Intraoperative and interventional MRI: Recommendations for a safe environment

REVIEW ARTICLE

Intraoperative and interventional MRI: Recommendations for a safeenvironment’’

JOACHIM KETTENBACH1, DANIEL F. KACHER2, ANGELA R. KANAN2,

BILL ROSTENBERG3, JANICE FAIRHURST2, ALFRED STADLER1, K. KIENREICH1 &

FERENC A. JOLESZ2

1Department of Radiology, Medical University Vienna, General Hospital, Vienna, Austria, 2Brigham and Women’s

Hospital, MRT Department, Boston, MA, USA, and 3Anshen+Allen Architects, San Francisco, CA, USA

AbstractIn this paper we report on current experience and review magnetic resonance safety protocols and literature in order todefine practices surrounding MRI-guided interventional and surgical procedures. Direct experience, the American Collegeof Radiology White paper on MR Safety, and various other sources are summarized. Additional recommendations forinterventional and surgical MRI-guided procedures cover suite location/layout, accessibility, safety policy, personneltraining, and MRI compatibility issues. Further information is freely avalable for sites to establish practices to minimize riskand ensure safety. Interventional and intraoperative MRI is emerging from its infancy, with twelve years since the advent ofthe field and well over 10,000 cases collectively performed. Thus, users of interventional and intraoperative MRI shouldadapt guidelines utilizing universal standards and terminology and establish a site-specific policy. With policy enforcementand proper training, the interventional and intraoperative MR imaging suite can be a safe and effective environment.

Key words: Interventional MRI; intraoperative MR, MR safety, image-guided therapy, MRI

Introduction

Magnetic Resonance Imaging (MRI) has been

shown to be invaluable for guiding and monitoring

interventional and surgical procedures, as it is a

multi-planar cross sectional modality with exquisite

soft tissue contrast. The appeal increases due to the

absence of ionizing radiation and lack of need for a

nephrotoxic contrast agent, as well as its ability to

elucidate tissue changes due to thermal therapies

(1). The use of an MRI scanner, a powerful magnet,

in the interventional setting poses challenges in

ensuring a safe environment for the patient and

staff. Moreover, energy associated with image

acquisition has the potential to cause heating in

conductive objects and in the patient himself. Many

safety guidelines for interventional and intraopera-

tive MRI (iMRI) are based on safety protocols for

diagnostic MRI. However, particular care is neces-

sary to enhance these guidelines for MRI-guided

interventional or surgical procedures. Safe behavior

can be engrained into personnel by training and

policy enforcement. Alternatively, safe behavior can

be coerced by measures such as suite layout and

restriction of access. In this paper, these two means

will be illustrated and specific recent recommenda-

tions and site experience for interventional MR

departments will be summarized (2–4).

Safety via design

Architectural considerations

Safety begins before the MRI scanner is even

installed. The location and layout of an iMRI suite

must be planned not only for workflow issues, but

also for safety (4,5). With an increasing number of

scanners being installed in locations outside of the

diagnostic radiology area, training alone will not

ensure safety. Unlike in the diagnostic MRI area,

Minimally Invasive Therapy mit81867.3d 1/3/06 21:28:14The Charlesworth Group, Wakefield +44(0)1924 369598 - Rev 7.51n/W (Jan 20 2003) 164056

Correspondence: J. Kettenbach, Division of Angiography and Interventional Radiology, Department of Radiology, Medical University Vienna, General

Hospital of Vienna, Wahringer Gurtel 18-20, A-1090 Vienna, Austria. Fax: ++43-1-40400-4898. E-mail: [email protected]

Minimally Invasive Therapy. 2006; 000:000; 1–13

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ISSN 1364-5706 print/ISSN 1365-2931 online # 2006 Taylor & Francis

DOI: 10.1080/13645700600640774

personnel may not be aware of the risks of magnetic

field attraction and effects of the fringe field on

implants.

To limit risk of untrained personnel entering the

iMRI room, it is ideal to locate the scanner in a

dedicated suite. If the scanner is to be located in the

operating room, interventional radiology, or catheter

lab complex, the scanner should be in an isolated

area without general access. To further limit access,

only one entrance into the iMRI room should exist.

