Download - Radiation exposure in bone mineral density assessment

Radiation exposure in bone mineral density assessment

Christopher F. Njeha, *, Thomas Fuerst a, Didier Hans a, Glen M. Blakeb,Harry K. Genant a

aOsteoporosis and Arthritis Research Group, Department of Radiology, University of California, San Francisco, CA, U.S.A.bDepartment of Nuclear Medicine, Guy's Hospital, London, U.K.

Abstract

Osteoporosis is a systematic skeletal disease characterized by low bone mass and microarchitectural deteriorationof bone tissue which leads to diminished biomechanical competence of the skeleton and low-trauma or atraumaticfractures. Due to increased awareness of the impact of osteoporosis on the elderly population, the use of bone

densitometric techniques is becoming more widespread. Considerable progress has been made in the development ofnon-invasive methods for the assessment of the skeleton. While DXA and QCT are commonly used techniques, thepopularity of other approaches such as RA, SXA and QUS is gaining grounds. QCT has an advantage over the

other techniques in its ability to measure the true volumetric density of trabecular or cortical bone. We thereforepresent an overview of these current techniques for bone mineral density (BMD) measurements. In the secondsection we discuss the radiation doses incurred in BMD measurements by patients and methods for reducing patient

and sta� radiation exposure are given. Studies of radiation dose to patient from DXA con®rms that patient dose issmall (0.08±4.6 mSv) compared to that given by many other investigations involving ionizing radiation. Fan beamtechnology with increased resolution has resulted in increase patient dose radiation dose (6.7±31 mSv) but this is stillrelatively small. Carrying vertebral morphometry using DXA also incurs less radiation dose (<60 mSv) than

standard lateral radiographs QCT has radiation dose (25±360 mSv) comparable to simple radiological examinationsuch as chest X-ray but lower than imaging CT. Radiation dose from other techniques such as RA and SXA are inthe same order of magnitude as pencil beam DXA. For pencil beam DXA and SXA systems the time average dose

to sta� from scatter is very low even with the operator sitting as close as 1 m from the patient during measurement.However the scatter dose from fan beam DXA systems is considerable higher and approaches limits set by regulatorbodies for occupational exposure. # 1998 Elsevier Science Ltd. All rights reserved.

1. Osteoporosis and metabolic bone diseases

Osteoporosis is the most common metabolic bone

disorder. It is a systemic skeletal disease characterized

by low bone mass and microarchitectural deterioration

of bone tissue, with a consequent increase in bone fra-

gility and susceptibility to fracture (Anonymous, 1993).

Such fractures can occur in any site but are most com-

mon in the spine, wrist and hip, and all regions of the

skeleton with a high percentage of trabecular bone.

Osteoporosis has frequently been called the silent epi-

demic because it is asymptomatic and not clinically

manifested until a patient su�ers a fracture. The dis-

ease represents a major world-wide public health pro-

blem with an estimated 1.3 million fractures per year

in the U.S.A. In 1995, the estimated medical, nursing

home and social cost of osteoporotic fractures

exceeded $13.8 billion in the United States alone

(NOF, 1997). As life expectancy increases a resulting

increase in osteoporosis-related problems is expected.

Hence, the prevention of osteoporotic fractures with

consequent reductions in health care costs and excess

morbidity and mortality is an important clinical goal.

The susceptibility to fracture depends on a variety of

factors including the propensity to fall, visual acuity,

Applied Radiation and Isotopes 50 (1999) 215±236

0969-8043/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved.

PII: S0969-8043(98 )00026-8

AppliedRadiation andIsotopes

PERGAMON

* To whom all correspondence should be addressed.

protective response to falling and bone strength(Prudham and Evans, 1981; Kelsey and Ho�man,

1987). However, studies have shown that bone mass isone of the most important determinants of bonestrength and accounts for up to 80% of its variance

(Hodgskinson et al., 1997). Reduced bone mass, whichis readily accessible, is therefore a useful predictor ofincreased fracture risk (Ross et al., 1990). Many pro-

spective studies have shown that a decrease in bonedensity at the spine or hip of one population standarddeviation increases the risk by a factor of two to three

(Cummings et al., 1993). Methods of measuring bonemineral density are pertinent to the detection of lowbone mass (osteopenia), identi®cation of those individ-uals at risk of atraumatic fracture and assessment of

the e�cacy of the prevention and treatment of osteo-porosis. Estimation of spinal bone mineral contentfrom the visual assessment of bone density on conven-

tional radiographs is unreliable if vertebral fracturesare not present, since the subjective assessment is in¯u-enced by radiographic exposure factors, patient size

and ®lm processing techniques (Banks et al., 1995;Masud et al., 1996). Additionally the presence of clini-cal risk factors such as lifestyle, diet and family history

of osteoporosis is relatively ine�ective in predictingosteopenia (Cooper et al., 1991). These factors havenecessitated the need for objective, non-invasivemethods of bone density measurements. Such methods,

ideally, should be accurate, precise (reproducible), sen-sitive, inexpensive and involve a minimal exposure toionizing radiation.

In the past several decades, considerable progresshas been made in the development of methods forassessing the skeleton non-invasively. These include:

radiographic absorptiometry (RA), single photon andsingle X-ray absorptiometry (SPA/SXA), dual photonand dual X-ray absorptiometry (DPA/DXA), spinaland peripheral quantitative computed tomography

(QCT/pQCT), quantitative ultrasound (QUS), andquantitative magnetic resonance (QMR) and magneticresonance microscopy (mMR) (Genant et al., 1996;

Adams, 1997; Blake and Fogelman, 1997; Njeh et al.,1997a). These techniques vary in precision, accuracyand discrimination, and di�er substantially in funda-

mental methodology, clinical and research utility, andgeneral availability. We will limit our discussion tothose techniques that use ionizing radiation.

2. Technical principles of absorptiometry

2.1. Radiographic absorptiometry (RA)

This technique, also known as photo-densitometry,was the ®rst quantitative technique to assess integral(trabecular and cortical) bone mass (Morgan et al.,

1967). In RA, a standardized hand radiograph is takenwith an aluminum step-wedge placed on the ®lm and

analyzed using an optical densitometer. The bone min-eral density (BMD) is determined by comparison withthe de®ned density of the aluminum step-wedge. The

results are expressed in aluminum equivalent values orarbitrary units (Cosman et al., 1991; Matsumoto et al.,1994). RA is a low cost and readily accessible tech-

nique. The attenuation of an X-ray beam in an absorb-ing medium is highly dependent on its compositionand thickness. In RA applications, the variation in the

amount of soft tissue overlying the bone is notaccounted for. Therefore RA is restricted to the appen-dicular bones such as the metacarpals and phalangeswhich are surrounded by a relatively small amount of

soft tissue. The physical factors that in¯uence theradiographic image such as ¯uctuation in beam qual-ity, instability of X-ray source, ®lm response, proces-

sing conditions, radiation scattering and beamhardening e�ects also have adverse e�ects on precisionand accuracy of RA. Hence, RA implementation was

initially characterized by signi®cant precision errors ofabout 9±10% (Morgan et al., 1967). Recently devel-oped computer-assisted methods have reduced oper-

ator errors and improved precision (Matsumoto et al.,1994; Yang et al., 1994) (Trouerbach et al., 1987).Cadaveric studies have demonstrated that RA is accu-rate and suitable for the measurements of the BMD of

phalanges and metacarpals and is used in about 500centers worldwide (Yang et al., 1994). New systems arebecoming available that are entirely self contained and

highly automated.

