age 74 years (range 67-92 years). 27 patients had PTFE 1®* generation aorto-uni-

iliac prostheses while 41 patients had Talent endografts: two configurations were used: aorto-uni-

iliac (7 patients) and bifurcated modular (34 patients).

Patients with complications such as endoleak or requiring conversion to open repair were excluded

from the study.

Techniques used for the manufacture of the UGL PTFE endograft sytem have been previously

described'^^’ The PTFE graft was unsupported apart from the two fixation sites where it was

anchored to the aorta with Palmaz stents (Figure 41).

118

Patients had SCTA on the 5’ postoperative day and 6 monthly intervals thereafter using the

protocol already described (Table 6). Two longitudinal distances were measured and compared for

each patient (Figure 38):

• The shortest distance between the two points joining the lowermost renal artery and the aortic

bifurcation (the vertical bociy length)

• The length of the intra-aneurysmal flow channel (luminal centre line). This was calculated by

placing markers in the centre of the aneurysm or graft blood flow channel, perpendicular to it’s

waU and then measuring the distances between these markers starting at the upper margin of

the stent and finishing at the aortic bifurcation. These, landmarks were relatively easily

identified using a combination of 3-dimensional reconstruction and multiplanar reformation.

S t a t i s t i c a l a n a l y s i s

All results are displayed in mm as the median with the interquartile range in parentheses.

Comparison of patient outcomes within groups was performed using a Wücoxon sum of ranks

test. A Mann-Whimey U test was used to compare preoperative differences between the PTFE and

Talent group. P less than 0.05 was regarded as attaining significance.

R e s u l t s

Data fcom 12 PTFE and 30 Talent patients was available for analysis up to 3 and 2 years follow up

respectively. These patients showed no Type I endoleaks or evidence of late rupture. There were 7

cases of Type II endoleak in the PTFE group and 5 cases in the Talent group but all resolved

spontaneously within 6 months without intervention.

1. Changes in vertical body length for PTFE-treated patients

There was a significant (P=0.04) median increase in vertical body length from an initial 111.5 mm

(percentiles 103.7 to 125.9 mm) to 114.7 mm at day 5 (106.3 to 125.4 mm). The increase was seen

for 10 out of 12 patients, of which six had a difference greater than 2.5 mm, ie beyond the range of

observer error (Table 10).

The gradual increase in length continued, becoming significant (P= 0.03) when there was a further

median increase from 119.7 mm (109.1 to 125.8mm) at 12 months to 126.1 mm (111.9 to 135.7

mm) at 18 months. This occurred in 6 out of 7 patients. Further significant change was not seen up

to 3 years follow up (Figure 39).

119

2. Changes in luminal centre line length (LCL) for PTFE-treated patients

In contrast to vertical body length, there was no significant change in luminal centre line length

from the initial median value of 116.6 mm percentiles 111.1 to 131.8 mm) until 18 months. At this

time, median length had increased by 16.4 mm to 133.0 mm (127.8 to 158.9 mm; P=0.01). The

new graft length subsequently remained unchanged. The distance between the lowest renal artery

and the top of the stent was also measured. This did not show any significant change excluding

graft migration from this site as a cause.

3. Changes in vertical body length for Talent-treated patients

The initial median length for Talent-treated patients was 123 mm percentiles 104.5 to 133.2 mm).

This did not significandy differ firom the PTFE group preoperative lengths (Mann-Whitney U test).

Although, there was an apparent decrease in aneurysm length at 1 year, this failed to reach

significance (Figure 40). When Talent patients with the aorto-uni-iliac configuration were analysed

as a separate group, the results remained unaltered.

4. Changes in luminal centre line length for Talent-treated patients

The initial length in the Talent group was 130.6 mm percentiles 113.4 to 139.7 mm). This also did

not differ significandy from the PTFE-treated initial length (Mann-Whitney U test). There was no

significant change in graft length after treatment (Figure 40) for the study period (2 years).

120

V e r t i c a l b o d y l e n g t h

Figure 38: Length measurements made on aneurysms. The vertical body length was the shortest linear distance between the lowest renal artery and the aortic bifurcation. The luminal centre line length was measured in 3- dimensional space using electronic markers (black circles) placed in the center o f the flow channel from the

beginning of the top stent to the aortic bifurcation. The distance between each marker was calculated and summedto give this length

121

C o n c l u s i o n

Aneurysm formation is a continuous process characterised by destruction and loss of aortic elastin

and collagen. Biochemical analysis of AAA tissue has shown significant alterations in collagen and

elastin content along with activation of many proteolytic enzymes^^^» No more elastin is formed

after embryogenesis^^" thus reducing amounts further. On this premise, one would expect

lengthening and increasing tortuosity of the aortoüiac segment with time, borne out by simple

observation in vascular surgical practice^^ and the frequent findings of elongation and tortuosity of

the ihac arteries that make for difficulties of access in endoluminal repair.

The results obtained here, using the gold standard of validated spiral CT angiography, support this

concept.

For PTFE patients there is an immediate increase in aneurysm length that is not reflected by a

simultaneous increase in the LCL or graft length. This is followed by a second significant increase

in aneurysm length, seen at 18 months.

Lengthening of the PTFE graft at 18 months would be in keeping with the increase in LCL length

and the characteristics of the graft material — pre-expanded PTFE which would “creep” back to a

lesser diameter and a concomitant increase in length with time.

No significant change in length was observed for the Talent-treated patients up to 2 years. These

patients received a graft that was fully supported throughout its length with interconnected, self­

expanding nitinol stents oversized by up to 4 mm in diameter. This stiff graft may have had the

effect of anchoring the aneurysm wall to the prosthesis, thus preventing any lengthening fcom

taking place.

Based on these results, it is unlikely that prosthesis distortion or disruption can be attributed to

aneurysm length contracture.

122

Change in vertical body length after PTFE endografting

175 1

-5150

f2 5

oS i

1 100

;75

□□□

0 5 183 365 548 730 913

Time (days)

Change in luminal centre line length after PTFE endografting

200 -I

f 175

125 -

100 -

75

T T▼▼

183 365 548Time (days)

730 913

F igu re 39: Column scatter graphs showing the change in vertical body and luminal centre line lengths that occur after exclusion with P T F E endograft.

Each point represents an individual patient whüe the horizontal line represents the median length for each group. P values are in the text.

123

Change in vertical body length length after Talentendografting

175n

g 125-Io 100- !

75-V

183 365time (days)

)m

g

tn?

200n

175-

150-

125-

100 -

75-

50

Change in luminal centre line length after T aient endografting

5 183time (days)

365

F ig u re 40: G raphs show ing the changes in vertical bocfy and lum inal cen tre line lengths tha t occur after exclusion w ith the T alen t endograft. T h e horizontal

Hne indicates the m edian length while the points rep resen t individual patients. P values are in the text.

124

7.8

Natural history of the aneurysm neck after endoluminal repair using balloon and self-exnanding endoeraft systems.

A i m

To determine the natural history of the proximal aneurysm neck after endovascular repair

deploying balloon and self-expanding endografts.

B a c k g r o u n d

The durability of endovascular repair is still unknown. One concern is expansion of the aortic neck

at the site of endograft attachment. Studies of inffa-renal aortic diameter in healthy individuals have

shown a gradual dilatati.on '^ of this segment with age (Table 22). The magnitude of the increase is

dependent on both the body surface area and the sex of the patient. The inffarenal diameter

increases 26% in males compared to 24% in females between the ages of 25 and 70 years * .

Patients with aneurysms demonstrate an even greater dilatation of this region over time with

significant implications for the longer-term integrity of the graft-vessel wall seal^^. In contrast to

studies of neck dilatation after open repair, only a few studies have looked specifically at neck

changes post-endovascular repair. Many of these have either short term follow up (1 year or less) or

are performed using axial CT slices and non-calibrated measurement techniques.

Neck measurements performed using 3D reconstruction and MPR have the advantage of

recording dimensions that are perpendicular to the central lumen (or flow line) of the aneurysm.

These are more accurate as plain slices may capture the aorta in an elliptical section and provide

erroneous results.

Aortic region Expansion (m m /year)Thoracic 0.14Coeliac 0.11Renal 0.08

Inffa-renal 0.07

T a b le 22: C hanges in aortic d iam eter at d ifferent levels w ith time.

P a t i e n t s a n d m e t h o d s

From a total experience of 88 patients undergoing endovascular repair of their abdominal aortic

aneurysms, 30 were chosen for this study. All patients reached a minimum of 2 or more years

follow up. 12 patients had pre-expanded PTFE balloon-expandable, aorto-uni-iliac grafts (Figure

125

41) and 18 had Talent, self-expanding, fully supported Dacron grafts (World Medical). There were

24 males and 6 females with a mean age of 72.9 years (range 55-93).

Spiral CT angiography was performed with a Siemens Somatom plus 4 scanner using the protocol

in Table 6 for all subjects.

Neck diameter and length measurements were carried out on all patients using 3D reformatting

and MPR respectively (Table 23). The neck of the aneurysm was defined as starting at the level of

the lowest renal artery and ending at the beginning of the aneurysm chamber, usually identifiable by

the presence of a gradual dilatation containing intra-luminal thrombus. This length is referred to

here as the 'neck length. ”

Neck diameters were recorded perpendicular to the true centre or flow-line of the vascular channel

within the aneurysm. Diameters were recorded at 3 levels (Figure 42):

• At the level just below the lowest renal artery

• The mid-neck diameter was calculated at the halfway point once the neck length was known.

• At the level just above the beginning of the aneurysm chamber

The aorta is never a perfect circle in cross-section. In some instances a true ellipse may be present.

In such a case an over or underestimation of diameter is easily made. To avoid this type of

assumption, two diameters perpendicular to each other were measured. The average of these was

used to represent the diameter at that level.

Measurements of aneurysm neck

proximal neck length

maximal proximal neck diameter maximal mid neck diameter

maximal distal neck diameter

T a b le 23: M easurem ents carried o u t on aneurysm patients (mm)

S t a t i s t i c a l a n a l y s i s

All values were recorded in mm with the median followed by the interquartile range in parentheses.

Comparison of patient outcomes within the two groups was performed using a Wücoxon sum of

ranks test. Median neck lengths and diameters were compared to the previous values with P less

than 0.05 regarded as attaining significance.

126

R e s u l t s

Results of neck length and diameter changes for the two groups are shown in Tables 24 -26.

No significant change in neck length was apparent during the course of the study.

In contrast, marked changes in neck diameters were recorded. For the PTFE-treated patients, both

the upper and mid-neck diameters of the proximal neck had increased (P=0.03) by day 5 post-

endografting (Table 25). The lower neck diameter showed a significant increase at 6 months

compared to the 5-day median value (P=0.05). Following these initial changes no further change in

neck dimensions was seen to the conclusion of the study.

For Talent patients, significant increases in all neck diameters were seen at day 5 and again at 6

months (Table 26). No further change occurred thereafter.

Days 0 5 183 365 548N um ber of patients 12 12 10 9 7

M edian 35.1 32.9 30.5 32.1 30.9Interquartile range 27.0-47.9 27.6-39.8 25.1-36.8 26.7-35.2 19.0-47.6

T a b le 24a: Change in m edian aneurysm neck length after endografdng w ith P T F E anchored w ith balloon-expandable stents.

E ach value is com pared w ith its preceding value using the W ilcoxon sum o f ranks test.

Days 0 5 183 365 548Num ber of patients 18 18 11 10 7

M edian 23.5 25.4 28.1 26.9 24.4Interquartile range 20.1-28.5 22.8-27.6 21.5-33.3 20.9-31.1 23.1-30.8

T a b le 24b: Change in aneurysm neck length after endografdng w ith the T alent self-expanding system

127

Figure 41: P T F E endograft. M ade w ith a standard length o f P T F E pre-expanded to three times its ongm al diam eter, two

Palm az balloon-expandable stents (S) are sutured w ith prolene stitch to each end o f the endograft (G). This is deployed w ithin

the aneurysm as described. T he contralateral com m on iliac artery was occluded with tungsten em bolisation coils.

Upperdiameter

Mid­diameter

Neck length

Lowerdiameter

Diameter=A+B/2Diameter A

Diameter B

Figure 42: Method o f calculation of neck, diameters at three levels within the neck o f the aneurysm. The mid-neck diameter was recorded at the

point hallway along the neck length.

128

Days 0 5 183 365 548N um ber of patients 12 12 10 9 7

M edian 23.5 25.0 25.8 24.6 24.2Interquartile range 22.1-24.6 22.9-26.3 24.0-26.5 23.2-25.6 23.1-24.4

P value - 0.03 0.79 0.11 0.73

T a b le 25a: Change in upper neck diam eter after endografting w ith P T F E anchored using Palm az stents. E ach value is com pared

w ith its preceding value using the W ilcoxon sum o f ranks test.

Days 0 5 183 365 548N um ber of patients 12 12 10 9 7

Median 23.6 26.1 25.5 24.9 25.1Interquartile range 22.4-25.5 23.8-26.7 23.7-26.7 23.6-25.4 21.2-25.6

P value - 0.03 0.52 0.10 0.73

T a b le 25b: C hange in m id-neck diam eter after endografting w ith P T F E anchored using Palm az stents

Days 0 5 183 365 548Num ber of patients 12 12 10 9 7

Median 24.5 26.0 25.7 25.5 24.9Interquartile range 24.2-29.0 22.7-29.7 22.7-27.9 23.6-28.0 23.4-25.1

P value - 0.12 0.05 0.77 0.43

T a b le 25c: Change in low er neck diam eter after endografting w ith P T F E anchored using Palm az stents

129

Days 0 5 183 365 548N um ber of patients 18 18 11 10 7

Median 22.9 25.5 26.1 27.9 25.1Interquartile range 21.8-24.0 24.0-27.7 25.6-29.5 23.7-29.8 21.9-29.8

P value - 0.007 0.05 0.31 0.63

T able 26a: Change in upper neck diameter after endografting with self­expanding Talent system. E ach value is com pared w ith its preceding

value using the W ilcoxon sum o f ranks test.

Days 0 5 183 365 548N um ber of patients 18 18 11 10 7

Median 23.5 26.6 26.7 28.0 25.1Interquartile range 22.7-25.7 23.9-28.6 24.8-28.8 25.1-30.2 22.2-28.0

P value - 0.03 0.03 0.38 1.00

T a b le 26b: Change in m id-neck diam eter after endografting w ith self-expanding T alent system

Days 0 5 183 365 548Num ber of patients 18 18 11 10 7

Median 25.6 27.8 26.5 27.4 25.5Interquartile range 24.1-27.0 26.0-31.2 24.8-31.1 25.4-30.6 24.7-30.7

P value - 0.01 0.05 0.64 0.63T able 26c: Change in lower neck diameter after endografting with self­

expanding Talent system

C o n c l u s i o n

Endografting with PTFE or Talent endografts did not appear to have any significant influence on

the length of the aneurysm neck during the course of this study. Instead, the grafting systems

produced significant effects on aneurysm neck diameters.

Both grafts caused an initial increase in the neck diameters. This continued over a period of at least

6 months for the Talent but not PTFE patients. This observation may be explained by the

characteristics of the individual stent types:

In the case of the balloon-expandable stents used to anchor the PTFE endografts, neck

enlargement is expected as the stents are oversized by 2 mm in each case. Once deployed, the stent

remains stable at this diameter with no further longer-term stent expansion. Neck diameters should

hence remain unaltered.

For Talent-treated patients, the system of interconnected self-expanding stents continues to slowly

expand into the aortic wall for a period of time after deployment, thus simultaneously increasing

130

the neck diameters. It is possible that if the elastic recoil of the aortic wall is eventually exceeded,

this could lead to loosening of the graft causing endoleak, graft migration or both. It is important to

note that the results described here are recorded within the medium-term of 1.5 to 2 years.

Whether these trends continue in the longer-term remains to be determined.

131

7.9

The intra- and inter-observef differences in aneurysm neck length and diameters found during measurement with spiral CT angiography

A i m

This study determines the inter-observer and intra-observer variability in the assessment of

abdominal aortic aneurysm neck dimensions.

B a c k g r o u n d

The selection of patients for endovascular repair of AAA relies on critical anatomical features such

as the proximal neck dimensions and the tortuosity of the diac arteries. These dimensions are

currently measured using a combination of calibrated angiocatheter and spiral CT angiography.

Both methods have inherent errors resulting from the nature of the imaging technology and the

operator who has to interpret the end result. For example, with calibrated angiography system-

related errors due to magnification and the penumbra effect (Figure 66) may result in

overestimation of neck diameters. Operator error from parallax may also produce similar

measurement problems.

Because of the recognised disadvantages associated with calibrated angiography, 3D-spiral CT

angiography is increasingly beginning to play an important preoperative role in measuring

aneurysm dimensions prior to patient selection. It is still possible however, that a certain

proportion of patients may be denied the procedure on the basis of incorrect interpretation of

measurements or worse, may undergo the procedure with incorrect anatomy. This study

documents the magnitude of error, involved in patient selection for endovascular repair in our unit,

using spiral CT angiography.

M e t h o d

120 consecutive patients were assessed for endovascular repair of AAA between January 1996 and

January 1998. From these preoperative spiral CT scans, 12 data sets were chosen for repeat

analysis. Random selection was carried out by numbering each scan and then generating a string of

12 random numbers by measuring background radioactive decay with a Geiger counter interfaced

to a computer. Scans were reanalysed by two individuals judged to be of equivalent experience in

utilizing this technology.

132

For the intra-observer part of the study, the original investigator (RSR) repeated his analysis 6

months after the first measurements. A second investigator, judged to be of equivalent experience,

also re-analysed the same scans at this time. Scans were stored on optical disc (3M Imation

Enterprises Corp. Oakdale, Minn) and only the data file was made available to both investigators.

Repeat measurements were performed without any knowledge of the previous values.

The following parameters were measured:

1. Aneurysm neck length

2. Aneurysm neck diameter at the level of lowermost renal artery

3. Aneurysm neck diameter at the start of the aneurysm sac

S t a t i s t i c a l a n a l y s i s

The difference of means analysis described by Bland and Altman^ " was used to compare values.

The difference between each pair of measurements was plotted against their mean. By analysing

the differences between paired measurements, the only source of variability is the measurement

error and this should follow a Gaussian distribution. The standard deviation of the differences was

also calculated to work out a coefficient of repeatability, which was defined as equivalent to 1.96

times the standard deviation.

R e s u l t s

The intra- and inter-observer variations for neck length are listed in Table 27. Proximal neck length

could be measured with an accuracy of ±5 mm both within and between observers. Observer

errors for the neck diameters were much lower (Tables 28 and 29). The upper neck diameter could

be measured with an accuracy of ±2 mm on repeated occasions compared with an accuracy of ±3

mm for the lower neck diameter.

Study Mean of differences SD Coefficient of repeatabilityIntra-observer 5.09 2.26 4.51Inter-observer 7.01 2.50 4.99

T a b le 27: T he in tta-observer and in ter-observer differences obtained in m easurem ent o f aneurysm neck length.