The ideal location for a door is near the console area

where it is within direct line of sight of the MRI

technologist or other trained personnel (Figure 1).

Security cameras installed at the entrance to the

suite and MRI room enable the MR trained staff to

further monitor access and activity.

Choice of MRI scanner and procedural flow

Several scanner configurations are currently used for

iMRI procedures. The choice of scanner may

strongly influence architectural considerations and

how the imaging and intervention are performed.

Solutions for iMRI include

N moving the patient into a conventional closed

bore or horizontally open scanner by translating

or pivoting a table up to 180˚ to allow the head of

the patient to be placed outside the 5 Gauss line,

N moving the closed bore scanner to the patient,

N operating within a vertically open scanner from

which there is no need to move the patient, or

N using a dedicated portable scanner (Figure 2a–d).

Certain procedures are enabled by the capabilities of

the scanner. The higher field closed bore scanners

have inherently greater signal to noise ratio images

and faster imaging, which are necessary for cardiac

and vascular applications (6–8). The high field

scanners, moreover, have greater sensitivity to

temperature changes during thermal ablations,

which is ideal for procedures such as focused

ultrasound ablation (9,10). Lower field open scan-

ners offer greater access for therapeutic probe

placement. The tradeoffs of these scanners are listed

in Table I.

Complexity of controlling access increases in the

situation where a single iMRI room services multiple

operating rooms (Figure 3 a–c). Depending on

layout, more entrances into the iMRI room may be

necessary. Moreover, moving an intubated patient

between the surgical station and the scanner poses a

clinical safety concern. Safety concerns with single

room solutions, where the procedure occurs within

the iMRI room away from the scanner, will be

discussed later.

Site access restrictions

Signs, with warnings of the risks associated with the

MRI scanner, are necessary but in reality are seldom

effective. Physical barriers such as locked doors and

limited entrances are crucial to restrict access to the

iMRI suite (Table II). All of the concepts that apply

to restriction of access and zoning in diagnostic MRI

also apply to the iMRI suite and can be read

elsewhere (2,3) in this special issue. Zone 3 (area

immediately outside the scanner room) must be

extended to encompass the substerile corridor and

support space outside the scanner room. If operating

rooms adjoin the iMRI room, the operating room

may undergo a change of zone designation based on

use and barriers. If an OR is open to the iMRI room,

it must also be considered Zone 4 (MR scanner

room). If a door between the iMRI room and OR is

closed, the OR becomes part of Zone 3. If passage

between the OR and Zone 2, Zone 3, and Zone 4 is

eliminated, the OR may be designed Zone 1 (outside

the MR suite). Zone 3 should be restricted.

Keycards, keys, or numerical codes are means of

limiting access to only trained personnel. Many

disciplines involved in iMRI must be MR safety

trained, however, to limit potential hazards; a site-

specific policy should be developed to determine

which personnel should have access and which

should be escorted into the suite (Figure 4).

Safety via policy

Safety training

Before a procedure in iMRI can be attempted,

a multidisciplinary team must be designed to


Figure 1. Flexible procedure room clusters: this concept allows

the operator to control access for personnel and supplies from a

centrally located point. If needed nearby rooms and various

imaging modalities can be included for image-guided therapies

easily. Courtesy of B. Rostenberg, # 2005 Anshen+Allen.

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Figure 2. Possible solutions for patient positioning for interventional or surgical MR-guided procedures: (a) moving the patient into a

conventional closed bore or (b) a horizontally open scanner by translating or pivoting a table; (c) operating within a vertically open scanner

(GE Signa SP 0.5T-MRI) that permits access to the patient during imaging by a 56-cm gap; LCD screens to monitor imaging data during a

procedure are mounted on both sides of the work area at eye level; MR-compatible anesthesia equipment and MR-compatible instruments

can be placed nearby, thus there is no need to move the patient; (d) or with use of a dedicated portable vertical 0.12 Tesla scanner (PoleStar

N-10, Odin Medical Technologies, Inc; Newton, MA) that uses a pair of ceramic permanent magnets spaced 25 cm apart (Figure 2a,

Courtesy of Siemens Medical Systems, Figures 2b and 2d, Courtesy of Case Western Reserve University, Cleveland, Ohio).

