Chapter 30 Equipment for the Magnetic Resonance Imaging Environment P.872
Magnetic resonance imaging1 (MRI) is a noninvasive diagnost ic procedure tha t can
produce superior images wi thout us ing ionizing radiat ion. MRI s tudies are not
painful but do require patient immobili ty. Patien ts who are unab le to hold sti l l of ten
require sedation or genera l anesthesia. Med ically uns table patients such as those
f rom in tensive care may need scann ing. A more recent development is the use of
MRI to guide and moni tor inte rventional procedures (1 ,2,3,4,5,6). MRI-guided
procedures may resul t in smaller inc is ions as wel l as more accurate local ization
and t issue retrieval .
Administering anesthesia in the MRI un it poses a number of technical dif f icult ies.
Knowledge of this envi ronment as wel l as i ts r isks and problems are essential for
safe anesthesia pract ice. The prac tice guide lines for anesthesia care and
moni toring developed by the American Soc iety of Anesthes iolog ists and the
American Associat ion of Nurse Anesthetis ts apply to the MRI env ironment just as in
other parts of the hea lth care fac il i ty (8). In some states, these guidel ines are
codif ied in to law (9).
Definitions A device is considered to be MR safe if i t presents no addi tional risk to the patient
or operator. The presence of such a device may affect the quali ty of the diagnost ic
info rmation when i t is p laced in the MR environment (10,11,12,13,14).
A device is MR compatible i f i t is MR safe, its use in the MR environment does not
s ignif icant ly affect imaging quali ty, and there is no s ignif icant effect on its
operations. A device may be MR compatible or safe fo r certain MR environments
but not o thers. Therefore , using the terms MR compatib le and MR safe wi thout
specif ica tion of the MR environment to which the device was tested should be
avoided. The term MR environment is used to desc ribe the area wi th in the 5-gauss
l ine around the scanner (the perimeter around an MR scanner wi thin wh ich the
stat ic magnet f ie ld is higher than 5 gauss).
Basic Principles
There is a s tat ic magnetic fie ld ins ide the MRI scanner bore
(7,13,15,16,17,18,19,20,21,22,23). Once the magnetic f ield is establ ished, it is
usual ly no t tu rned OFF. If the magnet is deactivated, i t can take up to 96 hours to
reestabl ish the magnetic f ie ld. The un it of measurement of magnetic f ie ld s trength
is the Tesla. A field of 1 Tesla is roughly 10,000 times the magnetic field at the
earth's surface. One Tesla equals 10,000 gauss. The strength of a magnet is
quant if ied in the midd le of the magnet. However, the f ie ld ex tends beyond the
margins of the magnet (the fr inge f ield), dec reasing in strength with distance f rom
the bore.
Atomic nuclei wi th an odd number of protons and/or neutrons have a spin that
produces a weak loca l magnetic f ie ld. In the absence of a strong magnetic f ie ld,
these nuclei are randomly aligned. A strong magnetic f ie ld causes approximately
half of them to rotate and al ign para llel to th is appl ied f ie ld (the low-energy, or
ground, state). The remain ing nuclei align against the appl ied f ield (the high-
energy, or exci ted, s tate).
Resonance describes the process of inducing a change in energy s tates of the
nucle i caused by absorpt ion of a specif ic radio frequency (RF) radiation . Adding
energy wi th a short, control led burst of RF energy causes some low-energy
(parallel ) sp ins to jump to the exci ted (ant i -paral lel ) energy level . Immediately after
the RF pulse, the nuc le i ro tate back into al ignment wi th the static magnetic f ie ld. As
they re turn to their orig inal orienta tion, energy is released. A receiver coi l detects
this weak electrical s ignal and ampl if ies i t fo r processing and eventual image
formation. Re laxat ion rates vary fo r specif ic body t issues , al lowing d if fe rentiat ion of
body structures.