133

Study Mean of differences SD Coefficient of repeatabilityIntra-observer 0.83 0.95 1.91Inter-observer 1.00 1.00 2.00

T a b le 28: T he intra-observer and in ter-observer differences obtained in m easurem ent o f aneurysm neck up p er diam eter.

Study Mean of differences SD Coefficient of repeatabilityIntra-observer 1.86 1.36 2.73Inter-observer 1.53 1.23 2.47

T a b le 29: T he in tra-observer and in ter-observer differences obtained in m easurem ent o f aneurysm neck low er diam eter.

C o n c l u s i o n

The differences involved in aneurysm neck measurement between and within observers are slightly

larger for neck length and neck lower diameter than for measurement of upper diameter. Proximal

neck length can be estimated with an error of +5 mm, neck upper diameter with an error of +2

mm and neck lower diameter with an error of +3 mm.

These values become clinically important when considering aneurysms on the borderline of

selection for endovascular repair. A 5 mm error in neck length measurement means that some

individuals with necks too short for stent fixation may incorrectly undergo endovascular repair. A

neck that is sized as 15 mm may hence actually lie within the range 10-20 mm. Clearly at the lower

end of this range some units may not consider the patient suitable for this method of treatment at

all so that a proportion of patients may be erroneously selected. The converse of this argument is

also true: patients with necks that are just within the correct length may be denied endovascular

repair. The actual numbers of patients with these borderline dimensions may never be known. As

discussed, biological tissues are subject to post-mortem changes and desiccation that render

measurement of their dimensions after removal ftom the host subject to inaccuracies.

Analysis of the comments made for individual data sets by the investigators in this study produced

a possible explanation for the differences in diameter measurements. It was noted that the junction

where the aneurysm neck ended and the aneurysm sac began was quite often difficult to clearly

define. Marking the point at which the gradual widening of the neck began to give way to the

aneurysm chamber was very subjective, especially when there was no thrombus outlining this area.

The lower neck diameter was hence measured at a variety of locations with differing values. In

contrast the upper diameter of the aneurysm neck was always measured consistently immediately

below the lowest renal artery.

134

An alternative explanation is based on aortic wall thickness. Measurements of neck diameter

differed by 2-3 mm, a value similar in magnitude to the thickness of the aortic wall. It is equally

possible that the diameters may not have been consistently measured from external wall—to-

extemal wall or internal wall-to-intemal wall by the investigators thus further accounting for these

differences.

135

7.10

Measurement of distance and volume with MRI: An in vitro feasibility stud

A i m

The aim of this study was to assess the feasibility of aneurysm volume measurement using

gadolinium-enhanced Magnetic Resonance Angiography (gMRA).

B a c k g r o u n d

Breath-hold gadolinium-enhanced three-dimensional (3D) magnetic resonance imaging (MRI) has

been developed in the past few years as an alternative technique to conventional angiography,

primarily in the evaluation of arterial disease. The rapid acquisition times possible with high-

performance gradient systems allow 3D imaging of large volumes of anatomy during the arterial

and later phases of contrast material distribution through the body, similar to that in SCTA. This

technique is versatile and in many clinical settings, 3D gadolinium-enhanced MR imaging is being

used in lieu of computed tomography and conventional angiography.

In addition to the diagnostic information regarding arterial anatomy that this technique can

provide, additional quantitative information helpful in patient selection and follow up post-

endovascular repak can be obtained from a multiphasic examination. This includes measurement

of linear and volume data. Volume may be more accurate than single maximal diameters in

describing the morphology of AAA after endovascular repair. The feasibility of aneurysm volume

follow-up with MRA remains to be demonstrated.

In this study the volumes and linear dimensions of six phantom aneurysms were calculated

following imaging with gMRA. These data were compared with the known values to calculate the

accuracy of this technique.

M a t e r i a l s a n d m e t h o d

Six glass phantom aneurysms were hand blown using tubing 2 mm in wall thickness and 25 mm in

diameter. The physical dimensions (volume and maximal diameters) of these phantoms were

recorded in the laboratory using pyknometry (Figure 17) and electronic callipers (Mitutoyo absolute

digimatic, Japan). These dimensions were referred to as the true diameters and volumes.

Phantoms were then filled with a mixture of Gadopentetate dimeglurnine (Magnevist; Berlex

Laboratories, Wayne, NJ) and de-ionised water in proportions similar to those found in vivo during

gMRA (20 ml of Magnevist in 5 L of deionized water). Scanning was performed with a Siemens

Magnetom vision 1.5T scanner using the same MRA protocol as in clinical practice (Chapter 7.11).

136

Scanned data were transferred to our post-processing workstation for the calculation of scanned

diameters and volumes using the same techniques as previously described (Chapter 7.1).

S t a t i s t i c a l a n a l y s i s

All linear data are given as the mean of eight observations together with the standard deviation.

Volumes are given as the value in ml calculated using 3D gMRA data. The true and scanned

dimensions were compared using regression analysis. P less than or equal to 0.05 was regarded as

attaining significance.

R e s u l t s

The results obtained for diameter are presented in Table 30. These values are comparable to the

SCTA validation study (Table 7). In particular, the standard deviations are again at least 10 fold

higher for the scanned diameters compared to the true diameters.

The regression graphs (Figures 44 and 45) for this data have a slope of 1.01 for both diameter

calculations. This suggests that the agreement between true and scanned values is near perfect.

The results for volume are presented in Table 31. Similar values were again obtained as for the

SCTA validation study (Table 8). Two further points should however be noted:

Firsdy, MRA is not based on the principle of Hounsfield number and doe not define objects as

maps of their tissue attenuation profiles. In MRA images the signal intensity is proportional to the

proton density of the target tissue with enhanced signal from tissues containing gadolinium

contrast. MRA raw data consists of voxels that are converted to 2-dimensional pixels in the

corresponding image. The signal intensity of each pixel equates to the contents of the

corresponding voxel so that to calculate the volume of an object a pixel histogram is generated

(Figure 43).

The number of pixels that are contained within the volume of interest can be calculated to allow

estimation of volume on the available software. The procedure takes somewhat longer than SCTA

volume estimation by an additional 10-15 minutes on the total estimation time.

137

AneurysmTrue diameters (mm) Scanned diameters (mm)

anteroposterior transverse anteroposterior transverse

A 43.33±0.10 48.83±0.07 42.84±0.80 47.3510.46

B 55.44+0.06 54.81±0.05 55.49+0.63 53.7410.39

C 60.51+0.07 60.47±0.10 58.5010.56 59.7010.56

D 69.81±0.07 69.84±0.06 68.7110.04 68.7310.05

E 76.54±0.05 78.92+0.03 77.3410.92 77.2010

F 79.74+0.03 79.80±0.03 78.8510.48 79.2910.32

T a b le 30: A C om parison o f true and scanned diam eters o f glass p h an tom aneurysm s. T he regression plots and equations for this com parison are show n in Figures 44 and 45.

138

500 450 400

^ 350 - I 300 -wc 250 I 200 S 150

100 50

0

Figure 43a

%♦

50 100 150

Number of pixels

200

120

100Figure 43b ♦

%

■S' 80

60

r 40

20

♦♦

♦♦♦

%♦

50 100

MRI threshold

150 200

F ig u re 43: G raphs showing pixel intensities obtained from scanning Phantom aneurysm A. Figure 43a is a plot o f the whole data set showing

an early peak m ade up o f indeterm inate factors that affect scanning (“noise”). A second peak relating to phantom aneurysm volum e follows. Figure 43b is a p lo t o f the pixel intensities relating only to the phantom . The threshold chosen to calculate the volum e o f this object is obtained

at the edge o f the contour, in this case 50.

139

Secondly, MRI imaging also depends on the ability to realign protons using a magnetic field. MRI

cannot image objects that are very dense, such as calcium or glass, because the protons are too

fixed to move when a magnetic flux is applied. During scanning, the glass comprising each

phantom was not imaged and the calculations for scanned volume hence only apply to the solution

contained within the aneurysms (Table 31). Although the percentage errors for volume appear to

be much lower than for SCTA estimations, this is in fact not a valid comparison. As only the

volume of solution contained within each phantom was imaged, the percentage error for the total

volume (glass phantom + solution) in each case is therefore much higher.

The Table shows that the MRI threshold for volume calculations differs for each phantom. These

range from 50 to 180. This is quite unusual, as one would expect the histograms to have equivalent

thresholds for the water-Magnevist mixture, which was the same in aU cases. The explanation lies

with the subtleties of MRI scanning. Unlike spiral CT, the object being scanned has an

undetermined effect on the MRI magnet producing an inhomogeneity in the magnetic field. This is

turn affects the scan data. External factors such as humidity and patients scanned prior to the

object of interest also have minor effects. Curiously, although the phantom aneurysms were all

imaged at different time intervals using the same protocol, phantoms B and C imaged on the same

day, had the same MRI threshold for volume estimation compared to A, D, E and F that were

imaged separately.

ANEURYSMTRU E VOLUMES

(ml at 21.9°C)SCANNEDVOLUMES(threshold)

PERCEN TERRORvolume of glass volume of solution

A 39.78 133.96 132.32 (50) 1.22

B 40.13 180.59 180.60 (127) -0.01

C 39.06 216.44 216.73 (127) -0.13

D 47.28 233.66 234.02 (150) -0.15

E 46.83 274.36 274.74 (155) -0.14

F 49.78 308.71 307.45 (180) 0.41

T a b le 31: T he true volum es o f six phan tom aneurysm s com pared w ith their scanned volum es after gM RA imaging. T he figures in parentheses refer to the M RJ thresho lds used to segm ent

o u t and calculate the volum e o f interest. I t is im portan t to n o te tha t M RA is unable to im age the glass o f the phantom s so the scanned values refer only to the vo lum e o f so lu tion in each

phantom . F o r this reason, the percentage e rro r has been calculated fo r the volum e o f solution and n o t the total phan tom volum e, as was the case fo r the SCTA validation study.

140

R egression graph o f scan nedversus true AP diam eters

<uu

I’3( U

ccu(/)

lOOn

7 5 -

5 0 -

2 5 -

100T ru e A P d ia m e te r (m m )

Variables ValuesSlope

Y-interceptr2

1.01 ± 0.04 -1.09 ±2.30

0.99

F ig u re 44: G raph and Table showing the results obtained w ith linear regression analysis o f true and scanned AP diam eters after

gMRA. The dotted lines on the graph are the 95% confidence intervals for the slope.

141

Regression graph o f scanned versustrue transverse diameters

100aa

I(U

>CAC§13<uCCu

C / )

10025 50 750True transverse diameter (mm)

Variables ValuesSlope

Y-interceptr2

1.01 ±0.02 -1.46± 1.14

0.99

F ig u re 45: G raph and Table showing the results obtained w ith linear regression analysis o f true and scanned transverse diam eters after gMRA. The dotted lines on the graph are the 95% confidence

intervals for the slope.

C o n c l u s i o n

These results demonstrate that gMRA is accurate for estimating the linear dimensions of

aneurysms. The gradient of the regression Hne is close to unity for both AP and transverse diameter

estimation. This suggests, as for SCTA, near perfect agreement with the true values. Although,

gMRA is able to calculate volume, this is very time consuming and dependent on the quantity of

protons and density of the object under scrutiny. In practical terms, this may cause problems with

aneurysms that are very calcified or contain large amounts of old, laminated thrombus.

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7 .1 1

A comparison of gadolinium-enhanced MR angiography versus spiral CT angiography in the evaluation of patients for endovascular repair

A i m

The aim of this study was to assess the accuracy of gadolinium-enhanced 3D MR angiography

(gMRA) in comparison with 3D spiral computed tomographic angiography (SCTA) in a clinical

setting.

B a c k g r o u n d

Accurate preoperative imaging of AAA is an absolute prerequisite in determining patient suitability

for endovascular exclusion. It is vital for stent construction, to demonstrate aneurysm morphology

and dimensions accurately, as well as identifying the possible complications that may be

encountered during repair. 3D spiral computed tomographic angiography (SCTA) has previously

been validated as the gold standard for aneurysm assessment in both these situations. The main

disadvantage of SCTA is that intravenous iodinated contrast may be a source of nephrotoxicity in

patients with renal compromise^^^> This is potentiated if intravenous iodinated contrast agents

are also used in preoperative and per-operative angiography.

Magnetic Resonance Angiography (MRA) has shown varied success in the evaluation of

aneurysms. Difficulties in imaging the slow, swirling flow within aneurysms, turbulent flow in

stenoses, and tortuous iliac arteries ^® previously limited its usefulness in providing the detailed

information necessary for preoperative planning.

Recent developments using gadolinium-enhanced MRA have overcome some of these

problems^^^. Gadolinium, a paramagnetic contrast agent, shortens the Ti (spin-lattice) relaxation

time of blood, making blood distinct from surrounding tissues regardless of its flow rate or

direction. The technique provides true anatomic images that have an appearance similar to

conventional or digital subtraction contrast arteriograms. Furthermore gMRA offers the potential

of an imaging modality that uses non-nephrotoxic contrast and provides both anatomical and

physiological information about renal function.

This study will assess the accuracy of gMRA in comparison with SCTA in the preoperative

evaluation of abdominal aortic aneurysms for endovascular repair.

143

P a t i e n t s a n d M e t h o d s

7 patients, refeired for endovascular repair, underwent both SCTA and gMRA for the evaluation of

their abdominal aortic aneurysms. There were 5 men and 2 women with a mean age of 68.6 years

(range 63-72 years).

Spiral CT angiography was performed with a Siemens Somatom plus 4 scanner using the protocol

described in Table 6.

All MRA imaging was performed with a Siemens Magnetom 1.5-T MR system (peak gradient

strength was 23 mT/m, and maximal slew rate was 120 mT/m/msec). Patients were imaged in the

supine position using a phased-array torso coil. An antecubital vein was cannulated for

administration of contrast. Axial and coronal 2D T l and T2 breathhold sequences and a coronal

3D spoiled gradient echo block. Approximate acquisition time for the pre-contrast data sets was

17-25 seconds.

Bolus tracking was carried out on all patients to optimize image acquisition to the phase of

maximal arterial enhancement following intravenous injection of gadolinium. Median delay from

the antecubital vein was 18 seconds.

Gadopentetate dimeglurnine (Magnevist; Berlex Laboratories, Wayne, NJ) was administered

intravenously by hand injection at a rate of 2.5-3 mL/sec for a total volume of 20 mL (dose of 0.1

mmol/kg body weight). The intravenous line was flushed with 15 mL of sterile saline following

contrast material injection. The breath-hold time was approximately 20-28 seconds for each 3D

volume acquisition. Image acquisition of the first post-contrast 3D volume set, with use of the

same pulse sequence parameters as for pre-contrast imaging, was begun after approximately half of

the contrast dose was injected. K-space data were collected in sequential order.

The 256 x 256 matrix images were reconstructed to a 512 x 512 matrix by using a process called

zero-fill interpolation to enhance spatial resolution. Section thickness varied between 5 and 7 mm,

with no intervening gap. The section thickness depended on the size of the patient, with the goal of

covering the anatomic space of the major visceral abdominal vessels. A total of 56 sections with an

individual thickness of 2.5 mm (using the 5-mm-thick sections) or 3.5 mm (using the 7-mm-thick

sections) were created.

Image acquisition in the arterial phase was performed in the coronal plane with use of a square field

of view. Owing to the anisotropy of the imaging voxel that was used (1.4 mm in the frequency and

phase directions and 5-7 mm in the z direction with a field of view of 36 cm), a diagnostic arterial

MIP could be generated only in the plane in which the data were acquired.

Data from gMRA and SCTA were independendy transferred to the post-processing MGI (UCL

imaging group) work station, where bone and venous structures were electronically edited from the

144

coronal and axial images respectively. Maximum intensity projection (MIP) technique was used to

reconstruct the gMRA coronal images into an arteriogram that could be projected from any angle

(Figure 46). Using the measuring tool within the software, markers were placed as for the spiral CT

reconstruction to measure distances between points.

gMRA images were analyzed separately to the SCTA scans with the reader blinded to the SCTA

findings. There was a gap of 2-3 weeks between analyses of data from the same patient. The

following parameters were recorded:

1. Aneurysm neck measurement

Aneurysm neck length (from the lowest renal artery to the beginning of the aneurysm sac) and

diameter at 3 levels was measured. The upper neck diameter was measured at the level of the

lowest renal artery; the lower neck diameter was measured at the level of the start of the aneurysm

sac; the mid-neck diameter was measured at the middle of the neck once the length was known.

Two measurements of the greatest and least diameters were made to avoid bias if the aortic neck

was not a perfect circle (Figure 42).

2. Aneurysm flow channel measurement

The blood flow channel was measured in true 3 dimensional space using multiplanar reformation

linked to a 3 dimensional surface shaded display of the aneurysm. Electronic markers were placed

in the centre of the flow channel perpendicular to its walls. The distance between these markers

was summed for the total length (Figure 38).

3. Common iliac artery measurement

Measurement of the common iliac arteries was carried out in a similar fashion to the flow channel.

The maximal diameter perpendicular to the vessel lumen at that point was also measured.

145

I U . C M O I - M a O l e a l O r m * « T l = # W o r k a c a t l o n

Figure 46: G adolinium -enhanced MRA o f an abdom inal aortic aneurysm. T he left picture shows the M IP view while the right figure shows the 3D reconstruction.

4. A n e u r y s m v o l u m e m e a s r e m e n t

Volumetric assessment of AAA did not prove successful in any of the patients chosen for this study.

ITie main difficulties arose because of data collection in coronal as opposed to axial slices due to the

anisotropy of the imaging voxel required for volumetric study, lliere was particular difficulty in

finding and manually delineating small segments of sac that had just been sliced off at their tip in

some sections. ITiis type of difficulty would not exist had slices been axial.

Further explanation is given below and in Chapter 10.

S t a t i s t i c s

Data are expressed as the median in mm with the interquartile range in parentheses. The Wilcoxon

paired test was used to evaluate the significance of the differences between measurements obtained

with two methods, and Bland-Altman analysis was performed for agreement assessment^. limits

of agreement were defined as mean ±1.96 times SD of differences. A value of P less than or equal

to 0.05 was considered to be statistically significant.

R e s u l t s

The mean aneurysm AP and transverse diameters for the whole group, measured using SCTA, were

57.74 (interquartile range 47.80 to 62.40) mm and 48.84 (33.60 to 76.40) mm respectively.

One of the main differences between SCI A and gMRA was the poor resolution, in the case of

gMRA, for the 3D data sets. Although the gMRA flow channel was very clearly delineated, the

transition between the outer edge of the sac and surrounding soft tissues was sometimes difficult to

distinguish at the resolution available (256 x 256 pixels). This was further confounded by the

146

difference in voxel shape between SCTA and gMRA. As described, the 3D-gMRA voxels were

rectangular with the long axis of each rectangle stacked vertically so that longitudinal resolution was

lower than transverse definition. In contrast SCTA voxels were cuboid with the same resolution in

aU visual planes.