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formalize a safety policy for the suite. A mechanism

must be set up to communicate the policy to

personnel. On-line or written testing in addition to

practical training is mandatory and must be annually

renewed at some facilities (4).

Screening

Patient screening must be conducted by an MRI

safety trained staff member (11,12). The screening

form should help to rule out contraindications for

MRI scans such as pacemakers, some aneurysm

clips, metal fragments in the eye, mechanical device

implants (e.g. drug infusion pump, neurostimula-

tor). Personnel who work in the iMRI suite should

be subject to the same screening procedure.

However, since the personnel will not be imaged,

some contraindications (e.g. tattoos, medication

patches) do not need to be considered.

Patients should change into a hospital gown and

remove hearing aids, hairpins, barrettes, jewelry

(including body piercing), watches, and medication

patches prior to entering the iMRI room. At some

centers, personnel are required to wear pocketless

scrubs to decrease the likelihood of ferrous objects

entering the scanner room.

Patient positioning

In many iMRI cases, the patient will be under

general anesthesia and unable to move. It is essential

that the patient is positioned properly and all

pressure points should be appropriately padded to

maintain good circulation. In neurosurgical cases

where the head is pinned in a head holder, trade-offs

must sometimes be made between centering the

patient in the suite spot of the imaging volume and

positioning the patient in the ideal surgical position.

Necessary padding, with larger patients, can some-

times prohibit fitting the patient into the bore of the

scanner.

Padding must also be used to eliminate closed

loops created by skin-to-skin contact or contact with

the skin and an ECG cable or the cable of the MRI

coil (Figure 5). The integrity of the insulation and/or

housing of all components, including coils, leads,

cables and wires need to be checked regularly.

A patient-warming system can be used to maintain

body temperature by placing the heater in the room

adjacent to the iMRI room and introducing the hot

air tubing into the room via a penetration panel.

Compression boots to maintain circulation can be

used with a similar method.

Static magnetic field and iso-gauss lines

The magnetic field decreases with distance from the

scanner. Closed-loop lines of iso-gauss (constant

magnetic field strength) exist, and can be drawn on

the floor of the iMRI suite to serve as a warning to

staff. The criterion of the 5-Gauss (G) (0.0005-

Telsa) line was established early to limit long-term

occupation exposure. The 5-G line must be con-

tained within the MRI room. The line also serves to

limit exposure to individuals with implants (e.g.

cardiac pacemaker, etc.) (11). For example, some

modern pacemakers switch into asynchronous mode

at roughly 5–7 G, whereas some pumps may be

affected only by far greater magnetic fields.

Additional information is available at www.fda.gov.

Ferromagnetic surgical tools and devices are not

affected until much greater field strength. Many

iMRI sites mark lines on the floor to indicate an area

where non-MRI safe tools can be safely used and no

attraction or torque (alignment of a ferrous object

with the magnetic field) are observed (Figures 3 and

4). No strict criteria exist at the time of this

publication to indicate exactly at what strength the

line should be drawn. Magnets with active shielding

exhibit a very low fringe field far from the iso-center,

with a dramatic increase in field strength within feet

of the opening of the bore. This sudden change in


Table I. Possible MRI configurations for intraoperative or interventional imaging.

Configuration Field (Tesla) Bore length and diameter Access to patient Advantages

Long bore (closed) 1-3 T 200–300 cm length Poor but not impossible Highest image quality

60 cm diameter

Short bore (closed) 1.0 – 1.5 T 125 cm length Limited but not poor Fast Gradients/cardiac/

vascular applications70 cm diameter*

Horizontal open gap 0.2 – 1.0 T Height 45 – 70 cm Good access, but limited

in vertical direction

Safety and instrument

compatibility

Vertical open gap 0.5 T Height approx. 70 cm Good access but not

fully unlimited

Intraoperative imaging;

flexible positioningPartially limited, width 56 cm

*Note: With this particular design, the distance to the isocentre is 62.5 cm from the bore entrance and the gap from the patient table to the

bore top is 55.5 cm.