2.2. Single photon absorptiometry

This was ®rst introduced by Cameron and Sorensonin 1963 (Cameron and Sorenson, 1963). A highly colli-mated photon beam from a single-energy radionuclide

source, such as 125I (photon energy 27.3 keV) or 241Am(60 keV) coupled to a radiation detector (usually asodium iodide crystal mounted on a photomultiplier

tube), is used to measure radiation attenuation at themeasurement site. To correct for overlying soft tissue,the anatomical site at which BMD is being measuredhas to be immersed in a water bath or surrounded by

water bags or water equivalent moldable materials(Adams, 1997). The method overcame the problemsfor RA caused by polychromatic X-rays (temporal and

spatial non-uniformity and beam hardening) and non-uniformity of ®lm sensitivity and development. Theamount of bone mineral in the beam can be derived

from Eq. (1) (Wahner and Fogelman, 1994).

MB � rB ln�I0=II��mBrB ÿ mSrS�

�1�

C.F. Njeh et al. / Applied Radiation and Isotopes 50 (1999) 215±236216

Where I0 is the transmitted intensity through soft tis-sue only, Ii is the transmitted intensity through bone

and soft tissue, m is the mass attenuation coe�cientand r is the density, the subscripts B and S refer tobone and soft tissue, respectively.

SPA was a widely used and established bone densitytechnique but it has now been superseded by single X-ray absorptiometry or peripheral dual X-ray absorptio-

metry.

2.3. Dual photon absorptiometry (DPA)

Single photon measurements are not possible at siteswith variable soft tissue thickness and composition (i.e.the axial skeleton, hip or whole body). Dual photonabsorptiometry (DPA) techniques were therefore intro-

duced to overcome the restriction of constant overallthickness of measurement site. This approach uses aradionuclide source, typically 153Gd with photon ener-

gies of 44 and 100 keV (Mazess and Barden, 1987).The simultaneous measurement of gamma radiation at

two di�erent energies allows for the correction of softtissue and fat of the torso without the need for a water

bath. The bone mineral density (g/cm2) can be derivedfrom:

MB � ln�IOL=IIL� ÿ �ln�IOH=IIH��mSL=mSH��mBL ÿ mBH�mSL=mSH�

�2�

Where symbols are as for the single photon case, with

the addition of subscripts L and H for lower andhigher photon energies, respectively. Eq. (2) is knownas the DPA equation.

2.4. Single X-ray absorptiometry (SXA)

Photon absorptiometric techniques are limited inthat the photon source is a radionuclide. This source

decays and requires regular replacement. It also has alow photon ¯uence which causes scanning times to belong and spatial resolution to be poor. These short-

comings have been overcome by the introduction of alow dose X-ray source. The physical principles of SXA

Fig. 1. A calcaneus SXA scanner (The OsteoAnalyzer SXA 3000) (Courtesy of Norland Medical systems).

C.F. Njeh et al. / Applied Radiation and Isotopes 50 (1999) 215±236 217

are the same as SPA except that the radionuclidesource has been replaced by an X-ray tube (55 kVp,

300 mA). This has imparted better precision andimproved spatial resolution to these systems and hasreduced examination time (Kelly et al., 1994). SXA

makes possible a quantitative assessment of bone min-eral content at peripheral sites of the skeleton (e.g. dis-tal or ultradistal radius and calcaneus) (Fig. 1). SXA

has proven to be a valuable method in the diagnosis ofosteoporosis, providing reasonable precision andexceptionally low radiation exposure. However,

recently peripheral DXA systems have been supple-menting SXA systems, eliminating the need for a waterbath or water bolus.

2.5. Dual X-ray absorptiometry (DXA)

2.5.1. Pencil beam DXADual X-ray absorptiometry (DXA) is based on the

method of X-ray spectrophotometry developed in the1970s. It was introduced commercially as the directsuccessor to DPA in 1987. Similar to DPA the funda-mental physical principle behind DXA is the measure-

ment of the transmission of X-rays with high- andlow-photon energies. The main advantages of an X-ray

system over a DPA radionuclide system are shortened

examination time due to an increased photon ¯uence

of the X-ray tube and greater accuracy and precision

resulting from higher resolution and removal of errors

due to source decay (Kelly et al., 1988). The preferred

anatomic sites for DXA measurement of bone mineral

include the lumbar spine, the proximal femur and the

whole body, but peripheral sites can also be scanned.

Using the dependence of the X-ray attenuation coe�-

cient on atomic number and photon energy, measure-

ment of the transmission factors at two di�erent

energies enables the areal densities (i.e. mass per unit

projected area) of two di�erent types of tissues to be

inferred. In DXA scans these are taken to be bone

mineral (hydoxyapatite) and soft tissue, respectively.

This basic principle can be explained simply using

Eq. (2).

Unlike monoenergetic radionuclide sources, the

bremsstrahlung X-rays consist of a broad spectrum of

photon energies, with beam hardening as a potential

source of error. Two methods of generating dual

energy X-ray spectrum have been implemented using

either K-edge ®lters or kVp switching. K-edge absorp-

tion ®lters made of a rare earth material such as cer-

ium (Ce) and samarium (Sm) are used to split the

Fig. 2. A pencil beam DXA Scanner (Norland XR-36) illustrating total body patient positioning (Courtesy of Norland Medical

Systems).


polyenergetic X-ray beam into high and low energy

components that mimic the emissions from 154Gd. The

two components have inherently narrow spectral distri-

bution and hence the problems associated with beam

hardening are minimized. Lunar DPX (Lunar,

Madison) systems have a cerium ®lter and use pulse

height analysis at the detector to discriminate between

high and low energy photons. Norland XR (Atkinson,

Wisconsin) systems use a samarium ®lter and separate

detectors for high and low energy X-rays (Blake and

Fogelman, 1997).

The second way of producing a dual energy X-ray

beam is to switch the high voltage generator between

high and low kVp during alternate half cycles of the

mains supply. Hologic (Waltham, U.S.A.) uses this

method in its QDR series of DXA systems (Wahner

and Fogelman, 1994). The spectra distribution is wider

than with the K-edge ®lter method and the consequent

e�ect of beam hardening is corrected by a rotating

calibration wheel containing bone and soft tissue

equivalent ®lters that measure the attenuation coe�-

cient in Eq. (2) and calibrates the scanned image pixel

by pixel (Blake et al., 1992). Pulse height analysis is

not required, giving the instrument an inherently wide

dynamic range.

First generation DXA systems, such as the Hologic

QDR 1000, Norland XR-36 (Fig. 2) and Lunar DPX

scan patients rectilinearly using a pin hole collimator

producing a pencil beam coupled to a single detector

in the scanning arm. When a DXA scan is analyzedthe basic data process create a pixel-by-pixel map of

BMD over the entire scanned ®eld calculated fromEq. (2). An edge detection algorithm is ®rst used to®nd the bone edges. The total projected area of bone

is then derived by summing the pixels within the boneedges and a reported value of BMD calculated as themean BMD over all the pixels identi®ed as bones.