Facility Design A member of the anesthesia department should be involved in p lanning the MRI uni t
(24,25). An anes thesia induction room that adjoins the scanning area is useful . The
anesthet ic can be in it ia ted at this locat ion, where ferromagnetic objects can be
safely used. The anesthet ized patient can then be moved into the scanning area.
Consideration should be given to plac ing a postanes thesia care room near the MRI
unit .
Wave gu ides, special ly designed condu its in the wal ls , can be used to pass pipes ,
cables, ducts, tubings, and electrica l wires through the wal l whi le maintaining RF
sh ielding (26,27) (Fig. 30.1). They are commonly p laced low in the room at the
farthes t point f rom the magnet and RF coils.
Fluorescent l ight ing emits RF energy that inte rferes with imaging (21). Therefore,
incandescent l ight ing a t a low wattage is used. Isola ted e lec trical power is used to
reduce the problem of leakage currents (10). I t is important that there are an
adequate number of elec trical plugs a t convenient locat ions for portab le mon itors
and o ther equipment.
There are four bas ic op tions for locating moni to ring and other equipment tha t is
used to administer anesthes ia or sedation and the person attending the monitors
and patient:
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View Figure
Figure 30.1 Wave guides are used to pass cables, tubings, sires, and the like through the wall while maintaining radio frequency shielding.
• Both the monitors and the attendant a re ins ide the magnet room. This allows
di rect observat ion of the pat ien t. The attendant can both see and hear the
moni tors , but the attendant is subject to possible hazards (loud sounds,
magnetic forces, and possibly trace anes thetic gases or hypoxia). I t may be
necessary for the attendant to be in the room when there is an inabi l ity to
see the pat ient, particula rly if the pat ient has entered the scanner headfi rs t
and/or the patient is a child (28).
• The monitors are ins ide the room wi th the attendant outs ide. The pat ien t and
moni tors can be v iewed through a window or by us ing a telev ision camera.
Light-emitting diode (LED) d isp lays are usual ly easier to read from a
distance than l iquid c rystal displays (21). Remote auscul tation using
special ly designed equipment can be used to moni to r heart and respiratory
sounds (29). A drawback is that moni tor sounds and a la rm s ignals may not
be heard wel l by the attendant.
• The thi rd op tion is to have the moni tors ou ts ide and the attendant ins ide the
room. Mos t equipment can be kept ou ts ide the scanner room wi th cables and
such running through wave guides (30,31). The a ttendant can observe the
moni tor through a window or a te lev is ion screen but cannot hear moni to r and
alarm sounds we ll .
• The last opt ion is to have both the monitors and the attendant outs ide the
room. The a ttendant can see the monitors and can hear the alarms and
sounds but cannot observe the patient wel l.
I f the a ttendant is outs ide the room, he may fai l to detect dangerous s ituat ions in a
t imely manner (32). There is a report of a pat ient dying during an MRI procedure
when the pneumatically-driven venti la tor ran out of oxygen (33). MRI auditory alarm
s igna ls need to be much louder than those used in the operat ing room due to the
s ignif icant scanner noise .
Problems I t is important to note that some dev ices tha t are s ta ted to be MR compatib le have
l imi ta tions or res tric tions to the ir use in the MR environment, and if they are not
used in accordance with these restric t ions/ l imitat ions, they can pose the same
types of hazards as devices that are no t MR compatible (14,34).
Ferromagnetic Materials All materials can be c lassif ied as ei ther paramagnetic o r diamagnetic
(7,15,16,17,18,21,22,27,35,36,37,38,39,40,41,42). Paramagnetic materials a re
weakly a ttracted and diamagnetic materials are weakly repel led by magnetic f ields.
Ferromagnetism is an extreme form of paramagnetism exhibited by a small g roup of
materials that are powerfully attracted to magnetic f ie lds. Ferromagnetism is shown
by devices conta in ing i ron, i ron oxide, i ron-containing a lloys , nickel, and cobalt
(43).