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attraction gives little warning to a staff member

holding a ferrous object as he approaches the magnet

(13). Attraction and torques constitute the greatest

risks in MRI. An object may become a projectile as it

accelerates in the direction of the spatial gradients of

the static magnetic field. Large objects can generate

incredible force as they are drawn into the magnet.

Currently, various MR systems used for diagnostic

or research purposes operate with static magnetic

fields that range from 0.064–8.0 Tesla. At the time

of this publication, field strengths from 0.12T to 3T

are used for iMRI.

Most ex vivo tests to determine MR safety or MR

compatibility for implants and devices were per-

formed at static magnetic fields of 1.5 Tesla or lower

(14). This could be problematic for a patient with

implanted devices exposed to a higher static mag-

netic field such as 3-Tesla units. Inside the human

body, torque and attraction can cause an implant to

tear surrounding tissue (11). Tearing and pain is

most likely to be experienced in the strongest part of

the gradient field, while the patient approaches the

scanner before actually being placed within the bore.

Questions of how soon a patient may be safely

scanned after surgery often arise. Patients with

metallic ‘‘passive’’ implants (i.e. there is no power

associated with the operation of the object), made

from nonferromagnetic material (e.g. nitinol, tita-

nium, elgiloy, tantalum, etc.) may undergo an MR

procedure immediately after placement of the object

using an MR system operating at 1.5 Tesla or less

(12,14).


Figure 3. Complexity of controlling access (a) increases in

situations when other imaging modalities such as angiography

(Axiom Artis, Siemens Medical Systems, Erlangen, Germany) are

located next to the MR suite to allow rapid transfer of the patient

for MR imaging; (b) magnetic fringe field of the closed bore

1.5-Tesla MR system (Magnetom Avanto, Siemens Medical

Systems, Erlangen, Germany); (c) magnetic fringe field lines

beginning at the center of the magnetic field (a) of an open 0.2-

Tesla MR system (Magnetom Open, Siemens Medical Systems,

Erlangen, Germany). (Figure 3b and 3c courtesy of Siemens

Medical Systems, 3b has been modified to enhance the magnetic

fringe field lines).

Table II. Operational Safety Rules should implement the follow-

ing security and safety guidelines:

N All personnel must be trained and frequently updated on MRI

safety.

1 Access to the MR scanner room (high-magnetic-field area) is

limited to trained personnel, screened patients or visitors

accompanied by trained personnel.

2 Entrance to the MR scanner room is controlled by a lockable

door, and keys to the area are issued only to trained personnel.

3 All entrances to the MR scanner room are visible to the system

operator.

4 All visitors are screened by the operator before entry.

N Appropriate warning signs must be posted.

Optional use of a scanner for ferromagnetic objects should be

considered.

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For objects that are weakly magnetic (typically

certain types of coils, filters, stents and vessel

occluders), a waiting period of 6–8 weeks prior to a

MR procedure is recommended. In this case, tissue

growth, scarring or granulation provides retentive or

counter forces to prevent the object from hazardous

movement within a field strength of 1,5 Tesla or less.

Implants that are weakly magnetic but are rigidly

fixed in the body, such as a bone screw, may be

studied immediately after surgery (12). If there is

any concern about the integrity of tissue with respect

to its ability to retain the object in place during a MR

procedure, the patient should not be exposed to the

MR environment (14).

Emergency response

Nursing, anesthesia, MRI technologists, and operat-

ing physicians should be trained to respond to

cardiopulmonary emergencies in the scanner room.

Since MR-compatible equipment to resuscitate a

patient does not exist, the patient should be brought

to a safe area away from the scanner room.

Designated personnel should be responsible for

maintaining access restrictions while other personnel

expeditiously move the patient to the location where

resuscitation will be performed. If it is impractical to

remove the patient from the scanner room, and life

saving equipment must be brought in, the magnetic

field can be eliminated (run down). However, it is

costly and time intensive to restore the magnetic

field.