Finally, bone mineral content (BMC) is derived bymultiplying BMD by projected area.

2.5.2. Fan beam DXA

A recent signi®cant development in DXA technologyhas been the introduction of a new generation of sys-tems, such as the Hologic QDR 4500 and the Lunar

Expert-XL (Fig. 3), which use a slit collimator to gen-erate a fan beam coupled to a linear array of detectors.Fan beam studies are acquired by the scanning armperforming a single sweep across the patient instead of

the two dimensional raster scan required by pencilbeam geometry. As a result, scan time has been shor-tened from about 10 to 5 min for the early pencil beam

scanners to 10 to 30 s for the latest fan beam system.Shorter scan time and higher patient throughput,around two±three times that for ®rst generation scan-

ners, are the major advantages of the fan beam DXAsystem. Another advantage of the fan beam system isthe higher image resolution. This allows easier identi®-

Fig. 3. A fan beam DXA Scanner (Lunar EXPERT-XL) (Courtesy of Lunar Corp.) illustrating measurement positioning.


cation of vertebral structure and artifacts resultingfrom degenerative disease.

2.5.3. pDXA

DXA is also employed for measurements of theappendicular skeleton. Most standard DXA densi-tometers allow for highly precise measurement of the

radius or calcaneus using regions of interest like thosederived from SPA and SXA measurements as well asuser-de®ned subregions (Faulkner et al., 1994; Yamada

et al., 1994). Recently peripheral DXA (pDXA) densi-tometers specially designed for the forearm or calca-neus have been introduced and may provide these

measurements at a lower cost (Fig. 4). They have theadvantage compared with SXA systems that they dis-pense with the need for a water bath.

2.6. Quantitative computed tomography (QCT)

2.6.1. Spinal QCTQCT is the only method available with the ability to

determine in three dimensions the true volumetric den-sity (mg/cm3) of trabecular or cortical bone. Generally,spinal QCT is performed on standard clinical CT scan-

ners. An external bone mineral reference phantom isused to calibrate the CT number measurements tobone-equivalent values. Special software is used toanalyze the image and place regions of interest inside

the vertebral bodies. Typically two to four vertebra inthe range of L1±L3 are evaluated. Based on an initiallateral scout image, single 8±10 mm thick sections are

obtained through the mid-plane of each of these ver-tebrae using a low-dose technique and the gantry

angled parallel to the vertebral endplates. In order toimprove precision and reduce acquisition and analysistime, the sagittal location of mid-vertebral slices and

the axial placement of regions of interest can be highlyautomated (GluÈ er et al., 1988; Kalender et al., 1988).Due to the high responsiveness of spinal trabecular

bone, and its importance for vertebral strength, QCThas been principally employed to determine trabecularbone density in the vertebral centrum (Genant et al.,

1982). The two-dimensional QCT methods cannot beused to assess the proximal femur because of the proxi-mal femur's complex architecture and dramatic three-dimensional variation in its density. QCT can be per-

formed in single-energy (SEQCT) or dual-energy(DEQCT) modes, which di�er in accuracy, precisionand radiation (Genant and Boyd, 1977). Although it is

possible to improve accuracy by employing DEQCT,this approach incurs reduced in vivo precision andhigher dose, and thus is recommended only for

research studies that require higher accuracy (VanKuijk et al., 1990). The validity of QCT for measure-ment of vertebral cancellous bone is widely accepted

and it is available at over 4000 centers worldwide.

2.6.2. Peripheral quantitative computed tomography(pQCT)

Special purpose pQCT scanners have been employedfor measurements of BMC and BMD of the peripheralskeleton. Initially, a radionuclide source (usually 125I)

Fig. 4. Peripheral DXA scanner (Lunar PIXI) illustrating forearm measurement position (Courtesy of Lunar Corp.).


was used. However, state of the art scanners employ

X-ray sources (Genant et al., 1996; Gluer et al., 1997).

pQCT allows for a true volumetric density measure-

ment of appendicular bone without superimposition of

other tissues and provides exact three-dimensional

localization of the target-volume. Ease of use and the

ability to separately assess cortical and trabecular bone

and to measure BMD, BMC, and axial cross-sectional

area make the method an interesting alternative to

SPA or SXA. With the commonly used clinical pQCT

scanner, measurements in the distal radius are per-

formed at only one site with a single axial slice of

2.5 mm thickness located at the level that represents

4% of the ulnar length from the distal radial cortical

endplate. Modern pQCT scanners also incorporate a

multi-slice data acquisition capability covering a larger

volume of bone as compared with the commonly used

single slice technique (Kaisel and RuÈ egsegger, 1991).

The measurement of several slices is potentially more

representative of changes in the distal radius and may

therefore re¯ect the bone status of an individual more

accurately. If studies employing this multi-slice pQCT

technique are successful, they may contribute to more

extensive use of this already promising technique.

2.7. Sites measured

The choice of the optimum skeletal site for the pre-diction of osteoporosis is a subject of great debate.However, the consensus is that sites with higher con-

tents of cancellous (high turnover) bone are more sen-sitive to osteoporotic changes. Longitudinal studiessuggest that most of the skeletal sites currently

measured (spine, femur, radius and calcaneus) are use-ful for predicting the risk of fracture at any site. Thebest assessment of risk at a speci®c site, however is to

measure the BMD at that site (Miller et al., 1996).

2.7.1. Lumbar spine

For the lumbar spine the patient is placed in the su-pine position on the scanning table, with the hips andknees ¯exed over a support to eliminate lumber lordo-

sis and to ¯atten the spine against the table top(Fig. 3). DXA systems normally measure the lumbarspine from L1±L4 in either PA or AP projection

(Fig. 5). In DXA of the spine, the BMD is the arealdensity of the integral bone which includes the ver-tebral body. Extraneous calci®cation such as in the

walls of the aorta, particularly with degenerative discand apophyseal joint disease with consequent hyperos-

Fig. 5. A spine DXA scan showing the ROI analyzed (Lumbar spine L1±L4).


tosis, will cause inaccuracies and over estimation of

BMD (Franck et al., 1995). Falsely elevated spinal

BMD on PA-DXA may be caused by other etiologies

such as vertebral wedge (crush fracture), Paget's dis-

ease of bone, sclerotic metastases and vertebral haemo-

globin.

The development of lateral DXA scanning of the

lumbar spine was aimed at reducing the errors intrinsic

in the posteroanterior examination of the lumbar

spine. A lateral examination of the lumbar spine

makes possible an evaluation of the vertebral body

also ensuring almost exclusive measurement of trabecu-

lar bone. Therefore, the correlation between lateral

DXA and quantitative computed tomography (QCT),

both measures of the vertebral body, has been found

to be stronger than that between PA-DXA and QCT

(Yu et al., 1994). However, overlap of the iliac crest at

level L4 and ribs at level L2 may substantially increase

the measured bone density. Nevertheless, the inclusion

of L2 through L4 usually yields the best precision and

diagnostic sensitivity (Rupich et al., 1992; Jergas et al.,

1995).