1. Aneurysm neck measurement

The median upper neck diameter measured using SCTA was 19.48 (interquartile range 18.78 to

21.75) mm. The median mid-neck diameter values were 22.43 (21.90 to 23.25) mm. The equivalent

lower neck values were 24.00 (23.10 to 25.03) mm.

The limits of agreement for neck diameter measurements between the two techniques were similar

for all three levels measured (Table 32). There was a disagreement of approximately -4 to +1 mm

using gMRA compared with SCTA. This was higher than the disagreement for neck length

measurement (-2 to +1 mm).

Apart from one case (mid-neck diameter, P=0.03), the measurement differences between gMRA

and CT were not statistically significant. The reason for this particular discrepancy was not

apparent and may have been due to statistical artefact.

2. Aneurysm flow channel (lAFC) measurement

The median lAFC length measured using SCTA was 119.7 (115.8 to 124.3) mm. The agreement

for this region approximated to ±3 mm between the methods. The greatest differences in length

estimation, between the two methods, were found to occur in situations where the flow channel

was very tortuous and a larger number of markers therefore had to be placed within the lumen for

an accurate assessment. There was no significant difference between the actual values obtained

using either SCTA or gMRA.

3. Common iliac artery measurement

The median right common iliac artery length and diameter measured with SCTA were 52.55 (46.90

to 76.85) mm and 14.75 (14.35 to 18.65) mm respectively. The equivalent left common iliac

dimensions were 51.70 (46.70 to 69.20) mm and 14.20 (13.70 to 15.85) mm respectively.

Results contrasted with the neck measurement data. In this case, limits of agreement comparing

gMRA to SCTA, were better for diameters (-3 to +2 mm) than for length (-3 to +7 mm). Again,

there was no significant difference between the data obtained with the two methods.

147

Mean SCTA value

Mean gMRA value P value Limits of

agreementUpper neck diameter 20.53 18.89 0.06 -3.84 to 0.56

Mid neck diameter 22.20 20.50 0.03 -3.48 to 0.08

Lower neck diameter 23.68 22.67 0.44 -2.44 to 0.68

N eck length 30.80 31.20 0.69 -1.67 to 0.79

lAFC length 120.5 121.2 1.00 -3.35 to 3.23

Right Common iliac artery length

Left Common iliac artery length

Right Common iliac artery diameter

71.23 71.20 1.00 -0.84 to 5.58

58.53 61.72 0.44 -2.82 to 6.47

16.43 15.23 0.16 -2.95 to 1.67

Left Common iliac artery diameter 15.30 14.00 0.09 -1.34 to 0.50

T ab le 32: A com parison o f the m easurem ents obtained using SCTA and gM RA in the preoperative assessm ent o f abdom inal aortic aneurysm s for endovascular repair

C o n c l u s i o n

These results demonstrate that gMRA can be used to assess abdominal aortic aneurysm dimensions

prior to endovascular repair. In a comparison with validated SCTA, there is an overall disagreement

of -4 to +7 mm depending on the region measured.

In general measurements over shorter distances produced better agreement compared to

measurements taken over longer distances. For example, the limits of agreement for aneurysm neck

dimensions (length and diameters) and common iliac artery diameters were much better

numerically than for flow channel and common iliac artery lengths. This was almost certainly due

to the anatomical tortuosity over longer distances.

Longitudinal measurement of a tortuous cylinder using two different methods will always result in a

discrepancy. This difference is related to the number of markers placed and their locations within

the lumen in 3D space. For example, a larger number of markers will be more accurate than a

lesser amount. Further, if markers are placed exacdy opposite points of wall deviation, this wül also

increase precision. We found this rule to especially apply to the common ihac arteries, which

demonstrated the greatest degree of tortuosity in our patients.

In the case of diameter measurements, discrepancies between the two methods may be explained

by the arterial wall thickness. Normal aortic wall thickness is about 2 mm and one would expect a

148

difference of about 4 mm in favour of SCTA if the gMRA scans did not display wall thickness. In

most subjects where this discrepancy was evident, subsequent simultaneous review of the gMRA

and SCTA scans showed the former to be simple “lumenograms” compared to their SCTA

counterparts. Wall thickness was more clearly defined with SCTA especially where MIP’s were

analysed for gMRA. Certainly, agreement between methods for diameter estimation was much

closer for the common iliac arteries compared to the aneurysm neck. It could be argued that the

arterial wall thickness at the level of the external iliac arteries is much thinner than in the main aorta

thus producing much less diameter disagreement between the two methods in this region.

In conclusion, we found gMRA to be a useful means of assessing aneurysm dimensions prior to

endovascular repair. Further discussion on this point is provided in Chapter 10.

149

C h a p t e r 8

Photogfammet

Endograft durability may be linked to motion occurring at the aneurysm neck. The endograft is a

rigid mechanical device that is placed within a moving biological system. A differential motion

between aneurysm neck and stent-graft may result in long-term strain on the weakest points of the

endograft with eventual distortion and/or disintegration.

To examine this hypothesis further, it is necessary to document and quantify the extent of

movement taking place at the aneurysm neck. At first glance, this concept would seem relatively

simple. Filming an aneurysm and measuring the direction and extent of displacement could be

quite easily performed using simple linear measurements calculating displacement from sequential

images of a moving object.

However, this reasoning is somewhat flawed. To explain further, it is necessary to summarize some

of the basic principles of photogrammetry.

8.1. B a s i c p h o t o g r a m m e t r y

Photogrammetry is the science of remote measurement. Its value stems from the fact that for

practical reasons it may not be possible to study the subject direcdy. For example, the

reconstruction of an archaeological site from available historical evidence cannot be brought

indoors into any sort of laboratory. The alternative is to use a series of images that can be

mathematically analysed to construct an accurate model remote from the original site.

Consequentiy, any sort of image that can be scanned into a computer is suitable for analysis. Some

of the sources that are commonly used include Polaroid's, 35 mm prints, radiographs and selected

frames from videotape.

Photogrammetry can be divided into two main fields, namely aerial photogrammetry and close-

range or terrestrial photogrammetry.

In aerial experiments, a special camera mounted on an aircraft takes photographs. The information

is used for topographical reconstructions such as the production of accurate terrain maps.

In close-range photogrammetry, the type of camera used can vary from the simplest 35-mm

pictorial model to highly specialised configurations. The resulting images are converted to 3D

format and are often input to a Computer Aided Design (CAD) system for further processing or

visualisation.

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Today, a dominating part of photogrammetric research is focused on digital photogrammetry. The

appearance of powerful computers has opened new possibilities for automatic recognition and

description alongside the established measurement process.

Terminology for the single photograph

Although both maps and aerial photos present a "bird's-eye" view of the earth, aerial photographs

are not maps. Maps are orthogonal representations of the earth's surface, meaning that they are

directionally and geometrically accurate (within the limitations imposed by projecting a 3-

dimensional object onto 2 dimensions). Aerial photographs, on the other hand, display a high degree

of radial distortion. ITieir topography is distorted (Figure 47), and until corrections are made for this

distortion, measurements made from a photograph are not accurate. This concept also applies to the

use of single images in close-range photogrammetry.

Only light rays that pass directly through the centre of the lens without distortion (along the

principal axis) are a true representation of the external scale of the object (Figure 48). lliese light

rays are incident at a point called the principal point o f autocollimation on the photographic

frame. Around the rest of the image there will be distortion, the degree of which depends on the

radial distance from the principal point and the height of the object.

Î)«

Figure 47: T he perspective view. In this pho tograph , the edges o f the road, which are actually parallel lines in 3D space, appear to converge as they recede tow ards the horizon. T he trees also get smaller tow ards the horizon form ed by the "infinitely d istant points" o r

vanishm g directions o f the ground plane. In reality the edges o f the road should never m eet and the trees do n o t get physically smaller. T his simple example show s how a perspective

view greatly differs from the true situation.

The centre of the photograph does not usually coincide with the principal point. The photographic

centre can be easily found by determining the intersection of imaginary lines drawn from opposite

pairs of fiducial marks in the sides or comers of the image plane. These marks are usually etched

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onto the photographic plate and in an ideal camera their intersection is referred to as the fiducial

centre.

The principal point is decentered from the fiducial centre by an increment referred to as xi, yi. This

is the most important part of the photograph as it is taken as the origin for calculations of radial

distortion.

Radial distortion

Distortion of images at the edges of photographs is now not as easily seen with the naked eye due

to vast improvements in lens manufacturing technology. Nevertheless, even with modem cameras

there is still a deviation from the true position of the object externally. This distortion depends on

the radial distance from the principal point and the height of the object. The edges of a photograph

therefore will contain the greatest distortion relative to the principal point and objects that lie above

the horizontal plane of reference (the datum plane) will exhibit varying degrees of distortion

depending on their height (Figure 49a and 49b).

Tangential/Tilt displacement

A third type of image distortion, due to internal camera geometry, results from two factors:

• The misalignment of individual lens elements in the camera during construction.

• The position of the camera during imaging which is seldom truly vertical and causes

displacement in addition to the radial deviation already described.

Tüt displacement is radial from the isocenter, a point midway between the principal point and the

nadir point. The nadir point is the line that is direcdy perpendicular to the image plane. The

different types of tilt are annotated in each dimension using the Greek expressions phi, kappa and

omega. Each describes tilt about a different axis: phi is tilt about j/, omega is tilt about x and kappa is

tilt about These parameters are always considered in the calibration of the internal geometry of

any camera used in the photogrammetric process.

8 .2 . C a m e r a c a l i b r a t i o n

Camera calibration is the process of determining the internal camera geometric and optical

characteristics (intrinsic parameters) and the 3D position and orientation of the camera frame

relative to a certain world coordinate system (extrinsic parameters).

Camera calibration provides the analytical software with critical information such as the focal

length of the camera, the size of the image, and distortion parameters of the lens.

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Pixel Index IJImage Coordinate System A

XWo rid Coo rdl nate System

Figure 48: T he projection o f a 3D p o in t o n to the image plane for a perspective camera. T he light ray form ed from the object P passes

through the centre o f the lens system and is incident on the principal point. T he external object co-ordinates are represented by x, y, z for

the three spatial dim ensions respectively. T he correspondm g pixel grid values, u and v, reflect the same co-ordinates. It should be no ted

that dep th (z-axis) canno t be m easured on a single image.

Figure 49a: Radial distortion increases with distance from the principal point. The left image is a test grid used to illustrate the effects o f radial lens distortion. The image on the right shows

how the grid would appear after distortion. Since radial distortion is a function only of the radius o f the image, not the angle, circular patterns remain circular. Straight lines that do not pass through the centre of the image tend to bow. This example shows barrel distortion (the

distorted boxes look like barrels) and results from off-axis magnification which is weaker than at the centre. If the magnification were stronger at the edges, the lines would bow the other way

(so that a box would distort into a pin cushion).

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Calibration is performed by photographing a series of known targets using the camera on a test rig

built specifically for the purpose (Figure 59). The co-ordinates of each target in 3D space are

known precisely and are then used to determine the events that alter the bundle of light rays

passing through the camera lens. Once the behaviour of the light bundle is known, mathematical

transformations can be used to derive the proportions of the object under study.

A second type of calibration, known as self-calibration, is used after completion of the

experiments. The camera is calibrated using only the data obtained from the new observations and

these values are then compared to the original calibration. The differences or residual values

between the two sets of data then act as a type of internal quality control.

Methods of camera calibration

To determine the six exterior orientation parameters defining the spatial location and orientation of

a camera, a minimum of 50-100 targets of known 3D location are required. The mathematics used

are quite complex but are based on a least squares solution where the different types of distortion

(radial and decentering) are represented by matrix transformations of the original point (x, y, z) and

used to derive the image co-ordinates. The system of equations that is used for this calibration is

referred to as a %undle adjustmenV\

Self-calibration uses discrete target points on the image as data required for both object point

determination and for the determination of the parameters of camera calibration. All other points

have to be defined relative to these co-ordinates. At the conclusion of the imaging, a scale is

entered and these control points then act as the parameters for calibration.

In this series of experimental observations, image analysis and bundle adjustment was carried out

using software available on The Vision Management System version 7 (VMS written by Dr. S.

Robson and Dr. M. Shortis, Department of Photogrammetry, UCL).

8 . 3 . T h e s t e r e o s c o p i c m o d e l

A single photograph only gives a 2-dimensional representation of an object, and consequently any

measurements made from this image are limited to these 2 dimensions (x and y co-ordinates).

Nevertheless, it is still possible to gauge depth using visual clues. Such clues are based on the

relative sizes of objects, hidden objects, shadows and differences in focusing the eye required for

viewing objects at varying distances. This type of depth estimation, known as monoscopic, only

enables rough estimations of distances to objects. For example the perspective view in Figure 47

implies correcdy, that the house is at the front of the horizon while the road continues on into the

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distance. ITiis is because the trees get smaller as the distance increases and the edges of the road

begin to converge.

Accurate information on the third dimension (z) requires extra information. This is achieved by

stereoscopic viewing where a much greater degree of accuracy in depth perception can be attained.

A stereomodel is constructed using an overlapping pair of photographs (Figure 50). An explanation

of how this objective is achieved will be the subject of the rest of this chapter.

ppimage plane

Figure 49b: Relationship betw een object height and degree o f radial distortion. T he objects A and B are b o th located equidistant from the optical axis o f the lens. A

how ever, lies o n a lower level than B w ith respect to the datum o r reference plane. WTien viewed w ith the smgle lens, A is displaced inward to p o m t A” rather than its

true location A ’. In contrast, B is displaced outw ard from its true location (B*) to B” .

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M :

§I

M

Figure 50: The stereoscopic model. This pair o f pho tographs, show ing the topography o f a hilly terrain, have been extracted

from their p roper positions in the full pho tos and placed in juxtaposition at a separation that should achieve stereo rendition. T he slight differences in shape (and shadowmg) o f the same hills in the tw o images result from the changed viewing positions. In a

stereo-viewer, the full 3D effect is visible.

The human eye

ITie human eye functions in much the same way as a camera. A pair of eyes is ideally suited to

appreciate depth, a function that is dependent on the parallax principle. When focused on a distant

point, the optical axes of both eyes converge on that point to intersect at an angle known as the

parallactic angle. The nearer the object the greater the parallactic angle and vice versa. The ability to

detect the change in this parallactic angle and thus judge difference in depth, is a subtle property that

occurs completely subconsciously. Indeed the average person is capable of discriminating parallactic

angle changes as small as 3 seconds of arc.

Similarly, by having two pictures of the same object, taken from two different positions, the "2 " co­

ordinate to the object can be calculated by using a mathematical representation of this parallax

relationship. Mathematical representation of this arc is the central concept in stereo modelling.

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The importance of parallax

Parallax is the apparent displacement in the position of an object, with respect to a frame of

reference, caused by a shift in the point of observation. Parallax wiU exist for images’ appearing on

successive overlapping photographs (Figure 50).

Two important points to note are that the parallax of any point is related to the elevation of that

point and secondly, parallax is greater for high points than for low points. This variation in parallax

with height provides the basis for determining depth in photographic images.

Essentially, the production of a stereo-pair image sequence tries to mimic the effect of a pair of

eyes. This can be achieved by the following:

1. Using a special stereo camera: such cameras were originally based on fihn-pktes and now are

quite difficult to obtain.

2. Using a normal camera with a stereo attachment: in the absence of specialised stereo cameras it

is possible to use a beam splitting attachment that can be fitted to a conventional camera.

These devices usually consist of a series of mirrors that can reproduce multiple images on a

single piece of film.

3. Using a normal camera to take successive pictures of a single object: a single camera can be

used to image an object firom fixed angles around its periphery. This is the method used in

aerial photogrammetry.

4. Using a pair of cameras to photograph simultaneously: this is the method that has been used in

most of the experiments discussed here. A pair of cameras is mounted on to a support, which

is in turn, attached to a tripod. The drive mechanisms of both cameras need to be synchronised

to obtain exposures at the same time especially if a moving object is being imaged. This has the

disadvantage that capture is only as fast as the shutter speed. However, newer forms of imaging

with digital cameras can image up to 30 frames/second.

Stereoscopic viewing

The sine qua non of stereo-viewing is that each eye should see only its own image. Trained

individuals can obtain this effect from a stereo-pair without any viewing aid. Some strain is

inevitable since the eyes have to be focused on separate images without convergence; an effort

must also be made by each eye to concentrate only on its own image.

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When the stereo pair is properly positioned left-right and then viewed through a stereoscope, the

eye-brain reaction is an impression of surface curvature or relief as though looking down ftom an

aeroplane at the ground.

Most commonly stereo-pairs are analysed in a purpose-built device known as a stereo- viewer. The

procedure requires that the stereo-pair are placed side-by-side at the principal focus of the viewing

lenses. The emergent beam is then parallel and the eye accepts it as originating ftom an infinitely

distant subject, allowing a relaxation of focusing and improved viewing comfort. The simplest type

of viewer is the pocket stereo-viewer. This consists of two lenses that can be expanded or

contracted along a slide bar to be as far apart as the distance between the observers eyes, placed in

a mount that is raised (on collapsible legs) approximately 6 inches above the central region of the

stereo pair. The stereo-viewer is able to measure the height of objects directly as well as differences

in height between two or more objects (relief).

Consider for example the determination of the height h of a tall building that lies near the edge of a

single aerial photograph (Figure 51a). If that photograph were taken as truly vertical, ie looking

straight down, then its center would lie at the nadir (the vertical line ftom camera perpendicular to

a surface point directly beneath). In this instance, the nadir would also coincide with the principal

point (pp) in the photograph, (the intersection of the lens' optical axis and the ground), so that the

pp would also be the true center of the photo. This point becomes off-center if the photograph

(and optical axis) is non-vertical.

If the building were to lie close to the pp, its top and bottom would appear to coincide in the

photo. If it were well away from pp at some radial distance r towards the edge, it would appear as

slanted with the top displaced further away ftom the pp than the bottom. This amount is

measurable in the photo as d. For an aircraft height H (calculable ftom the scale if a ground

distance is known), the value of h is:

h = (d /r )x (H ).

In Figure 51a, the lateral tilt of the tall buildings at the edges is obvious; d and r have been drawn

between the pp and one of the buildings near the edge:

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«

I I I !

Figure 51a: D em onstration o f the calculation o f absolute height o f a building w ith photogram m etry. T he principal point, distance to base o f

building (r) and height o f buildm g (d) are all marked. O nce the height o f the plane is know n, the calculation o f absolute height can be perform ed

as detailed in the text.

Height differences (Figure 51b) between points of different elevation along surfaces can be

calculated from stereo pairs using the formula, h = (H) dP/(P + dP).