In case of sudden loss of magnetic field (quench),

the cryogens that cool the magnet expand as they

turn to gas and may escape into the room. It is

prudent to vacate the room in case oxygen has been

displaced, even if oxygen monitors read normal

levels. The oxygen used in the OR setting as well as

therapeutic heat sources (e.g., laser) result in a risk

of fire. Non-ferrous fire extinguishers are available

for the management of small fires. Sites should

have a plan for rapid removal of patients in these

life-threatening situations, even when there is no

time to close a surgical wound. Policy and education

should exist to prevent responders from other areas

from bringing ferrous materials into the scanner

room.

Acoustic noise

Care should be taken to protect the patient’s

hearing, especially when under general anesthesia.

The sound pressure level of a gradient coil impacting

its mounting structure during scanning can exceed

the occupational limit of 100 dB (root mean square).

Hearing protection is mandatory in these cases

(11,12,15). Interventional staff within the scanner

room should also wear hearing protection (e.g.


Figure 4. Unauthorized access of non-trained personnel into a 1.0

Tesla-MR suite: in order to prepare oxygen supply for an

emergency patient, a physician moved a conventional (ferromag-

netic) oxygen cylinder into the MR room, while the MR staff

personnel was involved in screening the patient outside the MR

suite. No one was injured, however the magnet’s gradient coil was

severely damaged and the magnet had to be ramped down. Note

that the 5-Gauss line is highlighted as white line on the floor.

Figure 5. Appropriate padding to prevent skin-to-skin contact

and the formation of ‘‘closed-loops’’ is mandatory. Insulation

material (minimum recommended 1-cm thickness) should be

placed between the patient’s skin and the transmit RF coil and/or

body coil if appropriate.

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noise-abatement headphones or earplugs), particu-

larly if repeated imaging is necessary during a

surgical procedure. Non-essential staff should be

allowed to exit the room during scanning.

Time Varying Magnetic Field Exposure

In order to establish an allowable RF energy

deposition, the Specific Absorption Rate (SAR),

expressed in units of watts per kilogram of body

weight, is based on levels that induce a maximum

change in tissue temperature of 1˚C. According to

specific criteria, the SAR must be no greater than

4 W/kg averaged over the whole body for any 15-

minute period (15). The European Directive (ED)

however, sets the exposure limit values for whole

body average SAR to be as low as 0.4 W/kg. These

limits are far below those which may produce known

short-term negative health effects, following the

definition of the World Health Organization (16).

In the areas of tissue in contact with surgical tools

or conductive wires (e.g., for ECG, EEG, tempera-

ture measurement, cautery or radiofrequency

grounding pads, etc.), induced currents can be large

enough to cause significant burn (11). Care must be

taken that conductive wires, thoroughly tested and

determined MR compatible, do not overlap or form

loops, run in parallel as close to the center of the

magnet bore as possible, and are insulated from

contact with conductive materials or bare skin, in

particular in patients with conductive materials

applied to the skin (such as tattoos, eyeliner, or

some cosmetics). The maximum induction occurs

with the plane of a loop perpendicular to the

changing magnetic field, therefore surface coils need

to be insulated from the patient’s skin (11). In any

case patients should be informed of the possibility of

heat sensation (that usually occurs within the first

20–60 seconds).

Again, cables should be padded to eliminate

contact with the skin or, if possible, removed during

scanning. As a preventive measure a cold water

compress or ice packs can be applied to sites subject

to heating, such as RF electrode pads. Skin-to-skin

contact points should be avoided, since they have

been reported to cause burns as well (17). These

precautions are of particular importance in patients

under sedation or general anesthesia who are unable

to notice or report pain.

Occupational considerations

Several attempts have been made to provide

regulatory guidelines for the chronic exposure of

personnel required to work near strong magnetic

fields. An example is the guideline proposed by the

National Radiological Protection Board of the

United Kingdom, that customarily takes the form

of limits over a course of an 8-hour working day. In

this guideline, a worker could be in a 0.2-T field for

the entire working day, whereas in a field of 1.6 T

exposure should be restricted to one hour (18,19). If

only extremities are exposed to the field, a higher

average field (2T/day) is permitted (20).