2.7.2. Proximal femur

In scanning the femur, the leg is slightly abducted

and internally rotated using a positioning device in

order to bring the femoral neck parallel to the scan

table. This avoids foreshortening of the femoral neck

which would cause BMD to increase. Positioning of

the femoral neck is therefore critical to maintaininggood precision. The proximal femur is usually

measured with the BMD assessed at the femoral neck,

trochanter, Ward's triangle and total hip (Fig. 6).

Ward's triangle measures the earliest site of postmeno-

pausal bone loss in the hip and should in theory give

the best measure of trabecular bone in the proximal

femur. However, poor precision has limited the utility

of this site. Femoral neck BMD has been the hip par-

ameter most frequently used for making the diagnosisof osteoporosis.

The proximal femur's complex architecture and wide

3D variation in its density, precludes the use of QCT

methods widely used in the spine. However, it is

believed a well de®ned volume of interest selectivelymeasuring trabecular and cortical bone, as provided by

QCT, may have diagnostic signi®cance (Lang et al.,

1997).

Fig. 6. A proximal femur DXA scan, showing the ROI analyzed-femoral neck (oblong box), Ward's area (box) and trochanter.


2.7.3. Peripheral sites

Peripheral sites measured include: distal forearm,

calcaneus and phalanges. These sites are accessible for

SXA, DXA and some pQCT measurements. For the

distal radius, DXA scanning is performed with the

patient sitting on a chair next to the scanner table with

the forearm resting on the table top, the hand pro-

nated and, on some scanners, secured on the position-

ing board with a restraining strap (Fig. 4). BMD

measurements are made in the ultradistal, distal (Mid-

radius) and shaft (1/3-radius) (Fig. 7). The ultradistal

site is used because it contains the highest percentage

of trabecular bone in the forearm.

The calcaneus is also a site of interest, because it is

load-bearing and has a high content of cancellous

bone (Vogel et al., 1988). There are new pDXA ma-

chines such as the PIXI (Lunar) dedicated to measur-

ing the calcaneus and forearm. The phalanges on the

other hand are measured mostly by radiographic

absorptiometry techniques. However, special software

on DXA devices allows measurements of hand BMD.

This is of special interest for the study of rheumatoid

arthritis where the hand is a main site of involvement

(Peel et al., 1994).

2.7.4. Total body

Total body DXA is of interest because it o�ers a

comprehensive view of changes across the whole skel-

eton and in certain respects supplements calcium bal-

ance studies (Fig. 8). For precise results it is imperative

that the patient be placed on the scanning table in su-

pine position, with all parts of the body including the

arms included in the scan ®eld. Total body scans

measure BMC and average BMD in the total skeleton

together with subregions that include the skull, arms,

ribs, thoracic and lumbar spine, pelvis and legs (Herd

et al., 1993). Total body DXA has interesting appli-

cation in body composition (Laskey, 1996). In those

areas of a whole body scan where the X-ray beam

does not intersect bone, it is possible to use the R-

value (ratio of the attenuation at the two energies) to

estimate separately the masses of fat and lean tissue

(Pietrobelli et al., 1996). Over bone, however, only

BMD and total (fat and lean) soft tissue mass can be

measured. Extrapolation of measurements of percen-

tage of body fat in soft tissue over adjacent bone

means that a whole body DXA scan can provide esti-

mates of total body fat and lean mass as well as BMC

(Blake and Fogelman, 1997).

Fig. 7. A forearm scan showing the ROI analyzed, UD=ultradistal, MID=midshaft and 1/3,=shaft.


Fig. 8. DXA whole body scan with regional areas of analysis.


2.7.5. Vertebral morphometry

As a result of the relatively high resolution of fan-

beam DXA scanners anatomic details of the examined

region are depicted clearly. Using DXA to obtain lat-

eral images of the lumbar spine allows the scanning

beam Ð in contrast to conventional cone beam radi-

ography Ð to be generally parallel to the vertebral

endplates. This allows a better de®nition of vertebral

dimensions for a morphometric analysis, even though

resolution and signal to noise ratio are both worse

than X-ray. In reference to the DXA approach, this

method has been called morphometric X-ray absorp-

tiometry, or MXA (Steiger et al., 1994). Overlying

structures such as ribs or the iliac crest may have an

adverse e�ect on the morphometric analysis. To

enhance the accuracy of MXA, technical modi®cations

of the X-ray tube and the detector system may provide

images with higher resolution thereby enhancing the

analysis of vertebral deformities. These techniques are

still in the developmental and early clinical evaluation

phase.

3. Radiation dose to the patient

Previous sections have demonstrated the usefulness

of measuring BMD with the absorptiometric technique

in the assessment of bone integrity. Studies of the radi-

ation dose to patients from absorptiometry scans have

con®rmed that patient exposure is small compared to

many other sources of exposure including most radio-

logical investigations involving ionizing radiation

(Kalender, 1992; Lewis et al., 1994; Huda and Morin,

1996; Njeh et al., 1996). Due to the low radiation

doses encountered in BMD measurements, there have

been limited studies on doses incurred in this examin-

ation. In cases where doses have been measured, only

the entrance surface dose (ESD) has been reported

(Table 1). ESD does not permit a direct comparison of

the patient doses associated with absorptiometric and

other radiological examinations. Also, as explained

elsewhere in this volume the radiation hazard to the

patient is often expressed in terms of the e�ective dose

(ED). This is de®ned as the sum of the absorbed doses

Table 1

Manufacturer reported Entrance Skin Dose (ESD) for some of the more common densitometers

Technique Manufacturer Model Site ESD (mGy) Comments

DXA Hologic QDR 1000/1500 AP spine/femur <43 70 and 140 dual output, 43 and 110 keV

Total body <13

QDR 2000 AP spine/femur 192 fan beam mode

lateral spine 237

total body 8

DXA Lunar DPX-L/DPX-IQ AP spine/femur 11$ 76 kVp with cerium ®lter, 38 and 70 keV

Lateral spine 77

total body 0.2

EXPERT-XL Spine/femur-5 mA fast 530 Fan beam

Total body 1.5 mA fast 50

Forearm/hand 120

DXA PIXI Calcaneus/forearm <200

SPA Lunar SP2 forearm 150 7.4 GBq 125I

DXA Norland XR-36 AP spine 0.9±44.4* 100 kVp with samarium ®lter, 80 and 46.8 keV

Femur 1.8±89*

lateral spine 123

Total body 0.4

pDXA Norland pDEXA radius and ulna <25 60 kVp, tin ®lter, 28 and 48 keV

SXA Norland SXA300 calcaneus 13 36 kV, tin ®ltered, 27 keV

DXA Sopha Medical L-XRA AP spine 20 80 kVp with neodymium ®lter, 43 and 70 keV

lateral spine 40

pQCT Stratec XCT 960 Forearm 30 45 kVp, 0.3 mA, 38.5 keV

pDXA Osteometer DX-200 Forearm 55 kVp, 0.3 mA, tin (Sn) ®lter

DXA DMS Challenger Spine/femur <3.3 samarium ®lter, 35 and 75 kev

*The Norland XR-36 has a dynamic ®ltration which compensates for di�erent tissue thickness, so the ESD is dependent on the body

thickness of the patient.$ESD for DPX-IQ is similar to DPX-L, and is also dependent on the tube current, resolution and scan speed used. The ESD is linearly

proportional to these parameters if everything else remains constant.


to each irradiated organ weighted for the radiationtype and the radio-sensitivity of that organ (ICRP,

1991). The ED is equivalent to the uniform wholebody dose that will put the patient at equivalent riskfrom the carcinogenic and genetic e�ects of radiation

(ICRP, 1991). For low doses, as encountered in DXA,the principal risks to patients are the stochastic processof carcinogenesis and genetic e�ects. The following sec-

tions will discuss doses incurred in BMD measure-ments from di�erent techniques.