P is the absolute stereo parallax. For any standing object or separated points on a slope whose

height or relief is to be determined, the distance (in the direction parallel to the flight line) on one

photo between the base (here: point b) of that feature and the principal point (PPl) or nadir is

measured. The same is then done for that conjugate feature as it appears in some different position

in the other photograph. Note that the principal point of the first image PPl now lies off-center (at

PPl") in the second photograph whose equivalent is PP2. The average of the two distances (d +

d j/2) is the absolute stereo parallax.

dP is the differential parallax. It is determined by measuring the distance between the base and the

top of the feature or between two proximate terrain points at different elevations on a slope as

located in each of the two photos when the pair is in optimum alignment for stereo viewing. dP is

then the numerical difference between the two distance values (x - x j which will be different in

each of the photo pair.

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The value of dP is established optically through use of special devices built into the stereo-viewer,

commonly referred to as a parallax bar. Each uses a variant of the "floating point" method in which

reference marks (dot or cross) are visible when the aligned photos are viewed under a mirror

stereoscope.

The parallax bar is placed on the image plane so that a fixed mark over one photograph is made to

coincide with a movable mark on the other by turning a screw drive. The value of dP is then read

from a graduated scale. When this is done for a number of points, heights and relief values for

parts of the map can be calculated; these become absolute values (elevations) if points are

referenced to benchmarks.

PPlPPi

Flight Line

F ig u re 51b: Calculation o f relief o r difference in height using a stereo-viewer. T h e object is labelled as a-b in b o th photographs w here b is the base and a is the

top o f the object. A calculation o f the absolute and differential parallax values (see text) are required in o rder to determ ine the height o f the object.

Use of the manual stereoplotter to specify a large enough number of elevation points in stereo

photos to permit contouring is difficult and tedious. This process can be automated using digital

photography and newer optical-mechanical stereoplotters. These are capable of semi-automating

the contouring process through computer processing of mathematically transformed data.

Both types of stereo-plotter were used in this study. The Kem analytical stereo-plotter was used for

the manual analysis of image pairs obtained firom filming an aneurysm neck. These pairs were

further analysed by scanning into digital format and transfer to a digital workstation (Figure 60).

The digital workstation allows contouring or formation of terrain maps used to study natural land

shifts due to erosion or earthquakes.

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CHAPTER 9

EXPERIM ENTS II

DYNAMIC ARTERIAL IMAGING

9.2: Laboratory validation of a digital photogrammetric technique used to measure arterial dimensions.

9.1: Measurement of aneurysm neck and arterial motion during open surgical repair.

9.3: Measurement of arterial motion during open surgery using digital photography; a feasibility study

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9.1

Laboratory validation of a digital photogrammetric technique used to measure arterial dimensions

A i m

The aim of this experiment was, using digital photogrammetry, to describe a complex 3D shape

such as a glass aneurysm solely in terms of its volume.

M e t h o d :

A single glass aneurysm, forming a perfect sphere of measured diameter 79.8 mm and volume

358.49 ml, was imaged around its circumference using a digital camera. The steps involved in

imaging and subsequent photgrammetric analysis were as follows:

1. Image Acquisition

Using a Kodak Megaplus ES 1.6i digital camera, 42 images were acquired of glass phantom F

around its circumference and two stems. Image resolution was 1536 x 1024 pixels. The phantom

was imaged on a background grid marked with points that had known co-ordinates in 3D space. 8

of these were used as the initial control points. The phantom itself was covered with 13 rows of

403 retroreflective targets to delineate its shape. The co-ordinates of these targets were unknown.

2. Establishing ground control points

Reference points needed to calculate the location and orientation of the network of photographs in

3D space were established in the project area prior to photography. They consisted of clearly

labelled self-adhesive retroreflective targets that could easily be identified in the photographs.

A minimum of 4 (range 2 to 50) control points are usually required in every group of photographs,

although the control points need not be visible in each and every photograph. The first set of

images used 8 points on a special background grid as initial controls to establish the location of

points on the glass phantom. Once these locations on the phantom were known {with reference to the

initial controls points), they were then used to replace the existing control points which were deleted

from the file. The x, y and z co-ordinates of each target in the reference co-ordinate system was

calculated within the software (VMS) by simply measuring the distances between the various

control points and establishing an arbitrary, but correctly scaled co-ordinate system. This procedure

is called cantilevering.

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To permit 3D co-ordination each target measured from the photography had to be visible in two

or more images taken from different perspectives. Hence, each photograph taken had to contain a

fair amount of overlap with some of the other images. This overlap was used in the analytical

phase. This set of linked photographs is called a network. Networks consist of a group of images

that cover a certain area such as a single piece of equipment (Figure 52). In general, network sizes

range from 2 to 50 images. This grouping greatly reduces control requirements, and increases the

accuracy of the measurements.

3. Image Processing

A digital camera (Kodak Megaplus ES 1.6i) was used to streamline the image processing. Further

digital scanning or réseau measurement as is the case with traditional photogrammetry techniques

(Chapter 8) could hence be avoided. Images could be measured as soon as they had been acquired.

Camera Calibration

Camera calibration was accomplished prior to this experiment by using a number of test

photographs of a specially designed calibration field (Figure 59). The data obtained from this

calibration are shown in Table 33.

Camera calibration parameters Initial values from test grid (mm)

Focal len 15.68Principal point Px 0.09

offsets Py -0.03Radial lens K1 -1.55x10-3distortion K2 4.83 X 10-6

parameters K3 -1.86x10-"Tangential lens PI -0.002

distortion parameters P2 0.07

T a b le 33: D ata obtained from calibration o f K odak ES 1.6i digital cam era using a specially designed

test field (Figure 59). T h e term s relating to cam era calibration are explained in C hapter 8.

Data obtained from self-calibration are shown in Table 34. A comparison of Tables 33 and 34

shows similar calibration parameters for both experiments. Only the focal length differs by a

negligible amount.

Image location and orientation at the time of exposure were then calculated as discussed below.

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Image Point Measurement

The purpose of image point measurement was to match adjacent images in the network. This was

performed within the software, by manually identifying the centre of the same target point in each

image. These points, represented by the retroreflective targets, were then given a unique number.

In this network of 42 images, 403 targets were labelled 2978 times in total.

It was possible to measure the centre of circular targets to sub-pixel accuracy using the target mode

that automatically located the centroid. This procedure eliminated the errors associated with

manual selection.

Relative Orientation

Once tie points between images had been identified (at least 4 common points), the relative

position and orientation of two connected images was calculated.

The location and orientation of this base image was used to establish an arbitrary network co­

ordinate system. The orientations of all the other images connected to this base image were then

calculated in a common network co-ordinate system. This was continued until a complete 3-

dimensional image of the object was obtained, delineated by the retroreflective targets (Figure 53).

Camera calibration parameters Values from image points (mm)

Focal leneth: 15.78Principal point Px 0.09

offsets Py -0.03Radial lens K1 -1.55x10-3distortion K2 4.83 X 10-6

parameters K3 -1.86x10-"Tangential lens PI -0.002

distortion parameters P2 0.07

T a b le 34: VMS cam era self-calibration data. These data w ere derived by self-calibration o f the K odak

ES 1.6i digital cam era during this experim ent. T he values should be com pared w ith the data obtained

from the test grid (Table 33).

Bundle Adjustment

After a network has been formed, it must be refined for more accurate results. This is

accomplished by adjusting camera positions, orientations and the locations of ground control

points in such a manner as to uniformly distribute errors that exist in the network, providing an

overall minimum error solution at the expense of localised increases in error. The step does not add

any information to the photogrammetric solution, rather it modifies and refines an already existing

164

solution. It is very important to note that the bundle adjustment method does not reduce or

eliminate error but only redistributes it in a more statistically rigorous way.

The bundle adjustment technique employed here was a weighted, simultaneous, least-squares

solution to the photogrammetric equations relating image co-ordinates and control measurements.

Thus the solution transformed measurements into a mathematical description of the

photogrammetric network. Since there were many more measurements than required and each

measurement contained some measurement error, the software computed a “best fit” solution. The

inconsistency or residuals generated were the difference between the measured value and the value

predicted by the solution.

4. Measurement and 3D Modelling

Using the measurements determined firom the image network, A 3D solid model of the original

glass phantom was created using two further analytical software programmes. These packages

applied a solid surface over the original image (Figure 53), thus allowing estimation of volume and

maximal diameter of the original glass aneurysm. Both the software packages used. Microstation

and Surfer, are recognized for their universal photogrammetric applicability.

Microstation

Data regarding image point co-ordinates was transferred intact to a Computer Aided Design

package called Microstation. Here the aneurysm image was edited in two stages before

measurements were made.

In the first stage, a profile was defined along one-half of the phantom (Figure 54). This was then

rotated by 360° around a labelled axis of rotation that had been placed through the geometric

centre of the glass aneurysm. Rotation produced a series of different sized circles that were used to

generate the 3D-aneurysm shape. The volume of this shape (Figure 55) was then estimated by

integration.

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a ZTDf

a

Figure 52: An example o f a single netw ork consisting o f 8 images that cover the p h an to m aneurysm. In this experim ent, a smgle cam era was used to pho tog raph all the d ifferent perspectives o f the glass aneurysm shown.

Figure 53: 3D m odel o f glass p h an to m aneurysm F im aged using a K odak m egaplus ES 1.6i digital camera. T he green do ts are a representation o f the centre o f individual

retroreflective targets (the centroid). N ote tha t the image is m ade up only o f the dots. T he "gaps" betw een the dots do n o t contain any solid substance, as w ould be the case for the

actual object.

166

Figure 54: Image rendering using M icrostation software. T he red line is a profile w hich has been defined from the original green image co-ordinates dem onstrated in Figure 53 (and just

visible here). T his profile is then ro tated abou t the green central axis for 360 degrees to generate a series o f circles (white). I f the area o f every circle is calculated for the w hole shape, this gives the volume. N o te that, fo r illustrative purposes, only a skeleton o f the w hole shape

is p resented here.

Figure 55: Surface shaded display o f glass aneurysm F obtained from M icrostation using the image co-ordinates in Figure 53. As show n in Figure 54, a single profile is first generated to

form a volum e o f revolution about 360 degrees. A n artificial surface can be added to the com pleted volum e to outline the shape. T his shape should be com pared w ith the actual object

illustrated in Figures 16 and 24.

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Surfer volume calculation

Surfer uses a rninirnurn curvature algorithm to outline a shape. The volume contained within this

boundary is then summed.

To start, the computed object co-ordinates were transformed in 3D space so that the object was

completely aligned along the î -axis. The object was then divided into two equal halves. Minimal

curves were generated as a series of contours to form the whole shape (Figure 57). The volume of

each half shape was calculated to obtain the whole. Volume estimation within Surfer was obtained

with the two mathematical estimations of the trapezoidal rule and Simpson's rule.

The traper^idal rule calculates volume by fitting suitably chosen polygons with an increasing number

of sides to approximate the aneurysm shape. The area of each of these shapes is calculated and

then summed for the total volume.

Simpson's rule uses a suitably chosen parabolic shape to repeat the same process. In cases where

approximations to a circumference occur this may be a more accurate technique.

The difference between the three volumes obtained with the two different computer programmes

was used to obtain a relative error estimate of the accuracy of the volume measure, using the

following formula:

RE = (LR-SR) X100/average volume using the two programmes

RE is the relative error, LR was the largest result of the 3 volume estimations, and SR was the

smallest result.

R e s u l t s

Volumes of aneurysm F obtained by using the 2 methods are shown in Table 35. The actual

volume of F, calculated using mass and density values (Table 8) was 358.49 ml.

Microstationvolume 364 ml

SurferTrapezoidal rule: 368.4 ml Simpson's rule: 368.4 ml

T a b le 35: Volume estimation of aneurysm F. M icfostation and Surfer volum etric softw are was applied to reconstructed 3D im ages o f

aneurysm F. T he estim ation obtained from M icrostation is based on an integration m ethod, w hereas Surfer fills objects w ith shapes o f know n

area w hose sum approxim ates the to tal volum e.

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Using the above equation, the calculation of the relative error of these volume estimates was 1.20

ml.

Both software programmes estimated the same maximal diameters for this phantom aneurysm

namely 79.9 mm and 80.1 mm for AP and transverse diameters respectively. The laboratory-

measured values were 79.74+0.03 and 79.80+0.03 respectively.

C o n c l u s i o n

Estimates of volume using accepted photogrammetric techniques and software vary by 6-10 ml

from the laboratory values in this experiment. Those for diameter are much less. The differences

are approximately 1 mm. The errors are largely due to the remote nature of the estimation. The

object under scrutiny is first covered with retroreflective targets that are then imaged before being

mathematically altered (bundle adjustment) to form the aneurysm shape. This shape is then halved.

It is rotated around an imaginary axis to form a symmetrical object. The volume of the phantom is

then calculated using simple mathematical principles. Corruption of data is possible at all these 6

stages. Two major factors, which may produce error, include the image point density over the

object surface and the accuracy of the mathematical techniques involved in bundle adjustment and

volume estimation from the resulting networks.

1. Image point density

The 3D image produced at the end of the experiment is entirely dependent upon the absolute

number of targets that can be applied onto the original object. The presence of too many targets

leads to a failure of definition: the software is unable to recognise a single centroid when two or

more targets are placed within a critical distance of one another.

If too few targets are used, natural variations of the object surface that may affect volume, wiU be

missed (Figure 56).

F ig u re 56: E ffec t o f im age po in t density on volum e and o th e r geom etric calculations. T he m agnified surface o f a hypothetical object show ing th ree target points. T he density is such tha t the topographical inconsistency o f

the object will n o t be com pletely represen ted in the final image. T h e latter will simply be a com posite o f the three target points, in troducing an erro r

to all volum e and linear calculations.

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2. Mathematical estimates of volume

The bundle adjustment equation tends to redistribute the errors associated with each image point

measurement across the whole network in a more statistically acceptable fashion. In some cases

this may corrupt the original object shape by further removing the data obtained from the actual

object parameters. The remoteness can be judged by comparing the residual values obtained. These

were aU less than 0.1 mm suggesting only slight if any deviation from the original image point

measurements.

Both software packages estimated volume by adding artificial surfaces within the object under

study. In this case, the object used was symmetrical and a good volume of revolution estimate was

obtained from Microstation. This would have been subject to a greater error if a non-symmetrical

shape were used.

Surfer uses a different technique for volumes involving a basic shape-fitting exercise. In this

respect, although the volume estimate is slightly less accurate than for Microstation, this error may

be reduced when a non-uniform object, that cannot be successfully rotated around an axis, is

studied.

It is of interest to note that small errors of measurement distributed across the whole object surface

can become magnified to produce a much greater volume as opposed to diameter error. The

diameter is measured at a single point whereas volume is really a composite measure of all the given

diameters forming a spherical object.

170

450.00

400.00

350.00

300.00

250.00

200.00

0.00 50.00

450.00

400.00

350.00

300.00

250.00

200.00

0.00 50.00

F igu re 57: Minimal curves generated by Surfer to form the 2 halves o f aneurysm F. T he scale num bers are in m m and contour heights (in thicker font) are labelled for reference. The volum e o f each half was added to obtain the total

volume o f the aneurysm.

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9.2

Measurement of aneurysm neck and arterial motion during open surgicalrepair

A i m

The aim of this expenment was to define and quantify the types of motion exhibited by the neck of

an AAA and other smaller arteries during open vascular surgery.

B a c k g r o u n d

Endovascular treatment of aneurysms essentially results in the deployment of a rigid endograft

within a moving biological system. The extent and type of motion that occurs within this system

has never been quantified particularly with reference to the neck of the aneurysm, which is the

landing zone for graft fixation. Continuous differential motion, between the aneurysm neck and the

endograft has important implications for the graft-vessel seal. Loss of this seal may lead to

endoleak with catastrophic consequences.

Previous pilot experiments had identified that three types of motion may be occurring at the

aneurysm neck (Figure 58). These were not quantified due to an inability to gauge depth and the

inaccuracies inherent in direct measurements firom video-film. To overcome these difficulties,

calibrated cameras and 3-dimensional film stereo-pair modelling were used in this series of

experiments. The techniques were first tried on smaller arteries during vascular procedures

followed by actual filming of aneurysms during conventional repair. Ethical permission for these

studies was obtained from the UCL/UCLH Joint Ethics Committee.

P a t i e n t s a n d m e t h o d

Informed consent for filming was obtained from 9 patients scheduled for major vascular

procedures. The first group of 3 patients was used in a pilot study to confirm the presence and

nature of aneurysm neck motion. This experimental work was carried out with a co-worker in the

laboratory. During these patients’ operations with respiratory effort halted, aneurysm necks were

filmed using a high-speed video camera (NAC high speed HSV 400, NAC inc. Japan). Two

standard arterial clips (Ethicon proximate) were placed at two points on the anterior surface of the

aneurysm. These clips served as markers for measurement of longitudinal displacement and twist.

Results were analysed using the graphics tablet supplied with the video camera. We were unable to

determine a scale for the tablet and displacement was hence measured in arbitrary units.

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In the second group of patients, more sophisticated technology was used to estimate these

movements. There were 3 patients undergoing open aneurysm repair, 2 patients undergoing a

femoro-popliteal vein bypass and one patient undergoing carotid endarterectomy.

All patients had a general anaesthetic and respiration was suspended for the short interval over

which filming occurred.

Arteries were surgically exposed with care taken not to traumatise the vessels by direct handling,

application of vascular clamps or the passage of surgical sloops before photography had taken

place. During the course of this study, it was found that to enable recognition of identical points

during data analysis, the surface of each vessel filmed had to be gently marked with a regular array

of small dots made with a sterile spirit-based pen (vide injra).

All photography was performed with a pair of RoUei 6006 single reflex lens cameras containing

120-mm roll film. Cameras were electronically synchronized so that the triggering of one led to the

simultaneous firing of the other. The cameras were mounted on a steel bar, 1 metre apart and

angled inwards at 45 degrees. This bar was in turn fixed to a wheeled tripod that could be easily

manoeuvred over the patient when required.

2 types of film were used to test the quality of images obtained for measurement. Ilford PAN F

(Panchromatic) black and white film was used for half the exposures while Kodak Ektachrome was

used for the rest.

A strobe light was mounted in the centre of the bar between the two cameras and its flash

sequence was varied from 1 per second to 4 per second. The cameras had a 1-second exposure

time so each film had either 1 or 4 flashes per print. The idea was to capture movement as a series

of overlapping stills on the same photograph. However, it soon became apparent that the films

used were not sensitive enough to detect these subtle displacements. Instead, a series of image pairs

were obtained at recurrent 1-second intervals in time. These were analysed as described below:

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Figure 58: Graphs showing the three types of motion identified at the neck of an aneurysm using high-speed

video filming.

Longitudinal motion

9.85 -

9.75 -

I 9.7 -

Q 9.55 -

9.45 -

9.48 8 8 8

Time (ms)

Transverse m otion

3.75 T

3.7 --

1Î

3.55

3.5

8 i i 8 i i i i8 8T im e (m s)

174

Torque

33 T

32 --

1«

30 --

^ 28 --5

26 --

ii Îi i 8 ni8 isT im e (m s)

T hese experim ents w ere perfo rm ed w ith 3 patients undergo ing open aneurysm surgery. A fter filming, the d isplacem ent o f tw o

m arked points from one ano ther in differing directions was recorded using a graphics tablet. T here are 3 d ifferen t cyclical

m otions seen on the graphs. T orque was m easured by calculating the arctan o f the angle betw een the u pper and low er

m ark e rs / clips.