In a new European Directive (ED), accepted in

2004, new electromagnetic field (EMF) exposure

limits for workers in general are defined in the 0 –

300 GHz frequency range. The ED requires man-

datory adoption into national law for each EC

country within four years, i.e. by 2008. The new

ED follows the proposed guidelines from the

International Commission on Non-Ionizing

Radiation Protection (ICNIRP) that lists conserva-

tive low exposure limits and does not include a risk

versus benefit analysis of the acceptability of

practices that lead to EMF exposure above these

limits. Although the limit values for the static

magnetic field were excluded, the ED still includes

the ICNIRP limits for RF power and gradient

power, so that it may indeed become a limiting

factor for the clinical MR practice (20–30).

Although the underlying assumptions are currently

under question and great scrutiny, these restrictions

will severely limit interventional practice in the EU

(23). In any case, the MR user should be aware that

even well-known SAR exposure limit values do not

warrant a ‘‘healthy environment’’. Recently, experi-

ments have indicated that the proposed SAR

exposure limit values (10 mA m22), defined in

terms of induced current density to the head or

trunk, would be exceeded by a worker standing

within 130 cm of the isocenter (31). Also, the

exposure of staff to peak magnitude of the magnetic

flux density exceeds the limits up to a distance of

180 cm from the isocenter. The exposure is

expected to increase for modern short bore systems

with higher-powered gradients. To minimize the

SAR exposure efforts are underway to use combina-

tions of gradient axes, using real pulse sequences and

modern short bore MR systems (11,12,15).

MR-compatibility testing

Devices not marked as MR-compatible by a vendor

need to undergo meticulous testing before being

used in a MR environment. Any and all devices that

are or may contain metallic components that are to

be brought into Zones 3 or 4 should be positively

identified or tested for ferromagnetic properties prior

to their being granted access to these regions.


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Devices screened for safe access to Zone 3 or Zone

4 must be labeled with green ‘‘MR Compatible’’

labels or red ‘‘Not MR Compatible’’ labels. ‘‘MR-

conditional’’ is used for all other devices in which

testing conditions would be specified—for example,

‘‘MR-tested for up to x static magnetic field and up

to y static spatial gradient field’’ (32). Testing with a

powerful handheld magnet (> 1000 G recom-

mended) can be performed to detect if an object is

not safe. Other tests such as magnetic force and

torque tests can be employed, but need careful setup

of the tests and supporting gear such as rope brakes,

scales, etc. to avoid harm to the operator and

damage to the MR scanner (33,34). Electronic

devices used in the MR suite need to pass a series

of tests before they enter into regular preventive

maintenance schedule (4). Further tests include

signal-to-noise ratio and homogeneity tests while

the device is operating and exposed to various MR

scanner modes. This topic is expanded in this special

edition.

Anesthesia management for MR imaging

The iMRI environment is the most challenging

setting for out of OR anesthesia. In general, off-the-

shelf equipment, familiar to all staff anesthesiolo-

gists, cannot be used. Access and line of site to the

patient is limited when the patient is inside the bore

of the scanner. ECG lead placement necessitated to

achieve a clean trace in the MRI, prohibits true Lead

I,II and III waveforms (35). Moreover, waveforms

are distorted by the effects of the magnetic field and

are subject to induced noise during scanning.

As blood, an electrical conductor, jets through the

aortic arch in the presence of an external magnetic

field, a voltage is induced at the skin surface. This so

called magnetohydrodynamic effect masks S-T seg-

ment changes, which is an indication of cardiac

ischemia (36). Thus all patients should be carefully

screened for risk of ischemia before being consented

for a procedure in the iMRI area. Anesthesia staff

must be prepared to face problems involved with

moving a patient large distances as more modalities

are used in conjunction with MRI to guide a

procedure (29). MRI-compatible anesthesia equip-

ment is commercially available from several manu-

facturers, and general anesthesia services have been

established in many MRI centers worldwide (37).