3.1. Patient doses from DXA

The entrance radiation dose from a DXA examin-

ation is determined by machine dependent parameterssuch as source-detector geometry, focal spot size,source collimation, scan speed, tube current and sourcespectra. The operator can only change the mA and the

scan speed (resolution). However, the mA and scanspeed use can a�ect the precision of BMD measure-ment. E�ective dose from DXA measurements have

been reported in the literature (Pye et al., 1990;Kalender, 1992; Rawling et al., 1992). However, Lewiset al. (1994) presented the ®rst extensive e�ective dose

estimates for Hologic QDR 1000 and QDR 2000 DXAscanners. Similar studies have been reported by Njehet al. (1996) and Bezakova et al. (1997) for Lunar ma-

chines. Due to di�erences in the production of X-raysby these systems, their respective EDs are expected tobe di�erent. Lewis et al. reported the highest ED forthe total body of 3.6 mSv (4.6 mSv including the ovar-

ies) (Table 2). ESDs were found to be proportionate to

the speed of the scanning arm of QDR 1000. Njeh etal. (1996) and Bezakova et al. (1997) observed lowervalues of e�ective dose for a Lunar DPX-L machine

(Table 3). Clinically patients on one visit may receivescans of a number of di�erent regions (generally two

but not more than three) such as the posterior±anter-ior spine, lateral spine, left and right proximal femur,

hand, forearm, calcaneus and total body. The totale�ective dose per visit will therefore be a summationof the regions scanned. For example, a postmenopau-

sal patient undergoing scans of the spine and hip per-formed on a QDR-1000 would receive an e�ective

dose of 0.6 mSv (0.5 spine and 0.1 femur). However, apremenopausal woman scanned on the same scanner

would receive an e�ective dose of 1.9 mSv because ofinclusion of the ovaries. On the other hand, a postme-

nopausal woman and premenopausal woman scannedon a Lunar DPX-L will receive an e�ective dose of0.29 and 0.36 mSv, respectively.In lateral DXA scans, mAs values and therefore sur-

face dose values are increased typically by a factor of

2±4. However, due to the fact that organ doses are sig-ni®cantly lower relative to the ESD for the lateral

beam and that the length of the spine scanned is shor-tened in most cases, e�ective dose values for lateral

scans are comparable to PA projection scans. Lewis etal. (1994) reported EDs of 0.5 and 0.6 mSv for AP andlateral spine, respectively.

DXA measurement, if accompanied by a lateralradiograph, will signi®cantly change the magnitude of

Table 2

Scanning times, entrance surface doses and e�ective doses for postmenopausal women for a pencil beam mode

scans performed on the QDR-1000 (Hologic)

Scan type Scan time (min) Entrance surface dose (mGy) E�ective dose (mSv)

Total Body 17 18 3.6 (4.6)

AP spine (L1±L4) 8 60 0.5 (0.5)

Lateral spine(L2±L4) 20 238 0.6 (0.6)

Proximal femur 6 60 0.1 (1.4)

Distal forearm 6 113 0.07 (0.07)

Values in bracket includes the ovaries, therefore representing premenopausal women. Data from Lewis et al.

(1994).

Table 3

Entrance surface dose (ESD) and e�ective dose (ED) for Lunar DPX-L DXA scanner

E�ective dose (mSv)

Ref. Mode Entrance surface dose (mGy) PA-spine femur

(Njeh et al., 1996) Medium, 0.75 mA 10.25 0.21 0.08 (0.15)

(Bezakova et al., 1997) Medium, 0.75 mA 11.5 0.19 0.023 (0.14)


the dose involved. For a conventional lateral X-ray®lm examination, Kalender (1992) estimated an ED of

700 mSv for 90 kVp and radiation ®eld height of 25 cm.For larger ®elds, values of 2000 mSv can be expected(Richardson, 1990). Thus an e�ective dose of up to

2000 mSv will result from a DXA scan plus X-ray ®lm.On the other hand, Huda and Morin (1996) estimatedED for 80 kVp AP and lateral X-ray examination to

be in the range of 40±120 and 60±190 mSv, respectively.They assumed an ESD range of 2±4 mSv and an X-raybeam area between 400±600 cm2 at the patient

entrance.The dose associated with DXA BMD measurement

can be considered low or even insigni®cant in compari-son with natural background radiation levels. It is well

below the background value of about 7 mSv per day.

3.2. Patient doses from fan beam DXA

Fan beam DXA scanners were introduced to reducescanning time and increase image resolution. A conse-

quence has been an increase in radiation dose due tothe increase in photon ¯ux required. In the LunarExpert there is an increase in photon ¯ux by using a

higher tube potential (134 instead of 76 kVp) and ahigher tube current (5 instead of 0.75 mA). HologicQDR 4500 operates at 140 and 100 kVp compared to

140 and 70 kVp for QDR 1000. Entrance surface doses

have been reported for both the Lunar Expert, Expert

XL and Hologic QDR 2000 and QDR 4500. Stewart

et al. (1996) reported an ESD of 0.7, 1.1 and 0.12 mSv

for spine/femur, lateral spine and peripheral sites, re-

spectively, for the Lunar Expert-XL. E�ective dose for

these fan beams have been reported in the literature

(Lewis et al., 1994; Njeh et al., 1996; Blake and

Fogelman, 1997) (Table 4 and 5). The fan beam has

led to about a 100 fold increase in dose for the Lunar

and 10 fold increase for the hologic scanner. One of

the reasons for the di�erence in e�ective doses between

the Hologic QDR 4500 and the Expert-XL is that for

the Expert-XL the spine measurements are performed

in the AP direction. Hence the distance to the sensitive

organs such as the stomach is reduced and results in

approximate doubling of ED.

Despite the increases associated with advances in

technology, patient dose from DXA remains very low

when compared with natural background radiation

and from other common radiological procedures

(Fig. 9). For example the mean ED per radiograph for

AP lumbar spine X-ray examination has been quoted

as 690 mSv (Shrimpton et al., 1998) which are about

10 fold higher than fan beam ED. On the other hand

the fan beam DXA EDs are comparable to 17 mSvreported for chest PA X-ray examination (Shrimpton

et al., 1998).

Table 4

Scan times and doses (where values in brackets include the ovaries) for a Hologic QDR-2000 fan beam mode

reported by Lewis et al. (1994)

Scan type Scan time (min) Entrance skin dose (mGy) E�ective dose (mSv)

PA Spine (L1±L4) 0.1 57 0.4 (0.4)

1 138 0.9 (0.9)

2 271 1.8 (1.8)

3 432 2.9 (2.9)

Lateral Spine (L2±L4) 3 684 1.2 (1.2)

6 1390 2.5 (2.5)

Proximal femur 1 138 0.3 (3.0)

2 271 0.6 (5.9)

Total body 6 11 2.7 (3.6)

Table 5

Doses from Fourth Generation (fan beam) DXA scanner


DXA system Mode Entrance surface dose (mGy) PA spine femur Ref.