M e t h o d o f i m a g e a n a l y s i s

1. Camera calibration

The internal geometry of both cameras was elucidated in a separate experiment using a wall with a

non-uniform surface and targets placed at known ordinates in the x , j and planes (Figure 59).

Exactly the same photographic protocol was used with the cameras mounted on a rigid support

and tripod.

The cameras were randomly tilted in the omega, phi and kappa angular directions and multiple

exposures were taken. The data from these pictures was used to model camera geometry using

VMS bundle adjustment software. This provided precise information about the principal point for

each camera as well as the amount of radial distortion expected on each photograph of a stereo-

pair.

2. Ground control point matching for stereo-model reconstruction

The paired images of arteries and the aneurysm neck were arranged in the analytical stereoplotter

(DSR, Kem, Switzerland) for initial viewing and marking. To facilitate stereo matching, 13 ground

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control points were marked on each photograph over common stationary points on both images.

The true co-ordinates of these points in 3D space were calculated using the calibration parameters

obtained for each camera from VMS as described above. Using the calibration parameters and

working backwards, it was possible to assign the true 3D co-ordinates to these 13 points.

3. Scanning of images and analysis using digital photogrammetric workstation

For digital terrain modelling (DTM), image pairs were first scanned into digital format using a Leica

scanner (Leica Technology B.V. Rijswijk Netherlands) to a resolution of 512 x 512 pixels.

This data was then analysed using the Phodis photogrammetric workstation (Zeiss, Germany), a

unique application for a system normally used in DTM from aerial photographs (Figure 60).

Post-scanning, the 13 ground control points were re-identified manually and their co-ordinates

were re-assigned. The residuals between the 2 sets of values (manually marked ground points

versus the points marked in digital format) were less than 1 mm in aU 13 cases. This represented

only a very minor loss of information judged as acceptable for further analysis.

To further improve the quality of the 3D digital models, 12 extra points were then placed over

other corresponding features in both images. The software used these tie points to match images

one stage further.

Final adjustments to improve the stereo matching were achieved automatically within the software

using a pixel intensity recognition sequence. Briefly, individual pixels from the pen marks on each

photo pair (Figure 61) were identified and matched with their pair with further continuous

mapping until the full image had been captured in 3D.

Two series of DTM were built over the 3D-aneurysm shape using a wire grid with intersections at

every 1 mm and 0.5 mm respectively. Each intersection had known x , j and ^ co-ordinate values.

One problem became apparent in the change in depth from the edge of the aneurysm to the

laparotomy wound. This was too subtie for the software to recognise and the area had to be

manually marked on screen with a break-line. This loss of recognition was most likely due to the

original function of the workstation software as a geographical mapping tool usually dealing with

large variations in depth (such as would occur between a mountain peak and its corresponding

valley).

The whole of the above procedure was then repeated for two more pairs of images. These depicted

the filmed arteries at 1 second and then 2 seconds later in time.

The software was able to subtract these images from one-another (Figure 62) expressing only the

differences between image pairs. In the case of the AAA neck, this difference was taken to imply

176

that some movement had occurred during that time. The procedure was repeated for each of the 3

pairs of DTM using the 1 mm and the 0.5 mm data respectively.

R e s u l t s

Only one set of results from all the arterial filming proved suitable for analysis. This was due to a

number of reasons:

For the initial filming, it proved impossible to match the 13 ground points between image pairs

using simple landmarks such as retractor tips or blood spots as these were found to move very

slightly between the time intervals used. It was thought that this might have been due to respiratory

motion from the patient and/or operating assistants.

In an earlier case with the carotid artery, the workstation was unable to recognise any change in

surface texture (and hence pixel intensity) for the image pairs. The vessels were too smooth and a

DTM could not be produced. To avoid this problem, small dots were made on the arterial surfaces

during surgery, using mdehble ink (Figure 61) as described above. Retro-reflective targets such as

those used in Chapter 9.2 could not be sterilized without destruction and so were not utilised.

The indelible dots were recognised as a surface texture by the software and could be used to

construct a DTM from the 3D-aneurysm model. The DTM obtained from the aneurysms in Figure

61 is shown in Figure 62. This map was constructed using the software available in the Phodis

workstation. A series of such maps were produced for the aneurysm neck at each 1-second interval

of filming. After subtraction there did not appear to be much qualitative difference between the

figures confirmed by mathematical analysis of the DTM in Figure 62b. There is hardly any change

visible. There may have been a possibility of approximately 1 mm displacement in the area of the

neck near its lateral boundaries (Figure 62c). However, this small degree of change could equally

represent artefact due to movement of the surgeon’s hands, retractor or "noise" within the

software. This could also have been introduced when adding the manual boundary between

aneurysm neck and laparotomy incision.

When the resolution was increased using 0.5 mm grid DTM, exactly similar results were obtained.

C o n c l u s i o n

Based on the above results, it has not been possible to confirm the types and calculate degree of

movement affecting human arteries or occurring at the aneurysm neck. The problem may lie with

the type of image capture used. It is possible that the single cameras have been unable to capture

motion simply because they are not fast enough. Equally, they may have fired during intervals

when the neck motion had returned to its baseline. This sampling error could be corrected by

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taking larger numbers of photographs but the analysis stage, which was already very time

consuming (1-2 weeks), would be considerably lengthened.

The experiment has been useful in the sense that it has illustrated the feasibility of using stereo

filming in theatre and subsequently analysing results using photogrammetric software. A further

advancement on the present technique would be to improve the speed of image capture while

preserving the resolution of the images obtained.

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F ig u re 59: T he calibration wall used in this experim ent. The position and distances betw een each o f the targets in 3D space

was precisely known.

PHO D IS

Figure 60: Digital photogrammetric workstation. These workstations are capable o f faster and more accurate image analysis than the manual stereo-viewers. Because, digital workstations read in pixel format, photographs require an intermediate scanning stage where some

information may be lost. In this case the ground point residuals were less than 1 mm suggesting only a very minor loss o f resolution.

179

r

Figure 61a: A stereo-pair o f pho tographs show ing the neck o f an abdom inal aortic aneurysm at open surgical repair. T he black dots were m ade using a sterile marker. T he differences in position betw een these dots were used in quantitative analysis o f

m otion at this site.

Figure 61b: Pixel intensity m atching to im prove the quality o f stereo modelling. T he opera to r has to select m atching pen marks

o r o th e r distinguishing surface features over b o th stereo-pairs. T he softw are then m atches these a t the pixel and sub-pixel levels.

180

M O . O O - p ^

8 M . I

• 40.00-

3840 00 3860.00 3880.00 4000.00 4020.00 4040.00 4060.00 4060 00

Figure 62a: C onstruction o f a digital terrain m ap. F o r every pho tog raph o f the aneurysm neck (right), a m ap o f the image po in ts is constructed (top left). This allows subtraction o f image points o r terrain m aps (bo ttom left) taken over d ifferent time intervals (in this case there was a 1-second interval betw een image pairs). VCTien this is done, the differences represent the degree o f m otion

over tha t surface.

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F ig u re 62b: Digital terrain map showing the differences after subtraction o f two 3D-aneurysm terrain models. There is not

much motion apparent within the area that would be occupied by the aneurysm neck.

Figure 62c: Pictorial representation o f the DTM in Figure 62b. The green lines on the photograph show degrees of motion detected over the aneurysm neck. These cannot be isolated from similar degrees o f motion at the edges of the laparotomy wound and immediately adjacent to the

aneurysm neck.

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9.3

Measurement of arterial motion during open surgery using digital hotography: a feasibility study_______________

A i m

The aim of this expemnent was, using digital photography, to define and quantify the types of

motion exhibited by a femoral artery.

I n t r o d u c t i o n

Conventional still photography is not able to capture arterial or indeed aneurysm motion with any

degree of accuracy. The problems have been outlined in previous experiments. The advantages of

digital capture are firstly, that the camera shutter speed can be increased with the possibility of real­

time imaging and secondly, elimination of the stage of digital scanning necessary with conventional

photographic film.

This experiment was designed to evaluate the feasibility of arterial motion capture using a pair of

Kodak Megaplus ES 1.6i high-resolution, high-speed digital cameras. The capture rate was up to 30

images per second with a spatial resolution of over one million pixels. Each pixel measured 9-

microns square with a 60% fill factor. The digital images obtained had up to 1024 gray levels per

pixel providing an excellent contrast detail. The built-in electronic shutter produced exposure times

as short as 127 microseconds, ideal for imaging moving blood vessels.

M a t e r i a l s a n d m e t h o d

Two patients, undergoing endovascular repair of their aneurysms, had both their femoral arteries

exposed with standard surgical technique. This dissection was limited to simple exposure and the

vessels were not traumatised any further with the passage of control sloops or endovascular wices

until the conclusion of filming.

A series of small dots were made over the arteries and skin edges in regular arrays to act as

recognition points for the software during manual measurement (Figure 63). The method used for

filming motion was exactly as described for experiment 9.2 with one exception: instead of the

conventional RoUei cameras, a pair of Kodak Megaplus ES 1.6i digital cameras was used.

These cameras were mounted onto the same iron bar, which was then fixed onto the tripod as

described. A photographic circular fluorescent light was placed around the lens of each camera,

which were simultaneously fired using an electronic triggering device. The tripod with cameras was

183

moved over the patients’ groin wound for filming of exposed femoral arteries at the appropriate

point.

A sequence of 40 photographs was taken at a rate of 15 frames per second. The image data, stored

in the camera memory, were downloaded onto computer for later analysis using the Vision

Management System (VMS) version 7 software.

Using the VMS software, four control points were chosen using the pen marks on the skin edges

together with 5 more points over the arteries themselves. These were manually identified and

marked on all 40 pairs of photographs, A 3-dimensional vector map was then constructed to

demonstrate the location of each of these points at every 1/15* of a second. From these maps, the

direction and magnitude of movement was calculated.

R e s u l t s

From the complete animation sequence of 2,7 seconds, 2 cycles of pulsation could be detected in

all 4 films. The skin edges appeared stationary. Subtracting the image pairs from one another and

constructing a simple vector diagram from the results was used to confirm this motion:

Figure 63 below, demonstrates the presence of arterial motion in 2-dimensions, The digital images

of the femoral artery taken 1 second apart have been subtracted to show any differences. The

subtraction photo demonstrates motion-related differences between the two images as a white

mottling. Most of this motion has taken place in the lower part of the femoral artery. The vector

diagram (Figure 64) shows the motion is taking place mainly over the center of the vessel and not

the skin edges where the control points were placed.

After image point identification and measurement (Figure 65a), a 3D-displacement diagram was

constructed as shown in Figure 65b, For all the arteries filmed in this experiment, the control

points at the wound edges were found to vary in an inconsistent fashion despite re-measurement

on three occasions. This measurement noise was not related to arterial movement but more likely

the control points. At least two points of difficulty were encountered during analysis:

• The camera focal length was not adequate for closer filming of the femoral arteries. Because of

this limitation, the cameras could not be brought closer to the wound hence allowing better

definition of each artery,

• The pen-marks used were too large occupying 5 to 6 pixel spaces. This made it difficult to

identify the same pixel point on each successive frame of measurement.

The displacement diagram (Figure 65b) shows that the control point measurement noise is of

similar magnitude to the presumed arterial motion. This artefact prevents accurate determination of

the magnitude and direction of motion.

184

C o n c l u s i o n

The experiments conducted are useful in the sense that the presence of arterial motion was

captured over the filming interval of 2.7 seconds. Although qualitative evidence of motion was

present in the animated sequences and in the simple subtraction diagrams (Figures 63 and 64), the

measurement noise was too great for quantitative analysis of motion over the femoral arteries.

185

Figure 63: This sequence of 2 photographs was taken 1 second apart. The difference between the two images is shown in the third frame.

Areas of black represent no motion while the white areas show movement of the vessel wall. This is confirmed on the displacement

diagram below.

Figure 64: This displacement vector map has been constructed from the subtracted images in Figure 63. It is made up of many small arrowheads. The

head of the arrow indicates the direction of motion. The diagram shows motion occurring only over the site of the femoral artery located in the lower tight comer in this patient. In contrast, there is no motion over the skin edges.

186

F ig u re 65a: P hotographs from VMS show ing the m anual m arking o f image points. C orresponding surface pom ts have to be

m arked with the same co-ordinates on each pair. T his is done at the pixel level using a m agnified view.

# f ■ f

F ig u re 65b: D iagram show ing the degree o f m otion estim ated for each image pom t calculated using VMS software. T here is a similar range and direction o f m otion for central po in ts located over the

arterial surface com pared to the tie-points at image corners. T he latter are supposed to have little if any m ovem ent.

187

C h a p t e r 10

Discussion

Considerable progress has been made into the endovascular method of repairing aortic aneurysms,

avoiding the need for a major open abdominal operation. The procedure involves passing a graft

into the aneurysm through the femoral artery in the groin. The graft is then fixed in place to normal

calibre aorta, above and below the aneurysm, by expandable metal stents.

Endovascular repair is heavily dependent on reliable and accurate imaging in the preoperative,

perioperative and postoperative periods.

Preoperative imaging is necessary to ensure that the endograft is accurately constructed to fit the

diameter and length of the aneurysmal aorta. Qualitative information about the arterial “landing

zones,” or site of endograft-vessel wall seal is also important to exclude heavy calcification or lining

thrombus. Finally, the tortuosity and diameters of the access vessels must be assessed to make

certain the introducer sytem can be passed up into the aneurysm sac and neck.

Postoperative imaging is required to assess complications, the efficacy of treatment and durability

of the endograft. It is recognised that the efficacy of therapy and the durability of the endograft are

inextricably linked to the morphology of the aneurysm and its sac after endovascular repair. This

may in turn be affected both by the endograft itself as well as the continuing pathology of

aneurysm formation.

There is currently, no universally accepted imaging method in assessment of patients before and

after endoluminal repair of AAA. It is not clear what morphological changes should be expected

after endovascular repair or indeed whether any of these alterations are graft-specific.

CT is used in the preliminary evaluation of candidates for endovascular repair, in order to screen

patients and for diameter measurements. Standard CT, although costly, is accurate and

reproducible in documenting aneurysm size and has the additional advantages of delineating

suprarenal extension and other abnormalities that may influence endovascular repair, such as heavy

calcification or neck lining thrombus. For a high-quality study, large doses of iodinated contrast

may be required to provide visualisation of the distal iliac vessels, at considerable risk of renal

compromise in susceptible individuals. Coronal reconstructions, useful in these instances, are often

of low resolution and do not display the arterial anatomy in an angiographic format.

Axial CT slices often do not cut through planes perpendicular to the vessel lumen. The resulting

elliptical cross-sections make diameter measurements difficult. Although it is often assumed that

the narrowest diameter of the ellipse is the true diameter, this is not always the case, as an aneurysm

188

does not have a simple cylindrical or conical shape. Simply using sagittal or coronal reconstructions

does not reduce this error, because these sections may also not be perpendicular to the vessel (thus

overestimating vessel diameter) or may not pass through the exact centre of the vessel (thus

underestimating diameter).

While diameter measurements with conventional CT may be difficult, estimating the length of a

proposed endograft is even more inaccurate. The current standard is to perform Digital Subtraction

Angiography (DSA) for length measurements and evaluation of occlusive disease prior to

endovascular repair.

Angiography provides a 2-dimensional image of a 3-dimensional structure. Often only two

orthogonal views are available, making interpretation of the true anatomical dimensions difficult.

To improve the accuracy of measurement, angiograms are performed with graduated marker

catheters (usually at 1 cm intervals). The measuring catheter seldom follows the proposed path of

the endograft. The prosthesis, equal in diameter to the aneurysm lumen, tends to follow the centre­

line of the blood-fiow channel, whereas the much narrower and more rigid angiocatheter will

follow the shortest intra-luminal path (Figure 67a).

SCTA with 3-dimensional reconstruction and postprocessing offers unique benefits. It can acquire

volumes of data within very short scan times: for routine studies a single 32-second exposure with

conventional 1 cm coUimation will cover 32 cm of tissue. The use of finer collimation, such as 5

mm, reduces the longitudinal coverage to 16 cm, but allows the aorta and its branches to be

reconstructed from the celiac axis to the iliac arteries. The location and spacing of the slice

reconstruction can be varied before or even after data acquisition up to that of the slice coUimation.

Overlapping sections wiU result if the reconstruction interval is less than the coUimation. These

slices are used for generation of the 3-dimensional model. Overlap is also necessary to create

smooth 3-dimensional models without artifacts and jagged edges. AU this is performed without the

need for additional radiation exposure. Spiral CT aUows the flexibUity to view image data as both

conventional axial slices and three-dimensional models.

Multiplanar reformatting allows the accurate measurement of intraluminal diameters and

longitudinal distances. Rather than measuring the length of a line drawn in a 2D projection, the

computer makes length measurements fcom actual 3D data sets along a centre-Une defined by the

user. Similarly, for diameter measurements, the MPR technique enables the display of axial slices

perpendicular to the vessel lumen.

This thesis was designed to evaluate the effects of endografting on the morphology of abdominal

aortic aneurysms. As part of this project an evaluation of our current imaging modalities for

189

suitable aneurysms was undertaken. In both laboratory and clinical studies, SCTA, graduated

angiocatheter and gadolinium-enhanced MRA were compared. The objective was to select a gold

standard, accurate as the sole imaging modality for both pre and postoperative assessment of

patients.

M e a s u r e m e n t t h e o r y

Two essential components of a measurement are its reliability and its validity.

Reliability is concerned with the consistency or repeatability of each measurement made. Validity

reflects the accuracy of measurement. For example, it is possible for a measuring tool to provide

the same value for a specific dimension on repeated occasions although the actual figure provided

in this case may be incorrect. The measure is hence reliable but not valid (that is it is consistent but

wrong).

The definition of reliability is based on the true score theoiy of measurement, which postulates that

every measurement consists of two components, the true ability (or true score) of the respondent on

that measure and random error.

The disadvantage of the true score theory is its simplistic approach to random error. It ignores the

possibility of a systematic component that may be an inherent part of the measuring device and

which may be repeated consistently across aU of the members of a group. True random error is

caused by factors which affect measurement and which vary in an indeterminate way. For example,

the measurement of aneurysm maximal AP diameter using ultrasound is known to be strongly

operator dependenfl*^. The same operator may produce different values for this dimension on the

same patient measured repeatedly. Part of this variability may depend on factors such as

interruptions and distractions from the screen or the amount of operator fatigue. This type of

indeterminate error is also referred to as “noise.”

Systematic error is caused by factors that consistently affect measurement of a variable across the

whole sample. In the case of an ultrasound scanner, this may occur because of incorrect calibration

of the machine.

In the first stage of this thesis, the quality of measurements obtained from two preoperative

imaging systems in our department, calibrated angiography and spiral CT angiography, was

investigated.