Further guidelines have been established by the

American College of Radiology, the American

Society of Anesthesiology and the Joint Commission

on Accreditation of Healthcare Organizations and

publications (4). Guidelines request two sources of

oxygen (preferably central source piped oxygen and a

backup full E cylinder), suction, an anesthesia

machine if administering volatile anesthetics, a

scavenging system for waste anesthetic gases, a self-

inflating hand resuscitator bag able to deliver 90%

oxygen and positive pressure ventilation, standard of

care monitors and equipment, sufficient electrical

outlets, and illumination. Mason recommended that

it is safer to position stocks for essential medications,

alternative airway devices, intravenous supplies,

endotracheal tubes (ETT), suction catheters, syringes

and needles on a non-MR compatible cart outside the

scanner and to bring only the ‘‘essentials’’ into the

suite, arranging them on the anesthesia machine (38).

Laryngoscope handles and blades are generally non-

ferrous but require lithium batteries. Beware that

some batteries labeled as ‘‘lithium’’ MRI may be

tainted with a ferrous-containing substance (38).

Despite being MRI compatible, personal experience

has demonstrated that the power source for MR-

compatible anesthesia machines may cause image

artifacts if left in the scanner (38).

Safety via development

MR-cmpatible instruments and surgical devices

Owing to the relatively high magnetic fields applied

in magnetic resonance scanners, standard medical-

grade stainless steel surgical instruments cannot be

used in these systems. Such instruments, with

paramagnetic or ferromagnetic content, cause dis-

turbing image artifacts owing to magnetic field

inhomogeneities that are created by the suscept-

ibility mismatch between tissue and the instrument.

In addition, these materials are subject to torque and

displacement in the magnetic field and may become

a projectile. Previous reports have shown that

titanium, titanium alloy, ceramic materials and

others cause relatively small artifacts that are the

direct result of the very low magnetic susceptibilities

(39). These materials have been identified to be safe

for use in an MRI environment (40), but may exhibit

artifacts at higher fields.

Passive visualization techniques take advantage of

some residual device-induced susceptibility artifacts,

generally sufficient for biopsy and drainage proce-

dures. It is also a well-known phenomenon that

artifacts related to the use of the T1-weighted, spin

echo pulse sequences are smaller than those seen

with the GRE pulse sequence. Fortunately, there are

pulse sequences (e.g., fast spin echo) and other

techniques (swapping phase and frequency) that can

be used to minimize artifacts associated with the

presence of metallic implants (41). For intravascular

interventional procedures, active tracking techniques


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are under development, integrated into devices such

as catheters and endoscopes (7,42).

Pulse sequences

Pulse sequences for guidance of interventional

procedures have to be tailored to the interventional

or surgical procedures (1,9). Procedures that require

relatively high tissue contrast (T2 weighting) but only

limited temporal resolution can be monitored with

Fast Spin Echo (FSE) techniques, which allow image

acquisition in approximately ten seconds Sequences

that provide higher temporal resolution do so at the

cost of spatial and contrast resolution. These

sequences include multiple gradient echo techniques

with an acquisition time typically in the range of few

seconds. Advancements in the last decade have made

it possible to acquire and display more than ten

images per second in real-time with millimeter

resolution in all three directions. This temporal and

spatial resolution is considered high enough to guide

most interventions (13,43). High-resolution images,

made possible by the advent of multichannel receiver

systems, improved graphics, higher processor speeds,

and increases in speed and quantity of memory can be

used to update pulse sequence parameters in real

time, thereby opening new opportunities for inter-

ventional MR imaging that extend from biopsy and

thermal therapy to image-guided vascular and cardiac

procedures. However, RF heating of wires used for

device localization and the noise generated by rapid

switching of MR gradients constitute significant

obstacles yet to overcome. It is anticipated that these

topics will emerge as critical concepts in the next

decade of interventional MR imaging research (13).

Interactive imaging and real-time guidance

The principles of interactive-image guidance, frame-

less stereotaxy, and navigation were established in

neurosurgery, where preoperative images were used.

Optical tracking devices integrated within the MR

scanner combine surgical navigation with nearly real-

time imaging and do not require a registration process

between the patient and the images because images

are acquired in the surgical position of the patient (9).