Depending on the conf igurat ion of the magnetic f ie ld and the shape and mass of the
object and i ts posi tion wi thin the magnetic fie ld, these forces can result in
rota tiona l (torque) and/or translationa l (a ttractive) motion of the object.
Ferromagnetic Materials External to the Patient
The attrac tive force exerted on a ferromagnetic object depends on the distance
between the object and the center of the magnet, the mass and geometry of the
object, the strength of the magnet, and fac tors that modify the f ie ld configurat ion
such as magnetic shielding.
The attrac tive force inc reases rapidly as one nears the magnet and can be several
t imes that of the earth's gravi ta tiona l f ie ld by the t ime i t reaches the center of the
magnet. When free, ferromagnetic objects can move toward the magnet center wi th
dangerous speed (“missi le ” or “pro jec ti le” effect). Th is can resu lt in equ ipment
damage as wel l as serious injury to pat ients and/or workers trying to restrain the
equipment or trapped between the equipment and the magnet (14 ,44,45). Since the
magnet is cont inuously ON, it can attract ferromagnetic devices even when no
imaging is occurring . In addit ion,
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s ignif icant masses of ferromagnetic material in proximi ty to the magnet can disturb
the homogenei ty of the s ta tic magnetic fie ld, resul t ing in dis torted images .
Ferromagnetic parts may work loose over t ime, so precautions should be taken
during product design to prevent the inclusion of components that could be pulled
loose and attracted to the magnet bore (10). Removable equ ipment covers should
use capt ive hardware (e.g., screws and fasteners ).
There are hundreds of fe rromagnetic objec ts that must be kept out of the MRI room.
These inc lude personal i tems (watches, sc issors, keys, paper c lips , nai l cl ippers,
hairpins, calculators, ident if icat ion badges , c igarette l ighters, s teel-t ipped/hee led
shoes, pens , jewelry, c lipboards, pagers, cell phones, f irearms, e tc.); pat ient i tems
(strap buckles, safety pins , jewelry, zippers, metal gown fasteners, contracept ive
diaphragms, cosmetics containing metall ic part icles (such as eye makeup), sk in
staples , superf ic ia l metall ic sutures, RF tagging brace lets, metall ic handcuffs or
ankle cuffs , e tc .), and medical dev ices (s tandard gas cylinders, hemostats,
needles, v ia ls , s te thoscopes, chest tube stands , in travenous po les, s tre tchers,
wheelchairs , mobi le s tands, carts, anes thesia machines , vaporizers, monitors,
defibri l la tors, sandbags, tract ion weights, etc.) as wel l as mop buckets, vacuum
cleaners , laundry carts, chairs, ladders, l ight f ix tures, f loor buffers, and parts of a
fork lif t (12 ,38,39,40,46,47,48,49,50,51). Non-l ithium batteries are strongly
magnetic .
The presence or absence of ferromagnetism in a given object depends on a number
of fac tors, including i ts composit ion and manufac ture. Some objects tha t are
ferromagnetic may sti l l be safe because their mass is too small for the forces
involved to be s ignif icant and/or because they are fi rmly anchored in posi tion at a
safe distance f rom the scanner. One method is to anchor a ll devices that have
ferrous material to a movable cei l ing pendant system with a predetermined l imited
range of motion (24). Even wi th a reduced ferrous load, some delicate instruments
are sti l l ferromagnetic and subject to to rque tha t can cause serious damage.
Therefore, al l equipment that is no t required should be removed f rom the s ite.
Equipment may be posi tioned away from the magnet, in a room adjacent to the MRI
room with tub ings and wires used to connect to the patient. The safe distance from
the center of the magnet depends on field strength and shielding. I t is usually
considered to be greater than the 5-gauss l ine (12). Equipment containing
ferromagnetic components should not be a llowed past the 5-gauss l ine unless i t has
been labeled MR safe for that specif ic MR environment.