E v a l u a t i o n o f t h e q u a l i t y o f m e a s u r e m e n t s m a d e w i t h s c t a

The quality of the measurements obtained with SCTA depend on three factors:

190

• SCANNER: extremes of temperature or power failures/surges may influence electrical

circuitry.

• PATIENT: Obesity, motion within the scanner and respiration may all introduce artefact.

• OBSERVER: errors in implementing the clinical protocol or poor technique for

measurements made from scans.

Scanner and observer-related factors were assessed with six glass phantom aneurysms that were

hand blown using tubes with similar internal dimensions to the human aorta. Linear dimensions

(maximal AP and transverse diameters) and volumes of these phantoms were recorded before

scanning using the accepted physical techniques of pyknometry and electronic callipers.

Data obtained from scanning aneurysms with the patient clinical protocol were transferred to a

post-processing station for volume and linear estimations. AP and transverse diameters were

calculated using the MPR facility that measured the distances between electronic markers placed on

the phantoms. Volume was estimated using a voxel summation technique at a segmentation

threshold of 0 HU.

Comparison of the true values with their scanned counterparts, revealed near perfect correlation. In

addition the gradients for the three correlation lines (AP diameter, transverse diameter and volume)

were close to unity, implying near perfect agreement with the true values.

Observer-related factors were calculated using an intra- and inter-observer analysis of linear and

volume data obtained from a series of randomly chosen SCTA scans stored on optical disc. The

results demonstrated that the spiral CT technique, with post-processing of data into 3D and

multiplanar formats, had acceptable reproducibility. For example, in terms of transverse diameter,

95% of the differences within and between observers were in the range 2.3 to 2.5 mm, for a

median diameter of 52.6 mm (range 39.2-85.9). This compared favourably with the results from a

similar study where a difference of 4 mm was obtained looking at the same parameters with spiral CT270.

H o w A C C U R A T E IS C A L IB R A T E D A N G IO G R A P H Y F O R A S S E S S M E N T O F AAA?

Calibrated angiography is currently used in conjunction with conventional or spiral CT for imaging

the abdominal aorta and its branches prior to selection for endovascular repair. Previous studies

have found a high level of agreement between length measurements made using calibrated

angiocatheter and SCTA^7 many centres, angiography is deemed sufficiendy accurate enough

to be used as the sole basis for the construction of endovascular grafts.

191

In this clinical study, diameter and length measurements obtained with calibrated angiocatheter

(Cook, Australia) were compared with those obtained using validated SCTA.

As expected, there was an excellent correlation between measurements obtained with the two

methods (r=0.85 to 0.99): both techniques were in effect measuring the same dimensions on the

same aneurysms. In contrast, there agreement for these values was poor. This was highlighted

during lAFC length and aneurysm neck measurements. In the former case, there were

disagreements as large as -15.82 to 13.54 mm between the two techniques. The confidence

intervals for these limits were even larger confirming the poor clinical accuracy obtained with

calibrated angiocatheter.

Sources of error occurring during calibrated angiography include those resulting from

magnification, parallax and poor edge definition.

Magnification

Magnification is greater for objects closer to the x-ray source. For example, magnification of the

distal aorta and its bifurcation is greater than for the proximal aneurysm neck. The distal aorta

bifurcates in the region of the lumbar lordosis and sacral promontory hence passing closer to the x-

ray source than the coeliac axis. This may improve the quality of measurement as the angiocatheter

appears much clearer and the observer can easily distinguish the graduated marks along its length.

This may be the explanation behind the fixed error obtained in the sizing of the distal cuff

parameters.

Parallax

Parallax varies with the position of the observer to the imaging screen as weU as the thickness of

that screen. To minimise this problem, the person taking measurements must stand directly in front

of the screen when using the measuring device. If there is even a slight angle between the observer

and screen, this is likely to cause problems especially when estimating diameter.

Edge definition

Edge definition on fluoroscopic images is subject to a further error that is defined by the size of the

x-ray source. A fundamental difference between an image intensifier and photographic camera lies

in the size of the focal spot. The intensifier generates images using a beam of x-rays produced from

a tungsten source. The image is affected by the differing amounts of attenuation that the beam

undergoes. Unlike a camera lens, the tungsten focus is not a mathematical point and its effective

size has a practical limit of 0.3 mm. The finite size produces a penumbra effect, causing a

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degradation of the object edges (Figure 66). This tends to spread out the edge gradients resulting in

a softer on screen image. The effect is greater for objects closer to the source. In experiment 7.4,

the distal cuff, which was closest to the screen, had its diameter oversized by 2 mm in most

patients. This may have been due to the penumbra effect causing thicker edge lines.

Discrepancies between measurements may also occur due to the physical nature of the measuring

device. In the situation of vascular channel measurement, the sizing pigtail catheter wiU always take

the shortest intra-luminal route whereas an endoluminal graft, equal in size to the vessel diameter, is

likely to follow the centre-Hne of the vascular channel (Figure 67a). Only spiral CT can mimic this

effect by "dropping" markers within the true luminal centre in 3D space (Figure 67b). This is

dependent on the MPR facility that allows 3 simultaneous orthogonal views. These views

themselves are linked to a 3D SSD that can be manipulated to ensure the true aortic lumen is

always sectioned perpendicular to its long axis. The rigid angiography catheter may hence undersize

the tme length of the vascular channel. Conversely, when the aneurysm contains litde in the way of

thrombus, oversizing may occur due to bowing of the catheter. This is probably because the sizing

catheter can bend in the aneurysm sac thus increasing the apparent length of the vascular channel.

This effect may remain unnoticed, especially if it occurs in the AP plane.

The fixed errors in sizing distal cuff diameter and length with angio-catheter may also be explained,

in this way. There is less chance for the catheter to bend or bow near the aortic bifurcation as this

represents the entrance into the aneurysm sac from below. Magnification is also likely to provide

improved visualisation in this area generally. Unfortunately the clinical applicability of these

observations is limited as aorto-aortic tube grafts are now seldom deployed.

These experiments demonstrate that even with the most optimistic interpretation, the quality of

measurements obtained using sizing catheter angiography is not clinically acceptable. Incorrect

measurements may have serious clinical repercussions. The limits of agreement and their

confidence intervals were potentially bad enough to result either in the erroneous rejection (due to

undersizing of neck length) or selection (due to overestimation of neck length) of patients for

endoluminal repair. If a patient with an aneurysm neck that is too short for endoluminal repair is

selected, the risks of postoperative endoleakage or graft migration are then high. Similarly, incorrect

estimation of aneurysm neck diameter is likely to result in a poor graft-vessel wall seal with

resulting endoleak and chance of aneurysm rupture.

Patients undergoing sizing angiography are also exposed to larger cumulative doses of intravenous

iodinated contrast with the potential of renal impairment, especially with pre-existing renal disease

or diabetes mellitus. Spiral CT angiography should hence be regarded as the method of choice for

measurement of aneurysms prior to endoluminal repair.

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Tungstensource

X-ray beamsObject

Figure 66: The penumbra effect leads to blurring and overestimation of the image diameter

T h e l i m i t a t i o n s o f p r e o p e r a t i v e a s s e s s m e n t u s i n g SCTA

Although SCTA has now been fully validated both in the laboratory and clinically, one source of

error stems from the observer who makes necessary measurements from scans using the post­

processing technology.

In Chapter 7.9, 12 preoperative scans were re-examined at 6 months after the first reading, by the

original and a separate observer of equal experience. Proximal neck length (Table 27) could be

measured with an accuracy of ±5 mm both within and between observers. Observer errors for the

neck diameters were much lower (Tables 28 and 29). ITie upper neck diameter could be measured

with an accuracy of ±2 mm on repeated occasions compared with an accuracy of ±3 mm for the

lower neck diameter.

These results demonstrate one limitation of SCTA. In the case of neck length, 15 mm is currently

accepted as the minimum for selection for endovascular repair. It is easy to see how a patient with a

borderline neck of 10 mm may be over- or under-measured (producing values of 15 mm and 5 mm)

respectively with possible sequelae such as endograft migration in the former case.

In the case of oversizing neck diameter, the endograft may initially appear mechanically sound with

no post-deployment endoleak. However, if the frictional forces between the stent and aortic wall are

at their limit, longer-term loss of wall elasticity and recoil is likely to result in disruption of the blood-

tight seal and produce endoleakage or graft slippage.

When neck diameter is undersized, the endograft will sit loosely and endoleak or slippage will be

immediately apparent. In more insidious situations, where the endograft just contacts the aortic wall,

further expansion of the aortic neck as may occur with a continuation of the aneurysmal diathesis

likely to results in long-term loosening with complications.

This experiment also serves to highlight the importance of standardising measurement techniques to

reduce error where more than one clinician is employed to measure and select patients for

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endovascular repair. At one level this would involve measuring aneurysm neck diameter from

internal wall to internal wall or reviewing scans, in difficult cases, with other members of the

endovascular team to decide on where the neck ends and aneurysm chamber begins.

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F ig u re 67a: D uring an angiogram , the rigid angiocatheter will n o t follow the intralumm al path o f an endovascular prosthesis bu t instead tends to pass along the shortest route betw een the two

poin ts dem onstrated. This results m an underestim ation o f I AFClength.

U . C . L . H C X - O r a p H l o *

F ig u re 67b: Accurate estim ation o f the flow channel length w ith SCTA. T he intra lum inal m arkers are placed exactly central in all dim ensions. T he length o f the channel is calculated from the sum o f their distances apart. The

left image is the AP view o f the lA FC while the right image is a left lateral.

196

A n e u r y s m v o l u m e : a m o r e u s e f u l i n d i c a t o r o f s a c m o r p h o l o g y t h a n

M A X IM A L D I A M E T E R S ?

Successful endovascular deployment is followed by an immediate reduction in aneurysm sac

pressure. With the present limitations however, intra-sac pressure, cannot be continuously

measured after this point. Instead sac shrinkage, measured using maximal sac diameters, takes over

as the morphological representation of a successful procedure. Diameters are relatively easy to

measure and have traditionally been used to describe aneurysms in follow-up prior to conventional

repair. The enigma for endovascular repair lies with the equivocal results for sac morphology

obtained from postoperative follow-up. In some reports, successful endovascular repair may be

followed by no change in sac diameter while in others there may even be enlargement in the

absence of endoleak. Explanations for such discrepancies include the following:

• The point at which the diameter measurement is made may be too soon in the natural history

of the sac to detect any change.

• The aneurysm sac may really remain unchanged or increase slightly in size after successful

repair in certain patients.

• The parameter being measured (usually the maximal AP diameter) may be impossible to

measure accurately or may vary inconsistently with time.

• The measuring tool may be inaccurate and unable to detect small differences in diameter.

Volume, an alternative means of describing aneurysm sac morphology, has a number of advantages

over diameter measurements.

Firstly, volume is inherently a more accurate descriptor of aneurysm morphology than a single

maximal diameter. This is because volume is a composite measure of all the cross-sectional

diameters within the aneurysm sac whereas a single diameter measurement can interrogate only one

cross-section of the sac at a time.

The volume of a sphere is proportional to the cube of its radius. This means that a small but not

necessarily measurable change in diameter will lead to a magnified and, by implication, more easily

measurable change in aneurysm volume.

A further problem arises when the same diameter is not quoted at each follow up interval. Every

diameter occupies a unique location in 3D space within the aneurysm sac. It is hence possible for a

change in sac shape to occur, without any change in the magnitude of the maximal diameter (Figure

68).

197

Finally, measurement of aneurysm volume, an entirely automated process, avoids the obvious

subjectivity associated with diameter measurements. In some studies, follow-up of sac dimensions is

litde more than measurement of hard-copy axial slices with rulers. Even electronic measurement,

using markers available with the spiral CT software, has its problems. ITie operator may not

accurately locate the edge of the sac when placing markers if the screen resolution is set too low.

ITe importance of spiral CT lies in the fact that it collects a finite volume of data reformatted into 2

dimensional images. It is surprising that the original volumetric data set is not usually analysed

further since all modem scanners with post-processing workstations, contain software that can be

used to calculate volume. These workstations also provide a digital representation of the aneurysm

in stacked cross-sectional images (Figure 18) called the “surface-shaded display.” Multiplanar

reformation (MPR) allows further assessment and measurement of the 2 dimensional anatomy in

any part of the aneurysm structure.

F ig u re 68: T he relationship betw een volum e and m axim um diam eter. The fruits have d ifferent shapes w ith d ifferent volum es yet bo th have the

same m axim um diameter. In the natural history o f aneurysm s post exclusion, a shape change may be missed if only maximal diam eters are

measured. T hese ignore any change occurring above o r below the region o f interest. V olum e m easurem ents may be a better way o f describm g the natural history o f excluded aneurysm s, as they are in effect, a sum m ation

o f all the diam eters com prising the whole shape.

The inaccuracy of diameter measurement was demonstrated at a number of stages in this study:

It was observed that the standard deviations for diameter measurements on glass phantoms made

using SCTA, with only three exceptions, were at least 10 fold larger than those obtained direcdy

with electronic calipers (Table 7). liais was possibly due to the subjectivity involved in edge

detection on the postprocessing workstation screen.

198

A second, subtier example was demonstrated in the intra/inter-observer studies (Table 10). The

important point to note are the units used in this table. Although the actual values assigned to the

errors for volume and linear measurements appear to be similar in magnitude, volume is actually

proportional to the cube of the diameter. Hence an intra-observer coefficient of ±2.31 mm for

transverse diameter translates to a volume difference of ±12.33 ml. With this in mind, the actual

coefficient of ±5.7 ml (46% less than 12.33 ml) demonstrates an excellent reproducibility of

volume estimation.

Other workers in this field have obtained similar results. The Utrecht group^^ analysed a series of

35 patients to find that a decrease in volume post-endovascular repair was missed in 14% of their

cohort with diameter measurements alone, while an increase in sac volume was missed in 19%.

More importantly, there were 4 patients with major endoleak of which two had an increase in sac

volume but no change in diameter. In a second smdy with nine patients, Baku et aP- found again

that aneurysm sac diameters did not always reflect the volume changes after endoluminal grafting.

Calculations of thrombus or lAFC volumes were also necessary to discriminate successful from

failed exclusion.

In this series of 88 patients, there were 2 cases of Type I endoleak and one case of late graft

migration (a Talent-treated patient) all of which continued to have an increase in their total

aneurysm volume (Figure 35) until treatment. Only two of these had an increase in one or both

maximal diameters.

Recording change in the morphology of the intra-aneurysmal flow channel after exclusion is also

important. Endoleak or graft migration may lead to an increase in the volume of this entity. A

possible disadvantage of this method is the amount of work involved in initially editing out the

flow-channel by hand on each axial slice. This was even more time consuming than for total

aneurysm volume {vide infra). To overcome this, a new method based on HU segmentation was

designed.

Due to the sharp change in density at the boundary between the lining thrombus and the blood-

contrast mix, the LAFC should have its own unique Hounsfield value. Calculation of this value

should display only the LAFC volume allowing its estimation on a post-processing work-station.

The change in HU/density at the thrombus-flow channel edge was calculated using Disp Image,

software that is available with aU modem spiral CT equipment. In the case of the glass validation

model this value was 165 HU. The percentage error for calculating the volume of the balloons used

was dependent on their true volumes. This decreased as the true volume increased and approached

a value similar to that found for the LAFC in vivo.

199

Although the same technique was employed in the clinical studies, differences between the glass

model and patients’ aneurysms were evident. The main difference was the sharp contrast to water

boundary in the phantom (Figure 25) compared to the somewhat less well-defined boundary in

patients. Biologically the intra-luminal thrombus was far more irregular at its boundary with the

flowing blood raising the possibility that an automated segmentation technique could have

problems in defining this zone accurately. In the representative axial slices that were individually

examined before and after segmentation however, this problem was not immediately apparent.

A second difficulty occurred while plotting the HU values comprising thrombus and the blood-

contrast interface. Due the partial volume effect of spiral CT, the HU values did not demonstrate a

sharp numerical change at this site, but instead a gentle slope across the region (Figures 25 and 32).

The point hahway along this slope is conventionally chosen as the segmentation Houns field value

for the interface both in vitro and in vivo. The somewhat greater subjectivity of this technique was

demonstrated in the intra/inter-observer comparisons. It was found that the inter-observer

comparison for total aneurysm volume (4.39 ml) was less than its counterpart for lAFC volume

(6.49 ml). Fortunately, the difference involved (2.1 ml) would probably not be of much significance

clinically.

In this study, the main disadvantage of volume estimation was the time taken to edit out the AAA

by hand, from the surrounding soft-tissues and bone. The sac and lumen had to be painted out on

every axial slice (95-110 slices) using a special editing tool available within the software (Figure 14)

taking an average of about 30-45 minutes per patient before the total aneurysm volume was then

segmented at 0 HU. Van Hoe et examined the effect of manual editing on the accuracy of

volume estimation using a series of ellipsoid and irregular phantoms. They found that this was

likely to produce poor volume estimation only for smaller objects (they used ginger roots)

compared to larger regular shaped phantoms (such as aneurysms). Other factors that may

potentially influence the accuracy of volume estimation with spiral CT include increasing axial slice

thickness^^ and inappropriate window center selection^^^ both of which may cause overestimation

of volume.

A n e u r y s m m o r p h o l o g y a f t e r e n d o v a s c u l a r e x c l u s i o n

Two groups of patients were studied over two generations of endografting experience. The first

group (27 patients) was treated with an in-house generation F IF E endograft fixed with balloon-

expandable Palmaz stents as previously described (Figure 41). The second group (61 patients)

received the custom tailored talent (World Medical) device.

200

In the absence of complications there was a small but significant increase in total aneurysm volume

and decrease in flow channel volume at day 5 after endografting. The median increases were 12.39

± 5.70 ml for the PTFE group and 15.1 ± 5.70 ml for the Talent group. It should be recognised

that the actual values obtained may have been up to 6 ml higher or lower according to the estimate

for intra-observer error (Table 10).

In their study of nine patients, Balm and coHeagues^^ found a postoperative increase of 17 ml in

median total aneurysm volume for patients treated with the EVT endograft. In contrast with the

findings here however, the volume change occurred concomitanfly with an increase of 0.9 mm in

the median maximal aneurysm diameter. An increase in maximal diameter of this small magnitude

is difficult to accept with confidence as it could quite easily He within the ranges of intra- and inter­

observer error in diameter measurements, between ±3.40 mm and ±4.42 mm respectively (Table

10).

Following this early increase, the subsequent sac morphology was entirely dependent on the

endografting system used. In the case of the PTFE group, no further significant change in volume

was seen to the conclusion of the study at 3 years. For Talent-treated patients, marked and obvious

shrinkage took place up to 1.5 years. The impHcations of these findings are not clear, and although

the PTFE group did not exhibit sac shrinkage, no ruptures have so far been noted up to 3.5 years

following implantation.

In this study, had diameter measurements alone been used in follow up, the initial increase in

volume would not have been discovered and sac shrinkage in the Talent group would only have

been detected up to 1 year. Significant decreases in maximal AP or transverse diameter could not

be detected at or beyond this point.