Thus, coordinates of the optical tracker can be used

to control the scan plane of the nearly real-time

imaging (Figure 6) and to display a virtual instrument

(e.g. biopsy needle or therapy probe) and its

trajectory, overlaid on the MR image (1,44).

Interactive, near real-time imaging can be accom-

plished by positioning an optically tracked hand piece

to determine an appropriate trajectory in three planes.

Once a trajectory is chosen, a burr hole or craniotomy

is drilled in the skull overlying the entry site. The ideal

approach to a lesion is determined by using a

combination of the nearly real-time imaging and

conventional serial-volume imaging (Figure 7).

During brain surgery, the operator can thereby

successfully avoid vital vascular structures and critical

brain regions. Currently, more than 2000 cranio-

tomies with MRI guidance have been performed

worldwide (4,9,44) without significant complications.

MR was in particular helpful in a case of residual

enhancing tumor that was visualized during surgery

and further resected. Serial intraoperative imaging

also clearly demonstrated that there are substantial

shifts and deformations during surgery, caused by

CSF drainage, tissue removal, and tissue reaction

(swelling) to the manipulations (45). Furthermore,

intraoperative, interactive MR-guidance has been

shown to enable the detection of intraprocedural

complications such as hemorrhage (10,46).

Conclusion

The potential for injury or death in the MRI

environment (2) should not be underestimated.

Experiments and reported accidents show the

environment contains potential risk (47). However,

the authors of this work have collectively performed

over 2000 cases without serious adverse incident, at

their two centers, over a twelve-year period using

0.2T and 0.5T scanners. Hill reported a similar

four-year experience using a 1.5T MR system

together with a cardiac c-arm x-ray system for MR

guided cardiac catheterisation (48). Hushek et al

reported another perfect safety record over more

than five years with more than 400 surgeries being

performed at 0.5T (4). These examples give promise

that the iMRI environment can be safe.

Safe behavior can be engrained into personnel by

training and policy enforcement. Safe behavior can

be coerced by measures such as suite layout and

restriction of access. These two means must act in

concert to ensure safety.

We echo the recommendation (29) of a gradual

progression in establishing an iMRI suite. The right

team members, well thought-out architectural plan-

ning, and proper policy and training are an essential

start. Human vigilance and enforcement of policy

must be ongoing. For any new procedure, a dry run

should be executed to identify the potential pitfalls.

A small core staff should be involved in the first

cases. We recommend video taping and reviewing

the procedure. Some architectural firms or external

centers will provide a safety survey at this stage. In

expanding the services, the experienced core staff

should train new staff members. To streamline


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cases, time stamps of each portion of the procedure

should be recorded. Analysis and optimization

processes to maximize case efficiency can be applied.

The less time a patient is in the iMRI environment,

the less time he and the staff are at risk.

As the field of iMRI evolves, magnetic field

strengths will grow, gradients and thus potential for

heating will increase, and the armamentarium of

interventional devices used will expand. As these

advancements happen, risk will increase. We must be

prepared to face these risks with increased vigilance,

advances in materials from which instruments are

made, and advances in scanner software that can

achieve fast imaging without excessive exposure.

Acknowledgment

We are grateful to Gabriele Bench, RT at the Dept.

of Radiology, Mecical University of Vienna, for

providing Figure 4.


Figure 6. T1-weighted magnetic resonance images obtained during biopsy of an intracranial mass (glioblastoma), localized by using the

interactive handheld tracking system with the integrated needle holder. (a,b) Parasagittal and axial images show the planned trajectory

(dashed line) of the needle. (c) The biopsy needle itself as it is advanced into the lesion.

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Figure 7. Intraoperative image-guidance during craniotomy of a brain tumor (meningioma). (a) T2-weighted magnetic resonance (MR)

images were acquired within the Signa SP, preoperatively. Localization of the best entry site for craniotomy was accomplished by

visualization of the surgeon’s finger in the MR images. (b) T2-weighted MR images show the resection cavity, partially filled with liquor. c)

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Intraoperative and interventional MRI: Recommendations for a safe environment

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Transcript of Intraoperative and interventional MRI: Recommendations for a safe environment