QDR 4500 Array (1 min) ÿ 6.7 (Patel et al., 1996b)

QDR-4500 ÿ 295 8 ÿ (Starritt et al., 1996)

Expert-XL 5 mA Fast 700 73 (max scan size) ÿ (Stewart et al., 1996)

Expert Medium (40 s) 895 31 4.7 (32.0) (Njeh et al., 1996)


3.3. Doses from vertebral morphometry using DXA

Lateral radiographs have a better spatial resolution

than DXA vertebral morphometry or MXA. However,

one advantage of MXA is its signi®cantly lower radi-

ation dose to the patient. Estimates of the e�ective

dose to a patient from MXA have been reported by

Lewis et al. (1994). Blake et al. (1997) observed that

although the scan length for a MXA study was ap-

Fig. 9. (A) Comparison of e�ective dose to the patient from DXA and some common radiological examinations. (B) A comparison

of time averaged scatter dose to an operator positioned 1 m from the scanning table for four DXA systems (data from Blake and

Fogelman, 1997).


proximately 3 times greater than a bone density scanof the lumbar spine, patient dose is approximately 10

times higher (assuming the same scan mA, collimationand scan speed) because of the inclusion of the lungsand breast in the scan ®eld. Table 6 shows that the

highest dose of 51 mSv is incurred from the mediummode using QDR 4500. This is signi®cantly lower thanan approximate dose of 600±800 mSv from lateral

radiographs (combined thoracic and lumbar)(Shrimpton et al., 1998).

3.4. Pediatric doses from DXA

Di�erent diseases such as juvenile rheumatoid arthri-

tis (Falcini et al., 1996) may directly or indirectlydecrease the bone mineral content in children.Therefore, an assessment of bone integrity may assistin the choice of the right therapeutic strategy and best

follow up for patients with diseases a�ecting bonemetabolism. Also, bone mass accrued during childhoodhas been proposed as a determinant of an individual's

susceptibility to osteoporotic fractures in adulthood(Seeman et al., 1993). DXA is presently the mostwidely used technique for the assessment of bone integ-

rity. As previously stated, DXA can also be used tostudy other aspects of body composition such as per-

centage of fat and lean body mass (Laskey, 1996).

Njeh et al. (1997b) assessed the dose incurred from

DXA scanning of a 5-year-old child and a 10-year-old

child using a Lunar DPX-L scanner. They measured

the entrance surface dose and percentage depth doses

for the total body and PA spine scan modes using

lithium borate thermoluminescent dosemeters located

at the surface and distributed throughout various

organ locations in anthropomorphic child phantoms.

The EDs were calculated from the percentage depth

doses, amount of each organ irradiated and tissue

weighting factors (ICRP, 1991). The results are rep-

resented in Table 7. The EDs are more than two

orders of magnitude lower than reported ESDs and

EDs for pediatric chest X-rays. Fewer studies have

reported DXA pediatric dose. Koo et al. (1995)

reported an ESD of 3 mGy for pediatric total body

using the Hologic 1000/W. Faulkner et al. (1993)

reported 10 mGy ESD and e�ective dose equivalent of

2.7 mSv for total body mode using the Hologic QDR-

2000 (fan beam mode). This is similar to adult total

body ED reported by Lewis et al. (1994) for the same

type of scanner. Therefore, one could extrapolate that

similar EDs could be derived for the pediatric mode as

were reported for the adult scanning mode, due to pro-

Table 6

E�ective dose for vertebral morphometry scans performed on DXA bone densitometer and on

conventional radiographs


System Scan mode PA centerline scan MXA scan

QDR-2000 34

QDR-4500 Dual-energy fast 4 (30 s) 20 (5 min)

QDR-4500 Dual-energy medium 10 (90 s) 41 (10 min)

QDR-4500 Dual-energy hi-res 20 (3 min) 20 (10 min)

QDR-4500 Single-energy ÿ 2 (10 s)

Expert-XL 5 mA fast ÿ 38 (40 s)

Lateral Radiograph ÿ ÿ 800

Scan times are given in brackets. For QDR-4500 study requires the acquisition of both a PA

centerline and a lateral morphometry scan. Data from (Blake et al., 1997) and (Rawlings et al.,

1996) for QDR 2000. hi-res=high resolution.

Table 7

E�ective dose for a pediatric scan mode using a lunar DPX-L (Njeh et al., 1997b)

Scan Mode Patient size Age

Scan time

(min) ESD (mGy) ED (mSv)

PA spine 6±16 cm 5 5 6.0 0.28

PA spine 10 6.0 0.20

Total body medium 15±25 kg 5 9 0.12 0.03

total body large 25±35 kg 10 12 0.1 0.02


portionate reduction in the scan ®eld for the pediatric

mode.Few studies have reported the e�ective doses for

common pediatric radiological examinations. However,ESDs for common pediatric radiological examinationshave been reported (Ruiz et al., 1991; Chapple et al.,

1993; Martin et al., 1994; Kyriou et al., 1996). ESDsare in the range of 50±400 mGy for PA chest X-ray

and 420±1990 mGy for an abdominal X-ray examin-ation. Njeh et al. (1997b) used these ESD values andapplied NRPB-SR279 (Hart et al., 1996) Monte Carlo

data to estimate the EDs. The results are presented inTable 8 which in comparison with Table 7 shows that

doses encountered in DXA are more than two ordersof magnitude less than common radiological examin-ations.

Exposure of children to ionizing radiation givesmore grounds for concern than that for adults, because

for children there is a longer period available for thedelayed e�ects of radiation to manifest. This meansthat the lifetime risk of radiation-induced cancer per

unit dose to the child is likely to be higher than for theadult. The probability of aggregated radiation detri-

ment can be related to ED and for children this is ageand sex related. For a 10-year-old female, the total life-

time risk of radiation-induced cancer is estimated as16�10ÿ2 Svÿ1 compared with 5.9�10ÿ2 Svÿ1 for a30-year-old adult (NRPB, 1993). A pediatric subject

referred for a bone densitometry examination mayreceive a PA spine and total body scan. The total e�ec-

tive dose will then be 0.31 mSv, giving a lifetime risk of5�10ÿ8 for fatal cancer. This is negligible comparedwith the possible bene®ts from the scan. In addition to

carcinogenesis, genetic e�ects are also of concern inthe total body examination because of the irradiation

of the gonads. The estimated dose to the ovaries was0.002 mSv, which is 9% of the total dose. Even thoughthese doses are quite low, good radiological practice

should be implemented.

3.5. Patient doses from QCT

As mentioned earlier, spinal QCT for BMDmeasurement can use standard CT machines or those

with specialized software. As discussed by Jessen et al.(1998) CT has problems with speci®cation of radiation

doses imparted. The main problem is the non-uniform-ity of dose distribution within the patient. This isbecause in most CT examination a number of CT

slices may be contiguous or overlapping. The radiationdose to patients undergoing QCT depends upon the

following factors: (1) scan parameters-kVp, mAs, slicethickness, number and spacing of slices, (2) patientsize, (3) X-ray source/detector geometry; and (4) detec-

tor collimation and calibration. Hence the dose deliv-ered is highly scanner speci®c. The X-ray beam of the

QCT systems has a slice thickness in the range of 8±10 mm; the radiation received by the patient is there-fore limited to a narrow band. BMD assessment using

CT involves in most cases 3-slices.The radiation dose from QCT has been reported

over the years, however, there has been no consistencyin the parameter reported-entrance surface dose, CT

dose index (CTDI), mean depth dose, etc. Banks andStevenson (1986) reported the approximate mean sur-face dose (multislice, 125 kVp, 230 mAs) as 16 mGy.