All persons entering the scan room must be rigorously sc reened for inte rnal and
ex ternal ferromagnetic material . Prominent warning s igns should be pos ted. MRI
centers should keep registers of commonly used devices and whether they are safe
to bring in to the magnet room. The use of metal de tec tors in MR env ironments may
help, but is not recommended by the American Col lege of Rad io logy (49).
Nonambulatory patients should be brought into the MR uni t by using a nonmagnetic
wheelchair or wheeled stretcher and transport equipment checked fo r magnetic
objects (12).
Materials considered safe in the MRI scanner sui te inc lude aluminum, brass, nickel ,
plast ic , t itanium, copper, bery ll ium, s i lver, and gold (43,52). Certain types of
s tainless steels are considered safe, but others are strongly a ttracted in the
magnetic f ie ld (11,38).
Battery-powered equipment shou ld be tested at i ts intended locat ion and maximum
f ield strength to ensure that there is no s ignif icant attract ion . A magnet can be used
to tes t equipment going into the MRI room (53). Mos t inst itut ions have a cardiac
pacemaker ring magnet, and this can be used.
Ferromagnetic screening does not evaluate the potent ial for RF-related pat ien t
injury (53). Nonferromagnetic metals (e .g., aluminum or copper) that would y ield a
negat ive magnetic screen can absorb RF energy, resu lt ing in MR image art ifact or
thermal inju ry to the patient.
Implanted or Inserted Ferromagnetic Objects
Ferromagnetic objects wi thin the patien t are subject to fo rces tha t try to bring them
into alignment wi th the magnetic field. The extent of injury wil l be affected by the
magnetic f ie ld strength, ferromagnetism of the objec t, the objec t's geometry and
orienta tion, the loca tion of the objec t in s i tu, and the length of t ime the object has
been indwel l ing (f ibrosis or granulat ion t issue can serve to s tab il ize the object).
There are documented cases of death and blindness resul t ing f rom MR imaging of
patients with fe rromagnetic in tracerebral aneurysm clips, cardiac pacemakers, and
c linical ly occul t metal l ic in traocular (39,54,55,56). MRI is contraind ica ted for a
patient wi th shrapnel located in a biologically sens it ive area. I t cou ld move and
injure the pat ient (10). People in certain occupations, such as sheet metal workers,
are at risk of having magnetic f ragments in their bod ies and in many cases are not
aware of thei r presence (39). Now tha t MR imaging is a f i rmly establ ished
diagnost ic modality, efforts are being made to use magnetical ly compatible
subst itutes fo r previously uti l ized ferromagnetic implants (39). For example , most
new cerebral aneurysm clips are made of nonferrous material (52).
The Food and Drug Adminis trat ion (FDA) requires that MR imagers be labeled to
indicate that the device is contraindicated fo r pat ients who have electrica l,
magnetic , or mechanical implants because the energies produced by MRI systems
may interfere wi th the opera tion of these devices (57). The composi t ion of the
device and i ts magn itude of magnetic def lection should be determined before these
patients are scanned. These
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devices inc lude pacemakers , cardioverter-def ibri l lators, elec tromechanical infus ion
pumps, cochlear implants, neurostimu lators, bone-growth st imula tors, dental
implants, bul lets, magnetic sphincters , magnetic stoma plugs, magnetic ocular
implants, t issue expanders wi th magnetic posts , and magnetic pros thetic appl iances
(57). Problems may also occur with magnetic objects that are attached to the
patient 's body (e .g ., body pierc ing) (12).
Equipment Malfunction Magnetic inte rference can cause computer o r oscil lometric images to be distorted
(7,21,27,58,59). Devices wi th rechargeable batteries may swi tch off and blank their
sc reens. MRI interference also causes transformers to become saturated and burn
out. RF pulses are also capable of inducing electrical eddy currents and short
ci rcui ting electrical equ ipment. The magnetic f ie ld may cause damage or erase data
on magnetic media such as digi tal tapes or f loppy disks.