Hypotheses for volume changes

The increase in total aneurysm volume at day 5 was accompanied by a simultaneous decrease in

lAFC volume, indicating a true increase in the volume of the perigraft space. Investigation for the

cause of this increase was beyond the remit of this work.

It was evident from an examination of individual scans however, that the increase resulted firom an

accumulation of new clot and inflammatory oedema, formed firom the pool of blood excluded by

the endograft during deployment. A semi-Hquid pool of blood and contrast collected outside the

prosthesis wall and was later incorporated into the intra-luminal thrombus. The contrast may have

resulted in hyperosmolarity of this region with the accumulation of more tissue fluid over time.

The nature of the graft material is another important consideration. An initial inflammatory

reaction may result in the accumulation of an exudate in the perigraft area that is later resorbed in

201

Talent but not PTFE patients. There is evidence that both PTFE and Dacron can initiate

inflammatory reactions within surrounding tissues " - It is possible that a similar reaction in the

sac causing an inflammatory collection may have added to the increase in the perigraft space. A

further hypothesis is based on the semi-permeable characteristics of the graft materials^^^. The

normal hydrophobic quality of the PTFE may be lost when the material is structurally altered by

pre-expansion during the in-house technique of graft manufacture, and both grafts may then allow

ultrafiltration of the flowing blood to occur after implantation. This may later be resorbed in the

Talent but not PTFE group.

Later follow up demonstrated a divergence in the morphology of aneurysms treated with the two

endografts. PTFE-treated sacs remained static whilst Talent-treated sacs demonstrated marked

shrinkage. A second structural difference between the Talent and PTFE systems, apart from the

graft material, is the self-expanding nature of the Talent system and the balloon-expandable Palmaz

stents used in the unsupported PTFE group. Balloon-expandable devices abolish the pulsatile

motion of the aneurysm neck whereas this is not seen with the self-expandable devices^®. It is

possible that the aboHtion of pulsatile motion at this site may favour the maintenance of intra­

luminal thrombus in the PTFE group. This would explain the decrease in volume for Talent-

treated sacs. However, the PTFE patient with a saddle embolus and no intra-aortic flow below the

renal arteries, also had marked sac shrinkage. This introduces a further possibility. The PTFE

device was free and unsupported in its central segment where the transmission of intra-luminal

pressure waves into the sac may have also had some effect on maintaining the intra-luminal

thrombus.

C h a n g e s i n i n t r a l u m i n a l t h r o m b u s a f t e r e n d o v a s c u l a r r e p a i r

Thrombus is formed within the lumen of an AAA due to the presence of turbulence and pockets

of stasis in the flow of blood. Platelets and activated clotting factors are deposited in layers adjacent

to the aortic wall. The centre of the sac maintains its patency and blood can pass through this

vascular channel into the lower Hmbs.

Various roles have been attributed to the presence of ILT. Originally, it was believed that thrombus

had no function in the aneurysm sac and its presence offered no protection from rupture ' >

Thrombus allowed the direct undamped transmission of the blood pressure through to the

weakened aortic wall.

Evidence suggests that thrombus may act as a double-edged sword during aneurysm formation. On

the one hand, it could be involved in the formation of aneurysms by acting either as a physical

barrier to diffusion of nutrients causing aortic wall anoxia, or as a source of inflammatory cells and

202

cytokines causing aortic wall destruction and weakening. On the other, a demonstration of its’

elastic behaviour in vitro and a protective effect against rupture in fine element models of AAA

implies it may delay the onset of rupture. When models of the aneurysm are lined with varying

thickness of lAT, the mechanical strength of the whole unit increases in direct proportion to the

thickness of this lining^^.

With the advent of endovascular repair, these theories have now become even more important. In

contrast to open repaie of abdominal aortic aneurysm, endovascular repair leaves ILT undisturbed.

A further role, for which there is no evidence as yet, may be for thrombus to act as a type of

buttress around the stent-graft forming a thrombus-endograft unit. Clearly, it is important to be

aware of changes in the amount of ILT with time if it has an influence on the aortic wall.

In this study, the preservation of ILT was dependent on the prosthesis used. In the case of the

balloon-expanding PTFE grafts, thrombus was conserved whereas for the Talent patients, its

volume was markedly reduced with time. In this case, although further significant resorption could

not be demonstrated after 6 months, for some patients thrombus was completely reabsorbed

(Tables 20 and 21). The explanations for these differences are at present uncertain, although any

ideas must run on similar lines to the hypotheses presented above.

The suggestion that ILT may protect against type II endoleakage is based on a qualitative analysis

of its location within the aneurysm sac^^h This has been mimicked by some investigators who have

plugged lumbar and mesenteric arterial orifices with gelatin sponges and coded sprtngs ^ »

Although the sequelae of type II endoleakage are still debated, its presence may nevertheless

become significant^^ . If thrombus does have a protective role, then it follows that aneurysms with

complete resorption may be at greater risk of complications with time.

A n e u r y s m l e n g t h a f t e r e n d o v a s c u l a r r e p a i r

Endovascular repair of abdominal aortic aneurysm has focused attention to the morphological

changes of the aorta and iliac arteries that accompany aneurysm growth. Because of the destruction

and loss of aortic elastin and collagen, aneurysms undergo both longitudinal and transverse

expansion. Lengthening results in increasing tortuosity of the aortodiac segment with important

implications in obtaining access to the aorta for endoluminal repair. Tortuosity in the proximal

aneurysm neck may lead to migration and endoleak if the neck is short as wed as angulated.

Prosthetic Distortion

Investigators have recently drawn attention to the distortion of aortic stent-grafts in the medium to

long-term after implantation. These shape changes commonly affect bifurcated endografts and

203

include kinking of their limbs at the junction with the trunk. Three proposals have been put

forward to explain these findings ^®:

• Shortening or shrinkage of aneurysms along their longitudinal axes results in endograft

buckling and disruption.

• Shortening of the prosthesis following deployment results in infolding and disruption of the

endograft

• Migration of the endograft from its proximal or distal anchoring sites results in endograft

distortion

Distortion is a serious problem that may ultimately lead to graft Hmb thrombosis, separation of

modular components, or migration of the limbs from the common ikac arteries. The latter two

sequelae inevitably result in endoleak.

Shortening or contracture of aneurysms along their longitudinal axes has been held as the cause for

the majority of endograft failures post-exclusion. Harris et described shortening of the aorta by

as much as 3.1-6.2 mm occurring up to 1 year after implantation. A further complicating feature in

this study was concomitant shortening of the endograft after deployment, by as much as 2.4 to 67.6

mm, which was seen in 94% of their cohort.

Shrinkage of endografts, measured immediately after deployment, has been described by White et

aP^. This study, using the AneuRx and Vanguard devices, found shortening up to 30 and 40 mm

respectively. Varying degrees of graft component separation, angulation and distal stent migration

were observed and ascribed to this shortening process.

In both these studies, the techniques for graft and aneurysm measurements were not validated. The

Liverpool group measured aneurysm lengths directly from the CT scans while the Sydney group

recorded graft length pre- and post-deployment by fluoroscopic comparison to a graduated

marking catheter. The problems involved with both these techniques have been highlighted above.

When validated SCTA is used to examine aneurysm lengths with time, neither of these hypotheses

can be supported. Instead, the sequence of changes that is observed is related to the type of

endograft deployed and the changing volume of the aneurysm sac with time.

When balloon expandable PTFE is used, both the aneurysm and endograft undergo a gradual

lengthening. In the case of the aneurysm, the first significant lengthening is seen almost

immediately at day 5 post-repair. It is unlikely that enough collagen/elastin remodeling would have

occurred within this short interval to produce this change. Another explanation may relate to the

concomitant changes in volume and AP/transverse diameter.

204

At day 5 there is a significant increase in total aneurysm volume in PTFE treated aneurysms, but no

change in diameter. In keeping with the law of mass conservation, an increase in length might be

required to accommodate an increase in volume. This would result in “unfolding” of the aneurysm

thus apparently increasing its length.

A second significant increase in aneurysm vertical body length was seen at 18 months. This

probably does reflect the gradual and continuing loss of collagen and elastin occurring within the

aneurysm wall. Lengthening of the PTFE graft also at 18 months would be in keeping with the

increase in luminal centre line length and the characteristics of the graft material — pre-expanded

PTFE which would “creep” back to a lesser diameter and greater length with time.

Of interest was the PTFE patient with a saddle embolus resulting in endograft thrombosis and

necessitating an axülo-bifemoral graft after failed thrombolysis. Although this was the only PTFE

patient who had marked and obvious shrinkage in total aneurysm volume (50% reduction at 1

year), the vertical body length continued to increase as described.

No significant change in length was observed for the Talent-treated patients up to 2 years. These

patients received a graft that was fuUy supported throughout its length with interconnected, self­

expanding nitinol stents oversized by up to 4 mm in diameter. This stiff graft may have had the

effect of anchoring the aneurysm wall to the prosthesis, thus preventing lengthening. With the

marked and obvious shrinkage in sac volume of Talent-treated aneurysms, more of the aortic wall

itself would come in to direct contact with the prosthesis improving the anchoring effect with time.

Graft lengthening was not observed. This is again not surprising given the robust nature of the

graft construction. In one study of 290 patients treated with Stentor, AneuRx and Talent

endoprostheses, only Talent grafts remained relatively unaltered at 1 year post-implantation^®^.

The results described here are also supported in part by investigations looking at changes in neck

length after both endovascular^®^’ ® and open repair ® . These show significant increases in neck

length firom 2 to 10 mm over follow up intervals extending between 1 and 10 years respectively.

Based on these results and explanations therefore, it is unlikely that prosthesis distortion or

disruption can be attributed to aneurysm length contracture. The destructive changes described for

some types of endograft ®®' ® may be due to the physical properties of the graft structure itself and

not due to changes in the aortic wall and sac. An alternative explanation is to consider the

possibility that differential motion between the aneurysm wall and the endograft may be

responsible for some of these developments. The aorta is not an inert conduit for carrying blood to

organs and limbs, but a complex and moving biological structure whose function may interfere

with the long-term integrity of an implanted prosthesis in ways that are as yet only poorly

understood.

205

N a t u r a l h i s t o r y o f t h e a n e u r y s m n e c k a f t e r e n d o l u m i n a l r e p a i r

Long-term success after endovascular repair depends on durable fixation at the aneurysm neck and

distal landing-zone. It is believed that gradual dilatation of the aorta at these locations may cause

leakage and graft migration. Some units have begun to try and bypass these potential problems by

deploying grafts in the inter- and supra-renal positions. In two separate series of 25 ^ and 32^^2

patients, there were no reported renal occlusions during this procedure or postoperatively. A

second advantage of the technique is a potential use in the treatment of aneurysms with very short

necks although this has yet to be formally assessed.

Many studies that measure aortic neck diameters in patients after open and endovascular repair

have now been published. Methodological differences must be carefully noted when comparing

their results because different measurement techniques, aortic regions and time periods are

described. For instance studies have assessed the aortic neck at various levels: halfway in the

neck/attachment system^^i, proximal neck suitable for “proximal stent implantation^®^, 1 cm below

the lowest renal artery and 1 cm above the aortic bifurcation^^®, the juxtarenal aorta ® , and at

multiple 5 mm intervals^®®. Time intervals are often short (less than 1 year for the majority of

patients) and the actual changes in size are often smaller than the sensitivity of the methods used.

Further complexity is added by the physical properties of the stents (balloon- versus self­

expanding) whose effects are seldom individually analysed.

In this study, the natural history of the aneurysm neck was recorded for up to 2 years using

validated SCTA. In the case of the balloon-expandable Palmaz stents used to anchor the PTFE

system, there was a significant increase in neck diameter at day 5 for the upper and mid-neck levels.

This was followed by a slower increase affecting the distal part of the neck at 6 months. The neck

diameters were then stable. An opposite situation was observed with the self-expanding Talent

endograft. Again there was an immediate increase in diameter for aU 3 neck levels but this was

followed by further gradual increases over the next six months.

The most likely explanation for these results is that they reflect the outward force generated by the

endovascular stents onto the aortic wall. The balloon-expandable stent reaches its’ maximal

diameter soon after deployment whereas the self-expanding nitinol stent continues to dilate for

some time after the procedure accounting for the differing patterns of neck enlargement described.

In the case of the self-expanding system, this poses an interesting dilemma. As the stent gradually

approaches its maximal size, the elastic recoil of the aortic wall may begin to deteriorate with time.

Frictional forces between the stent and aortic wall will then become dangerously low with possible

endograft migration as a result. The preferred stent type from this perspective would consist of a

206

stent with a low outward force after deployment and a fixation mechanism independent of friction

between stent and neck. This would imply a reversion back to the 1®* generation balloon-expanding

stents perhaps with hooks/barbs used as part of the fixation.

In this study, although further progressive enlargement of the neck diameters, a consequence of the

underlying aneurysmal disease, was not demonstrated, this may have been too insidious to have

been statistically significant or possibly even overshadowed by the effect of over sizing the stent-

graft relative to aneurysm neck diameter. Because of these uncertainties longer-term follow up

(greater than 5 years) is required to confirm this change.

In contrast to changes affecting neck diameters, no change in aneurysm neck length was observed

in either group of patients during this smdy.

C a n M R I b e u s e d f o r p r e o p e r a t i v e e n d o v a s c u l a r i m a g i n g ?

MRJ has already been used in the evaluation of aortic aneurysms prior to conventional and

endovascular repair. The main advantages are the use of non-nephrotoxic contrast and the absence

of ionising radiation during the imaging process. MRA may be the technique of choice in young

patients or those with real impairment. However, even MRI is not completely safe as use is limited

by the presence of ferromagnetic materials in the patient, such as pacemakers and metallic fixation

devices, and by artifacts created by surgical clips or the endografts themselves. Some patients

cannot tolerate the noise and confinement of the MRI scanner. Motion artifacts and spatial

resolution limit imaging of nonaxial vessels and, as in conventional CT, precise delineation of

relevant aortic branches is suboptimal. Further, MRA examination times are longer than those

typically required for SCTA, and therefore MRA is not suited to examinations of acute conditions

such as rupture.

MRI may also be used in the assessment of tissue volume although this application has so far been

limited to the brain. Absolute volumes of brain tumours have been followed to reflect the effect of

treatment^® . Conditions in which the encephalic ventricles become enlarged such as Alzheimer's

disease and hydrocephalus, also require accurate measurement of the ventricles to provide a safe

means of aiding the diagnosis and provide follow-up information in affected patients. In one study

measuring the volume of simulated ventricles (water-filled cavities in gelatin phantoms), the results

obtained using MRI were better than those obtained with CT ® . The average percent difference

between volumes obtained by CT or MRI compared to the actual volume determined by water

displacement was 15.8% and 8.3% respectively.

207

Laboratoty studies

The use of gMRA in the assessment of aneurysm volume and linear dimensions was examined in

the laboratory and in a sample of 7 patients prior to endovascular repair. The main laboratory

findings were that gMRA produced linear and volume data comparable to spkal CT angiography.

One problem with volume estimation however, was the inability of MRA to detect glass due to the

nature of the imaging mechanism. If this effect was ignored however, gMRA estimation of solution

volume only was highly accurate with percentage errors in the range -0.01 to 1.22. In clinical

practice possible volume errors could arise if the aneurysm walls were heavily calcified or contained

large amounts of old laminated and calcified thrombus.

Clinical studies

In the clinical section, the feasibility of gMRA for assessment of linear abdominal aortic aneurysm

dimensions prior to endovascular repair was investigated. In a comparison with validated SCTA,

there was an overall disagreement of -4 to +7 mm depending on the region of the aneurysm under

consideration. The disagreement was much higher for length measurement than for diameters

particularly in the presence of tortuosity. This was especially true for LAFC and common ihac

lengths. However, the results are not a reflection of the inability of MRA to measure these regions

accurately, but more a consequence of the complex 3D anatomy in the area. Any two imaging

methods would have discrepancies in this situation.

Compared to SCTA, two further disadvantages became apparent:

Firstly there was an inability to clinically estimate aneurysm volumes due to the form of data

acquisition. The software collected volumetric data in coronal slices compared to the axial slices of

SCTA. This was because the 3D volumetric MRA had different voxel geometry (rectangular)

compared to SCTA (cuboid).

Secondly, aneurysm neck wall thickness was a problem during diameter measurements. This was

especially true with the maximum intensity projections (MIP) that tended to improve luminal

resolution at the expense of wall thickness thus causing underestimation of the aortic neck

diameter.

These results suggest that the role of MRA in the endovascular field is evolving. While MR

angiography compares favourably with SCTA in the estimation of linear distances, volume

calculations are disadvantaged by the nature of imaging mechanism. Although it can be argued that

aneurysm volumes are not essential when measuring patients for endovascular grafts, theic

importance in follow-up after endovascular repair renders MRA less useful. Accordingly, it is

208

recommended that this technique be presently reserved for patients with contraindications for

SCTA such as contrast allergy or renal compromise.

M o t i o n a t t h e a n e u r y s m n e c k

In addition to distortion and changes in position, endoluminal stent-grafts are at risk of structural

deterioration. This may involve fatigue fracture in the metal frame, disruption of sutures between

the metal frame components or covering, and fabric deterioration ranging from small defects to

disintegration of the graft. For example, the Stentor (MinTec, Freeport) graft is composed of a

series of nitinol zigzag stents interconnected with a 7-0 polypropylene suture. Breakage of these

ligatures and dislocation of the frames has been shown to occur between 5 and 41 months after

deployment^^^. Suture breakage and stent separations have also been noted in its successor, the

Vanguard (Boston Scientific) device on plain radiographs of the abdomen^^^. These structural

alterations may lead to graft distortion. Umscheid and Stelter^^o provide the most comprehensive

report of these changes to date and emphasize that their incidence increases with time.

As demonstrated, longitudinal shrinkage of aneurysms is unlikely to be responsible for these

changes. An alternative is to consider that the damage may be device specific and/or related to

some other source of trauma. One possibility that incorporates both these suggestions is related to

the physical forces acting within the aortic lumen and the nature of the motion occurring at the

aneurysm neck.

The endograft, a rigid device, is inserted into a continuously mobile aneurysm. If the endograft

construction is not robust as was the case with original Vanguard, the repeated minor trauma

expected with a continuous low-grade motion could be responsible for some of the types of

endograft loosening and damage seen after deployment.

Malina and colleagues have identified two basic motions affecting the aneurysmal aorta. There is a

longitudinal pulsatile motion in the range 1.5 mm and a radial component estimated at Imm ®" .

These motions are altered by endograft deployment (the radial component is reduced to

approximately 0.2 mm) and by the shape of the endograft. Wider endografts pulsate more and

transmit a greater pulsation to the aneurysm sac.