However, Kalender (1992) reported a surface dose of2.5 mGy for a SOMATOM Plus (80 kVp, 125 mAs).

Note that the scan parameters are di�erent for thesetwo studies. This illustrates the fact that low dosevalues can be achieved via low kVp and low mAs.

Changing from 120 to 80 kVp with ®xed mAs settings,Kalender (1992) was able to measure a dose reduction

of a factor of about 4. QCT BMD examinations arealways accompanied by a lateral scout radiograph

(tomogram) which is required to ensure reproduciblepositioning and involves an additional radiation dose.Kalender (1992) measured an ESD of 0.46 mGy for

lateral scout radiographs. He further estimated the ED

Table 8

Reported mean ESDs per radiograph and calculated EDs using Monte Carlo simulation for two common radiological examination

for comparison with Table 7 (Njeh et al., 1997a,b)

PA chest Abdomen

Ref Age (years) ESD (mGy) ED (mSv) ESD (mGy) ED (mSv)

(Martin et al., 1994) 1±5 50 5.2 470 89.8

(Kyriou et al., 1996) 1±4 70 7.3 420 73.2

(Chapple et al., 1993) 1±5 200 37.8 1200 242

(Martin et al., 1994) 6±10 80 7.9 770 132

(Kyriou et al., 1996) 5±9 60 6.1 670 106

(Ruiz et al., 1991) 6±10 310 43.2 1280 245

(Kyriou et al., 1996) 10±15 80 5.7 1060 125

(Ruiz et al., 1991) 10±14 400 38.8 ÿ ÿ


from the tomogram and QCT to be 31.9 and 28.6 mSv,respectively. It is worth nothing that his methodologywas very crude. For instance, no depth dose was calcu-lated. A QCT organ dose of 1.5 mSv estimated from

the center of a phantom was assumed for all theorgans. This assumption is not valid since the center ofdi�erent organs/tissue varies and thus the organ dose

also varies. However, he acknowledged the fact thathis results were only accurate to within a factor of 2 to

3.Huda and Morin (1996) estimated the ED for

SEQCT and DEQCT using a GE 9800Q CT scanner.

The estimate, based on the methodology of Shrimptonet al. (1991), reported an e�ective dose of 10.8 mSv for

a GE 9800Q scanner (120 KVp, 400 mAs, 22 slices).By assuming that the e�ective dose was linearly pro-portional to the number of slices and mAs, they calcu-

lated the ED as a function of these parameters. Theyalso scaled the dose by a factor of 0.3 when 80 kVpwas used instead of 120 kVp. When a tomogram

(Scout) and 3 CT images are acquired at 80 kVp, ane�ective dose equivalent in the range of 200±360 mSv is

estimated, depending on mAs used. Similarly, for atomogram and six CT slices associated with the 80 and120 kVp DEQCT an ED of 700±1400 mSv is estimated.

This also represents an over simpli®cation in the e�ec-tive dose estimates. Their values are about 8 times

higher than those reported by Kalender (1992). Thescaling method used may have overestimated the doseconsidering that the radio-sensitive organs are not

located proportionally in the body. However,Karantanas et al. (1991) have also reported an e�ectivedose equivalent of 370 mSv (no details of how EDE

was calculated were given) for 4 slices and a scoutview, using a Siemens Somaton DR2-G. They used

125 kVp, 60 mAs and 2408 tube rotation in the dataacquisition. They reported a computed tomographicdosimetric index (CTDI) of 5 mGy. The QCT BMD

doses are similar in magnitude to other normal radio-logical examination such as abdominal AP X-ray of700 mSv, but lower than normal CT-examinations,

such as 3300 mSv for the lumbar spine (Shrimpton etal., 1998).

The high doses encountered during normal imagingCT examination is necessary in order to obtain the lowcontrast resolution required to separate lesions from

normal tissue and to use CT to its optimal diagnosticpotential. However when measuring high Z material

such as bone, high doses such as those used for ima-ging are generally not necessary. Also the use of alarge region of interest (ROI) to determine mean CT

numbers for these studies allows much lower X-raydoses. Because bone mineral sensitivity goes up anddose goes down at lower kVp, the lowest possible set-

ting is recommended for BMD measurements (Cann,1988).

3.6. Patient dose from other techniques

For other techniques such as RA, PQCT, and SXAno e�ective dose calculations have been reported, onlyentrance surface doses have been reported. Muller et

al. (1989) reported an ESD of 100 mGy for a low dosepQCT. For a SXA DTX-100 (Osteometer A/sRodovre, Denmark), an ESD of 16.6 mGy (40 kVp,

0.2 mA) has been reported (Kelly et al., 1994). Sincevery few radiosensitive organs are irradiated using per-ipheral techniques, the ED is bound to be very low.

For example only a small fraction of these radio-sensi-tive tissues are exposed: skin, red bone marrow andbone surfaces.For superseded techniques such as SPA and DPA at

the most only absorbed doses have been reported.These doses are dependent upon the strength of thesource and the beam characteristics. The peak skin

entrance dose for DPA ranged from 50±150 mGy,while for SPA of the radius it was about 50 mGy(Huddleston, 1988).

3.7. Sta� dose from DXA

In the United Kingdom, the maximum annual dosefor a non-classi®ed worker is 15 mSv, corresponding toa time averaged dose rate in the workplace of

7.5 mSv hÿ1 (IRR, 1985). If the dose rates approachthis limit then the working area is de®ned as aControlled Area and requires environmental monitor-

ing and a written system of work. This includes moni-toring the radiation exposure of sta� members enteringthis area by ways such as TLD batches. At lower dose

rates of up to 5 mSv yearÿ1 (2.5 mSv hÿ1) the lesserSupervised Area requirement applies. In 1990 theInternational Commission on Radiological Protection(ICRP) recommended an annual dose limit of

1 mSv yearÿ1 (equivalent to 0.5 mSv hÿ1 in the workplace) for members of the public (ICRP, 1991).For pencil beam systems such as the Lunar DPX

and the Hologic QPR 1000, a time-averaged dose tosta� from scatter is very low even with the operatorsitting as close as 1 m from the patient without shield-

ing during scanning (Njeh et al., 1996; Patel et al.,1996a). For the Lunar DPX-L scanner, the maximumscattered dose-rate to an operator at 1 m from thepatient for continuous exposure would be less than