Equipment may conta in pumps, e lec tric motors, elec tronic ci rcuitry , or analog
gauges that a re affected by the magnetic f ie ld (10). The magnets in motors may
become saturated. This can result in lack or s lowing of motor operation or
increased operat ing current, which could u ltimately burn the motor out.
There are two general methods to solve this p rob lem: locate the equipment ou t of
the magnetic f ie ld or make the equipment compatible wi th the magnetic f ie lds.
One way to make equipment MRI compatib le is to shield moni to rs and cables f rom
RF currents . Cables can be wrapped wi th a th in layer of aluminum fo il , and small
copper boxes can be used to house electrical equipment. Using appropriate RF
f il te rs in the magnet shie lding and iso la ted f i l tered a lternat ing-current power
c i rcui ts may permit effec tive monitoring (58).
Moni tors special ly designed for use in the MRI uni t combine low ferromagnetic
content, shielding, and f i l ters to min imize magnetic f ie ld inte rference. These
devices may have some limi tat ions to their use in the uni t. It is important tha t prior
to use, manufacturer dec laration and/or clearance by a recognized body such as
the FDA demons trates MR compatibil i ty .
Image Degradation Equipment can degrade the qual i ty of imag ing in two ways (7,10,17,27,30,58,60).
Fi rs t, ferrous meta l in mon itors and other equipment can cause distu rbances in the
magnetic f ie ld. Second, the equipment may genera te s ignals that interfere with the
MRI s ignals .
Any RF energy within the bandwidth of the rece iver, inc luding stray radiation f rom
outside RF sources, is amplif ied. Sources of RF noise include electrical machinery,
motors, computer hardware, disp lays, telev is ion transmitters , beeper-paging
systems, two-way rad ios, commercial rad io stat ions, and other electrical equ ipment.
I t is common practice, therefore, to enclose the scann ing area in an RF shield.
Nonconduct ive devices can safely be passed th rough the shield , but wires that
transgress the shield can ac t as aeria ls and feed noise from the ex ternal
environment into the examination area. This can be overcome by f i l tering wires that
run in and out of the sh ield. Care mus t be taken to match f i l ters to specif ic
moni tors .
To prevent RF in terference from degrading the MR image, appropriate measures to
provide sh ielding, such as housing the display in a RF-t igh t enclosure or locat ing
the uni t outside the room, shou ld be used. In ei ther case, the connect ions between
the display uni t and the patient should pass through f i l ters tha t attenuate
f requenc ies around the imaging f requency. All electrically conduc tive material tha t
is not required shou ld be removed f rom the MRI system bore .
The distance from the magnetic core necessary to prevent RF interference varies
wi th the strength of the magnet. Severa l meters be tween the scanner and other
equipment are usually necessary. This dis tance can make it dif f icult to read and
inte rac t wi th the equipment. Long spl iced l ines and tubings may disconnect, impose
high resistance, or become obs tructed.
Another solut ion is to adjust the frequency of the noise source so that i t is outs ide
the MRI-receiver system bandwidth. This can of ten be accomplished by adjust ing
the block that controls the operation of the microprocessor in the monitor.
Once the examination has started, the pos ition of the moni to ring equipment should
not be changed, because reposit ioning of a la rge metal l ic mass may degrade
magnetic f ie ld homogeneity.
Burns Apply ing intermit ten t RF f ields to metal lic objec ts wi th in the imaging area can
cause heating and resu lt in burns (7 ,10,16,26,48,57,59,61,62,63,64,65,66).
Equipment of p rimary concern is tha t which may be posit ioned di rect ly in the RF
transmission f ield. Th is includes electrocard iogram (ECG) electrodes and leads,
pulse pickup and pu lse oximetry sensors and cables , halo dev ices, and surface
co ils as wel l as metal lic components, connectors, and surface coils . These can
provide a path fo r curren t f low th rough the patien t or along the cab le sh ie ld to
ground. If this current is not l imi ted, patient burns can resul t at the loca tion of
equipment or at body parts located next to an insuff icient ly insulated RF shield.