A second force generated by the column of flowing blood above the endograft creates a

longitudinal force within the aortic lumen. Data based on mathematical calculations and

experimental perfusion models suggest that a longitudinal force of approximately 10 Newton acts

on the proximal stent of an implanted endograft ®" . This force is proportional to the blood pressure

but independent of the flow rate. The force of 10 Newton has been shown to be sufficient to

dislodge stent-grafts in cadaveric models. The risk of stent dislodgement is reduced with bifurcated

209

as opposed to aorto-uni-iliac endografts. Within the bifurcated design, the effects of the

longitudinal force on the proximal stent can be further abated by using a longer column for the

endograft body so that its component limbs separate as close as possible to the native aortic

bifurcation. Short graft Hmbs inside the aneurysm sac are less likely to buckle or twist.

In this study, a method was developed to record and accurately measure the types of motion taking

place at the aneurysm neck. A pilot study, utdHsing high-speed video camera had previously

confirmed the presence of the described aneurysm neck motions.

The method eventually chosen to record and measure neck movement accurately was based on the

same techniques employed by surveyors to record land shifts. Measurement of linear distances

from photographs is not as simple as it may first appear. As described in Chapter 8, the distortions

inherent in all camera lenses translate onto photographic images making measurements in two

dimensions inaccurate. Further problems occur in trying to measure depth from a perspective view.

To reproduce depth requires a 3-dimensional reconstruction.

AU of these difficulties can be overcome with the use of multi-camera imaging and analytical

mathematical techniques based on the least-squares method. These correct the measurements

made direcdy from photographic images either manuaUy in stereoplotters or with the use of

speciaHsed software, both of which predict the distortion a bundle of Hght undergoes as it passes

through the internal geometry of a camera (bundle-adjustment software).

Because this software was originaUy designed for geographical land mapping on a much larger

scale, its use for measurement of the much smaUer aneurysm was uncertain. A simple vaHdation

experiment was hence designed to measure the volume of a glass aneurysm using this technology.

Aneurysm F was coated with a series of retro-reflective targets and photographed around its

complete circumference. The images were then Hnked to form a 3D image on screen used to

measure diameter and volume. The values obtained were very similar to the known parameters of

this phantom with minimal error.

Still multi-camera photography was used to capture aneurysm neck and other arterial motion

during conventional vascular surgical procedures. Dual cameras were set to fire synchronously at 1-

second intervals. With this technique, although the presence of movement was confirmed, it

proved impossible to quantify its direction and magnitude. This was due to a number of problems.

It was beHeved that the shutter-speed of the cameras used was too slow. The cameras were fixed to

a simultaneous shutter speed of 1-second that could not be varied and it may have been possible

that aneurysm neck movement was either much faster than this or was cycHcal within the 1 second

intervals. To test this hypothesis, a strobe Hght was mounted on the camera bar and programmed

to produce 4 flashes per second. This would therefore result in 4 exposures during the time the

210

camera shutter was open. If there were any movement in this time, overlapping images of the

aneurysm would be seen. Unfortunately this did not prove possible probably due to the sensitivity

of the photographic film s used. Although a selection of films was tried within the budget allowed,

none were sensitive enough to record more than one exposure.

A further loss of data may have occurred in the analysis stage. Image data had to be labeled by

hand in the Kem stereo-plotter and then converted into digital format with a scanner. These steps

may have resulted in the loss of some information. Against this hypothesis were the figures for the

residual values of the ground point co-ordinates measured in the manual and digital stereo-plotters.

All were less than 1 mm.

Digital photography

Digital photography was used to increase the camera shutter speed and also to avoid the manual

analytical stage. Two pairs of femoral arteries were filmed in a preliminary experiment. The results

showed that while a degree of motion was present, its magnitude was again difficult to determine.

This time however, the problem was due to the measurement error or “noise” associated with the

tie points. The most likely explanation lies in the nature of these points. Individual points were

made with indelible ink on the surface of the native artery. These were then converted into pixel

format by the digital cameras. The accuracy of image point identification of the software used

(VMS) was at the pixel level. Hence larger points generated more inconsistency due to the errors

involved in recognising and marking the same pixel by hand on each photograph. This problem

could have been overcome by improving the marking process with smaller points or use of

photoreflective targets that could be sterilised without damage. This has to be contrasted against

the critical diameter of each point that makes it easily visible in every photograph. At the time of

writing, the VMS software was being altered to allow automated recognition of the same point at

the sub-pixel level in different image sequences.

In this series of experiments therefore, although aneurysm neck motion was observed, its capture

and accurate measurement proved difficult in the time allowed for this work. However, from these

preliminary results and discussion, it is apparent that a complex pattern of physical forces and

arterial movements must co-exist within the aorta. Future research will be directed towards

measuring and elucidating interactions between the continuous longitudinal force within the vessel

and the pulsatile wall motions to understand how these influence endograft durability.

211

C h a p t e r 11

S y n o p s i s

This thesis validates a new gold standard, 3D-spiral CT angiography with post-processing, for the

assessment and follow-up patients suitable for endovascular repair of their AAA. The difficulties

encountered with measurement of maximal diameter alone, suggest that volume measurements

should also become part of the routine assessment when describing aneurysms after endoluminal

repair. Using this technology, the intricate geometrical changes that affect aneurysm morphology

post-stenting can be shown to depend solely on the physical nature of the endograft (balloon

versus self-expanding/PTFE versus Dacron), a point that has previously not been recognised.

These observations may be used as a baseline in actual clinical practice and may also form the basis

for the timing of postoperative imaging. For example, the significant increase in sac volume to day

5 post-deployment would suggest that patients should have their first spiral CT scan at this time to

act as the baseline for comparisons with other scans during follow-up.

It is essential to state that these results do not commit to a relationship that may exist between a

critical volume and the risk of sac rupture. Volume has been examined purely in the context of

follow-up after endovascular repaie. This serves to demonstrate that the aim of aortic endografting,

which is to eliminate the threat of aneurysm rupture, is not necessarily synonymous with

aneurysmal shrinkage. PTFE-treated sacs appear ftozen in their natural history up to the medium-

term.

Previous work has suggested that aneurysms shrink along their longitudinal axes causing endograft

damage. From the results presented here, it can now be seen that aneurysm and graft lengths do

not shrink after endovascular repair, but instead may increase if unsupported grafts are deployed.

One consequence of this statement is that observations of medium to long-term endograft

buckling in situ must be due to reasons other than aneurysm contracture.

One possibility is the repeated minor trauma that must occur at the aneurysm neck due to

differential motion between the endograft and aortic wall. Over a period of time, this may result in

the disintegration of less robust constructions such as the Stentor endograft. This thesis has

identified aa accurate photographic method that may be used to investigate this hypothesis further.

Finally, there has been a clear demonstration of MRA as an alternative in the assessment of patients

for endovascular repair. Unfortunately, because of problems with volume rendering and loss of

wall definition with MIP images, the methodology is limited to cases where iodinated-contrast

injection is unsafe. For the time being, patients presenting for endovascular repair should be

212

assessed using SCTA as the sole imaging modality thus avoiding the complications of direct arterial

puncture seen with sizing-catheter angiography and intra-vascular ultrasound.

213

C h a p t e r 12

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243

A PPEN D IX I

LIST OF FIGURES

Viÿire number

Figure 1: Fragments of the Papyrus Ebers.

Figure 2: Bifurcated modular endograft system.

Figure 3: The Palmaz baUoon-expandable stent.

Figure 4: Upper end of Talent system showing two configurations.

Figure 5: The upper end of the EVT Aiicure graft.

Figure 6: CT scan of a patient with an upper stent endoleak.

Vage

9

13

14

15

16

31

Figure 7: Sites of endoleak after endovascular repair of aortic

aneurysms. 34

Figure 8: Cross section of aortic wall showing laminar arrangement of

elastin. 42

Figure 9: Haematoxylin and eosin stained section of aortic wall showing

thick tunica media. 42

Figure 10: Measurements required for the construction of an

endovascular graft. 50

Figure 11: Aortogram of the abdominal aorta in a patient with an

abdominal aortic aneurysm. 52

Figure 12: Principles of spiral CT scanning. 55

Figure 13: Demonstration of partial volume artefact. 55

Figure 14: Demonstration of manual editing of abdominal aortic

aneurysm from surrounding bone and soft tissues. 58

Figure 15: Three-dimensional modelling from spiral CT volume data. 65

244

Figure 16: Glass aneurysms used to validate spiral CT angiography data. 70

Figure 17: Pyknometer used to calculate fluid density. 70

Figure 18: A surface shaded display of an abdominal aortic aneurysm. 72

Figure 19: A multiplanar reformation (MPR) display. 72

Figure 20: Linear regression analysis. Anteroposterior diameters derived

from SCTA and postprocessing versus true anteroposterior diameters for

six phantom aneurysms. 76

Figure 21: Linear regression analysis. Transverse diameters derived ftom

derived from SCTA and postprocessing versus true transverse diameters

for six phantom aneurysms. 77

Figure 22: Linear regression analysis. Volumes derived from SCTA and

post-processing versus the true volumes of six phantom aneurysms. 78

Figure 23: 3D reconstruction of an abdominal aortic aneurysm treated

with an aorto-uni-iliac PTFE graft. 80

Figure 24: Spiral CT multiplanar reformation of glass aneurysm F filled

with contrast-deionized water and containing a central water-filled

balloon. 81

Figure 25: Hounsfield profile of a spiral CT slice taken from the glass

aneurysm in Figure 24. 83

Figure 26: Representative SCT slices taken from the phantom illustrated

in Figure 24. 84

Figure 27 a: Cross section of glass aneurysm F filled with deionized

water and containing a central contrast-deionized water-filled balloon. 84

Figure 27b: Graph of the HU profile across a Spical CT slice taken from

a glass phantom filled with water and containing contrast in it's central

balloon. 85

245

Figure 28: Graph showing the actual volumes within the central balloon

against the calculated volumes 86

Figure 29: In this graph, the percentage error obtained from scanning

different balloon volumes is plotted against the actual volumes. 87

Figure 30: Fluoroscopic image of a graduated angiocatheter. 93

Figure 31: Bland-Altmann graphs showing the degree of agreement

between SCTA and sizing angiocatheter. 97

Figure 32: Calculation of a Hounsfield profile using a spiral CT slice. 104

Figure 33: PTFE graft infection requiring removal and open repair. 107

Figure 34a: Column scatter graph showing the change in aneurysm

volume after endografring with PTFE. 108

Figure 34b: Column scatter graph showing the change in intra-

aneurysmal flow channel volume after endografring with PTFE. 108

Figure 34c: Column scatter graph showing the change in total aneurysm

volume after endografring with Talent. 109

Figure 34d: Column scatter graph showing the change in intra-

aneurysmal flow channel volume after endografring with Talent. 109

Figure 35: Graph showing changes in total aneurysm volume following

the onset of complications in patients with PTFE endografts. 110

Figure 36: This sequence of 3D reconstructions show the effects of

endografring with PTFE (top) and Talent (below) on aneurysm sac

morphology. 112

Figure 37: This graph shows the differences in preoperarive intraluminal

thrombus volume for the two populations used in this study. 116

Figure 38: Length measurements made on aneurysms. 121

246

Figure 39: Column scatter graphs showing the change in vertical body

and luminal centre line lengths that occur after exclusion with PTFE

endograft. 123

Figure 40: Graphs showing the changes in vertical body and luminal

centre line lengths that occur after exclusion with the Talent endograft, 124

Figure 41: PTFE endograft. 128

Figure 42: Method of calculation of neck diameters at three levels within

the neck of the aneurysm. 128

Figure 43: Graphs showing pixel intensities obtained from scanning

Phantom aneurysm A. 139

Figure 44: Graph and Table showing the results obtained with linear

regression analysis of true and scanned AP diameters obtained with

gMRA. 141

Figure 45: Graph and Table showing the results obtained with linear

regression analysis of true and scanned transverse diameters obtained

with gMRA. 142

Figure 46: Gadolinium-enhanced MRA of an abdominal aortic

aneurysm. 146

Figure 47 : The perspective view. 151

Figure 48: The projection of a 3D point onto the image plane for a

perspective camera. 153

Figure 49a: Radial distortion increases with distance from the principal

point. 153

Figure 49b: Relationship between object height and degree of radial

distortion. 155

Figure 50: The stereoscopic model. 156

247

Figure 51a: Demonstration of the calculation of absolute height of a

building with photogrammetry. 159

Figure 51b: Calculation of relief or difference in height using a stereo-

viewer. 160

Figure 52: An example of a single network consisting of 8 images that

cover the phantom aneurysm. 166

Figure 53: 3D model of glass phantom aneurysm F imaged using a

Kodak megaplus ES 1.6i digital camera. 166

Figure 54: Image rendering using Microstation software. 167

Figure 55: Surface shaded display of glass aneurysm F obtained from

Microstation using the image co-ordinates in Figure 53. 167

Figure 56: Effect of image point density on volume and other geometric

calculations. 169

Figure 57: Minimal curves generated by Surfer to form the 2 halves of

aneurysm F. 171

Figure 58: Graphs showing the three types of motion identified at the

neck of an aneurysm using high-speed video filming. 175

Figure 59: The calibration wall used in this experiment. 179

Figure 60: Digital photogrammetric workstation. 179

Figure 61a: A stereo-pair of photographs showing the neck of an

abdominal aortic aneurysm at open surgical repair. 180

Figure 61b: Pixel intensity matching to improve the quality of stereo

modelling 180

Figure 62a: Construction of a digital terrain map 181

248

Figure 62b: Digital terrain map showing the differences after subtraction

of two 3D-aneurysm terrain models 182

Figure 62c: Pictorial representation of the DTM in Figure 62b 182

Figure 63: This sequence of 2 photographs was taken 1 second apart.

The difference between the two images is shown in the third frame. 186

Figure 64: This vector map has been constructed from the subtracted

images in Figure 63. 186

Figure 65 a: Photographs from VMS showing the manual marking of

image points. 187

Figure 65b: Diagram showing the degree of motion estimated for each

image point calculated using VMS software. 187

Figure 66: The penumbra effect leads to blurring and overestimation of

the image diameter. 194

Figure 67a: During an angiogram, the rigid angiocatheter will not follow

the intraluminal path of an endovascular prosthesis. 196

Figure 67b: Accurate estimation of the flow channel length with SCTA. 196

Figure 68: The relationship between volume and maximum diameter. 198

249

A PPEN D IX II

LIST OF TABLES

Table number Page

Table 1; Yield and size of abdominal aortic aneurysms from 5 European

screening programmes. 22

Table 2: Early specific complications after conventional surgery for

abdominal aortic aneurysm. 24

Table 3: Characteristics of the neck that might be considered unsuitable

for endograft implantation. 25

Table 4: Clinical outcomes after endovascular repair in some of the

major series reported within the last 2 years. 28

Table 5: Anatomical classification of endoleak. 31

Table 6: Spiral CT angiography protocol used for the imaging of

patients and glass phantom aneurysms in this study. 73

Table 7: A Comparison of true and scanned diameters of glass phantom

aneurysms. 74

Table 8: Comparison of tme and scanned volumes of glass phantom

aneurysms. 74

Table 9: Volumes obtained after SCT scanning of glass aneurysm F

containing different volumes of water in a central balloon. 82

Table 10: Intra-observer and inter-observer coefficient of repeatability in

the measurement of aneurysm parameters. 91

Table 11: Comparison of aneurysm measurements between SCTA and

sizing catheter angiography. 94

Table 12: Values for r and P after Spearman’s correlation comparing

SCTA and sizing catheter angiography. 95

250

Table 13: Summary of the lack of agreement (bias) between SCTA and

sizing angiography. 98

Table 14: Precision of lack of agreement between SCTA and sizing

catheter angiography. 98

Table 15: Measurements carried out on the aneurysm sac (mm) after

endovascular repair. 102

Table 16: Starting sac diameters and aneurysm volumes for patients in

this study. 103

Table 17: Number of patients foUowed-up at each time interval within

the 2 groups selected for this study. 105

Table 18: Changes in aneurysm or flow channel volumes for patients

with PTFE or Talent endovascular grafts. 106

Table 19a: Changes in aneurysm maximal diameters for patients with

PTFE or Taient endovascular grafts. 110

Table 19b: This table shows the distance from the lowest renal artery (a

fixed anatomical point) to the level, within the aneurysm, at which each

maximal diameter was measured. I l l

Table 20: The effects of endografting with PTFE on volume of

intraluminal thrombus. 116

Table 21: The effects of endografting with Talent on volume of intra-

aneurysmal thrombus. 117

Table 22: Changes in aortic diameter at different levels with time. 125

Table 23: Measurements carried out on aneurysm patients (mm) 126

Table 24a: Change in median aneurysm neck length after endografting

with PTFE anchored with balloon-expandable stents. 127

251

Table 24b: Change in aneurysm neck length after endografting with the

Talent self-expanding system. 127

Table 25a: Change in upper neck diameter after endografting with

PTFE anchored using Palmaz stents. 129

Table 25b: Change in mid-neck diameter after endografting with PTFE

anchored using Palmaz stents. 129

Table 25c: Change in lower neck diameter after endografting with

PTFE anchored using Palmaz stents. 129

Table 26a: Change in upper neck diameter after endografting with self­

expanding Talent system. 130

Table 26b: Change in mid-neck diameter after endografting with self-

expanding Talent system. 130

Table 26c: Change in lower neck diameter after endografting with self­

expanding Talent system. 130

Table 27: The intra-observer and inter-observer differences obtained in

measurement of aneurysm neck length. 133

Table 28: The intra-observer and inter-observer differences obtained in

measurement of aneurysm neck upper diameter. 134

Table 29: The intra-observer and inter-observer differences obtained in

measurement of aneurysm neck lower diameter. 134

Table 30: A Comparison of true and scanned diameters of glass

phantom aneurysms. 138

Table 31: The true volumes of six phantom aneurysms compared with

their scanned volumes after gMRA. 140

252

Table 32: A comparison of the measurements obtained using SCTA and

gMRA in the preoperative assessment of abdominal aortic aneurysms for

endovascular repair. 148

Table 33: Data obtained from calibration of Kodak ES 1.6i digital

camera using a specially designed test field (Figure 59). 163

Table 34: VMS camera self-calibration data. 164

Table 35: Volume estimation of aneurysm F 168

253

APPENDIX III

CRITICAL VALUES FOR Z

The calculation of Z is shown in Chapter 7.3. The critical value of Z is noted below, where N is the

number of values in the group. If the value of Z is higher than the tabulated value, the P value is

less than 0.05.

N Critical Z N Critic

3 1.15 27 2.864 1.48 28 2.885 1.71 29 2.896 1.89 30 2.917 2.02 31 2.928 2.13 32 2.949 2.21 33 2.9510 2.29 34 2.9711 2.34 35 2.9812 2.41 36 2.9913 2.46 37 3.0014 2.51 38 3.0115 2.55 39 3.0316 2.59 40 3.0417 2.62 50 3.1318 2.65 60 3.2019 2.68 70 3.2620 2.71 80 3.3121 2.73 90 3.3522 2.76 100 3.3823 2.78 110 3.4224 2.80 120 3.4425 2.82 130 3.4726 2.84 140 3.49

254

255