1 mSv hÿ1 (Njeh et al., 1996). The typical maximumworkload for this type of scanner is 16 patients perday with two views per patient, with the X-ray beam

on for 4 min per view. Thus, the maximum annualwork-load would be approximately 533 h per year. Theannual dose to the operator at 1 m would therefore be

less than 0.4 mSv per year, which is similar to theNational Radiological Protection Board (U.K.)suggested constraints of 0.3 mSv per year to members


of the public from a single source (NRPB, 1993).Therefore, no protective screen for the operator, or ad-

ditional protection for the public in adjoining areas isnecessary.Associated with the increase in patient dose and

higher patient throughput with fan beam DXA is agreater occupational hazard to sta� from scatteredradiation (Njeh et al., 1996; Patel et al., 1996a). The

scattered dose from the fan beam systems such as theQDR 4500 and Lunar Expert XL is considerablyhigher and approaches limits set by the regulatory

authorities for occupational exposure. The scattereddose-rate for the Lunar EXPERT Scanner at 1 m fromthe patient is 64 mSv hÿ1 (scan width 180 mm) (Njeh etal., 1996). If one allows for a nominal scan width of

120 mm, the scattered dose rate reduces to 42 mSv hÿ1.Considering a maximum daily beam on time of60 min, the time average dose rate at 1 m would be ap-

proximately 5 mSv hÿ1 (using an 8 h day). On thisbasis, the Controlled Area, as required by IRR 1985(IRR, 1985) would not need to extend beyond 1 m

around the radiation beam. However, in order toensure that the operator receives an annual dose ofless than 1 mSv (the new ICRP proposed limit for

members of the public) (ICRP, 1991), the operatorwould need to have a protective screen or be at least3.5 m from the beam. Thus, in a relatively small room,to achieve doses as low as reasonably achievable, a

protective screen o�ering a small degree of attenuationwould be required. On the other hand, with a reducedscan time and workload, the daily beam-on time could

be as low as 20 min, in which case a protective screenwould not be necessary, provided the operator is atleast 2 m away from the beam. A similar sta� dose

analysis has been made for Hologic QDR 2000plusand QDR 4500 by Patel et al. (1996a) and found to be2.1 and 2.4 mSv/h, respectively. This occupational dosewas found to be similar to that from a 99Tcm MDP

radionuclide bone scan (Fig. 9).

3.8. Sta� dose from other techniques

Similar or lower scattered radiation dose have beenreported for peripheral scanners such SXA. Kelly et al.

(1994) reported the scattered radiation to be less than1 mSv at a distance of 10 cm from all sides of the scan-ner. Scattered dose assessment is important in the

examination of the radius because of the proximity ofsensitive organs such as the breast to the scatteredradiation.

3.9. Reduction of occupational dose

Reduction of occupational exposure can be achievedby making appropriate use of shielding, distance, timeand radiographic method as follows:

. Shielding: When installing a fan beam systemactive precautions to reduce dose to sta� such as use

of a radiation barrier should be considered.. Distance: Apply the inverse square law by having

a scanning room large enough to place the operator at

least 3 m from the patient.. Time: Use of scan modes with shorter scan times

has been observed to approximately halve the dose for

QDR systems (Lewis et al., 1994).. Method: Patel et al. (1996a) have observed that

scanning the patient right hip in preference to the left

can give a three-fold reduction in potential occu-pational dose.

3.10. Dose reduction techniques in DXA applications

To minimize patient doses, as for instance requiredby the U.K. Government Ionizing Radiation

Regulation (IRR) of 1988 (IRR, 1988), the smallestpossible scan width and length should be used. For theLunar Expert a range of collimation widths are avail-

able. Njeh et al. (1996) have been able to show that a60% reduction in e�ective dose can be achieved byusing smaller ®eld sizes. The Lunar DPX-L scanners

have an ``auto scan width'' option which automaticallynarrows the width of the scan path when it locates thepatient's bone mass. Using this facility will reduce thearea exposed and thus the e�ective dose.

For the femur, the greatest contributor to the EDare the ovaries, but since a large number of femalepatients are postmenopausal the radiation hazard of

concern is carcinogenesis, the genetic risk to future o�-spring no longer being a factor. Nevertheless, exclud-ing the ovaries from the calculation results in a greater

than 70% reduction in the ED and a good workingprotocol should exclude the ovaries from the scanning®eld for both femur and AP lumbar spine. This maynot be possible for the fan beam femur scans

The operator should be adequately trained in orderto be able to locate the landmark for proper position-ing. This will avoid repeated starting and terminating

of a scan. Repeat measurements should be justi®ed interms of fast bone losses, borderline bone mineral den-sity and monitoring for treatment e�cacy.

4. Problems with measuring patient and sta� dose from

absorptiometric techniques

Various detectors have been used to measure skin

entrance and depth doses from DXA. These includeionization chambers (Sorenson, 1991; Lewis et al.,1994; Njeh et al., 1997b), proportional counters

(Waker et al., 1992) GM-Tubes (Pye et al., 1990), ther-moluminescent dosimeters (such as LiF and LiBO)(Njeh et al., 1996; Njeh et al., 1997b) and ®lm


(Bezakova et al., 1997). Lithium borate (Li2B4O7:Mn)TLDs have an improved energy response compared

with lithium ¯uoride (LiF). It also has better tissueequivalence than LiF, with an e�ective atomic numberof 7.4 compared with 7.42 for tissue and 8.14 for LiF.

For the dosimetry of X-rays with energies below100 keV as used in DXA, the di�erence in e�ectiveatomic number between TLDs and tissue leads to a

di�erence in mass attenuation coe�cient and thuserrors in measured dose (Kron, 1994).The physical properties of the DXA machine pose a

number of practical problems with regards to dosim-etry. First, the dose rate during scanning is very lowand multiple scans have been used to increase the radi-ation ¯ux (Njeh et al., 1996) above the detection limit.

This is achievable for ESD but may be problematic fordepth dose as the dose rate drops exponentially. Forinstance, Njeh et al. (1997b) exposed TLDs to 60 and

167 scans for the PA spine and total body, respect-ively, using pediatric mode on a Lunar DPX-L inorder to achieve a measurable dose. For ®rst-gener-

ation DXA, the beam size is small and precise posi-tioning of the detector in relation to the small DXAbeam is required.

The accuracy of the e�ective dose estimates dependsnot only on the accuracy of the measurement of thedose rate at the surface and at depths but also on thecorrect determination of the relative positions, frac-

tions and depths of the individual radiosensitiveorgans/tissue exposed. It is apparent from the abovethat computation of e�ective doses is generally time

consuming and cumbersome. Current computer pro-grams compute EDs for common radiological examin-ation but not for BMD measurements. Huda and

Morin (1996) have suggested a method of estimatingED using ESD, kVp and ®ltration information

.

5. Summary

Osteoporosis is a consequence of decrease in bone

mass and leads to mechanical weakness of the bone.Management of this can be improved by assessment ofbone mass using absorptiometric techniques such asRA, SXA, DXA and QCT. Availability and ease of

use have made DXA the most widely used techniquefor measurements of bone density in clinical trials andepidemiological studies. Studies of radiation dose to

patients from DXA con®rms that patient dose is smallcompared to that given by many other diagnostic in-vestigations involving ionizing radiation. Despite the

increase in dose associated with the development offan beam technology patient dose is still relativelysmall. QCT gives rise to doses which are comparable

to simple radiological examination such as the chestX-ray but lower than imaging CT. Radiation dose

from other techniques such as RA and SXA are of thesame order of magnitude as pencil beam DXA.

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Banks, L. M., Stevenson, J. C., 1986. Modi®ed method of

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