Other devices that uti l ize a wire such as thermodilut ion Swan-Ganz ca theters or
epidural catheters can also sub jec t the patient to electric shocks or dangerous
heating (43).
Burns may also occur f rom contact wi th the scanner bore. High duty cyc le
sequences cause heat to bu ild up
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on the ins ide of the bore. Burns can be minimized by using the fo llowing measures:
• Electrica lly-conduct ive leads should be replaced with nonconduct ing paths
(e.g ., f iberopt ic cable or plast ic tubing) o r high-res is tance paths (e.g ., carbon
ECG leads).
• Electrica lly-conduct ive material (ECG leads, cables, e tc .) that must remain
wi th in the MR system bore should be pos it ioned to prevent “cross po ints.” A
cross poin t is where a cable c rosses another cab le , loops ac ross i tself , or
touches ei ther the patien t or s ides o f the magnetic bore more than once.
When the patient is moved, wires may coil and fo rm a loop, so i t is important
to check a ll wires each t ime the pat ient is moved.
• Cables and sensors should be kept away f rom the bore (e.g., by plac ing a
pulse oximeter sensor on the pat ient's toe when the head is being
examined).
• All sensors, wires, and cables should be checked to ensure that the electrical
insulat ion around them is intact. A small towel should be placed between the
patient and the wires or cab le to avoid contac t wi th the skin.
• All unnecessary conduct ive materials such as unused wires, leads, sensors,
cables, and surface coi ls should be removed f rom the MRI envi ronment.
• All electrica lly-conductive material that must remain in the MR system bore
should be kept from direct ly contacting the patient by placing thermal and/or
electrical insulat ion (inc luding ai r) between the conductive material and the
patient. Wires should not cross metall ic prostheses.
• Cables should be pos itioned so that they exi t as c lose as possible to the
center of the pat ient table of the MR system (63). I t is a lso important to avoid
cable contact wi th the sk in or the MRI scanner.
• Conscious patients should be ins tructed to call out if they experience
uncomfortable heat levels so that the procedure can be immediately
discont inued.
• Any monitor that does not appear to be operating properly during the MR
procedure should be removed.
• A cold compress/ice pack should be placed along skin stap les , superf ic ial
metall ic sutures, and lead attachment s ites, if this can be safely
accomplished (49).
Hypothermia
Hypothermia can be a problem, especial ly with smal l children (27). Ai r is often
c i rculated over the patient during the imaging sequence. Covering the patien t,
warming f luids, and using non-electrical heating pads and humidif ied inspired gases
can help to prevent hypothermia (52). Heating blankets and radiant l ights cannot be
used because they inte rfere wi th imaging.
Other Patient Problems Loud sounds in the MR environment may obscure cries for help from a patient in
dis tress (9). The patien t should be given a squeeze bal l to sound a distress signa l,
but this is of l imi ted usefulness in the sedated patient.
A major problem for anesthesia personnel is the relative inaccessibi l i ty of the
patient. This creates problems with moni to ring, airway management, intravenous
access , patient v isual ization, and moni tor applicat ion. Patients can only be di rectly
observed from ei ther end of the tunne l and can only be extricated by sl iding them
out.
Specific Equipment A number of devices special ly designed to be used in the MRI unit are avai lable . It
is important to demonstrate MR compatib il i ty by preuse tes ting, manufacturer
declarat ion, and/or c learance by a recognized body such as the FDA. For devices
that do not carry such credentials , i t is advisable to consul t the manufacturer or
biomedical personnel with expert ise in MRI before introduc ing them in to the MRI
room. I t is important to read all of the ins truct ions accompanying a p iece of
equipment, because some equipment may be MR safe or compatible only if
instal led a certain dis tance from the magnet (67).
MR is an evolv ing technology, and specif ications and perfo rmance characteris tics
of specif ic MR sys tems cont inual ly change. Products should be tes ted under
s imulated or ac tual use condi tions. Because upgrades to the MR sys tem to achieve
higher perfo rmance levels may affect the characteris t ics of the equ ipment,
yesterday's device tests may not be suf fic ient to ensure the safety and
ef fectiveness of a device wi th today's MR system (10). Since new equipment is
continually being introduced, it is on ly possible to make general iza tions about
which equipment is MRI compatible. I t is recommended tha t equipment intended for
use wi thin the magnet room be c learly labeled wi th the maximum field level wi th in
which i t can opera te effect ively (10).
Monitors wi th mul tiple funct ions decrease the amount of equipment that must be
transported, decreasing the risk of equipment damage and injury to the personnel
responsible for transport.
Any equipment that has an alarm should have a v isual alarm s ignal , because
auditory s ignals may not be heard (5).
Anesthesia Machines Standard anesthetic machines conta in varying amounts of fe rromagnetic
substances and electronically control led components, making them unsui table for
use in
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proximity to an MRI magnet (68 ,69). Non-MRI compatible anesthes ia machines
have been used wi thout problems by keeping them 20 to 30 feet away f rom the core
of the magnet (26). Some anesthesia pract itioners have used long tubings fed into
the scanning room f rom an anesthesia mach ine p laced outs ide the scanning room
or a standard anesthesia machine bol ted to the wa ll (31 ,52). Replacing
ferromagnetic components (e.g., support s tructures, castors, cy linders, and parts of
cylinder supports) with nonferromagnetic materia ls and aluminum cylinders can
make an anesthesia machine MRI compatible. A few ounces of magnetic material
may prove negl igible. MRI-compatible anesthesia machines are available (Fig.
30.2). Aluminum cyl inders have been avai lable for some time. Unfortunately,
confusion about cylinder composit ion can arise (70).
Vaporizers Most standard vaporizers perform accura tely when used in the scanning room
(36,68,69,71). However, a portable vaporizer may be dangerous due to
ferromagnetism (51).
Anesthesia Breathing Systems A number of breathing sys tems have been used successful ly in the MRI
environment. These inc lude the c i rc le and Mapleson systems (20,35,52,58,72,73).
Long corrugated or fresh gas tub ing wi l l be required if the anesthesia machine is
remote f rom the pat ient. Fortunate ly, most components of ci rc le breathing systems
used today are nonmetall ic .
View Figure
Figure 30.2 MRI-compatible anesthesia machine. (Picture courtesy of Ohmeda.)
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Anesthesia Ventilators Standard anesthesia venti lato rs usual ly contain ferromagnetic materia ls , pumps ,
electric motors, and analog gauges that can be affected by the magnetic field
unless specif ically adapted fo r the MRI. These may not function properly in a
magnetic f ie ld or may degrade the image. A remote venti lato r in the control room
wi th c i rcui t tubing fed into the scann ing room can be used, o r an incompatible
venti lato r can be attached to the wall (31 ,52).
Specially engineered MRI-compatible venti lators (bo th anesthesia and c ri tical care)
that have no electronic components are avai lab le (27,35,74,75,76,77). Fluidic
venti lato rs work wel l in the MRI envi ronment (75). Cur-rent ly avai lab le MRI-
compatible anesthesia machines are equipped wi th venti lators . An MR-compatible
posit ive end-expiratory pressure (PEEP) device is available (78).
Pulse Oximetry Most of the early pu lse oximeters were both suscept ible to interference from the RF
pulses and a source of interfe rence to the MRI s ignal (10 ,15,17,21,79,80). Burns
have been reported at both the sensor s i te and along the cable
(17,22,61,62,81,82). Heavy insulat ion and shielding can be used to isolate the
moni tor signa l (17,21). The moni to r should be p laced as far from the magnet as
possible.
Components, inc luding the probe, with a low ferromagnetic content are useful , as is
minimizing the number of electrica l connec tions and computer chips within the
moni tor. Most consoles have f i l te rs in the circui try and some models have f i l te rs in
the cable to help eliminate ex traneous s igna ls . These help to separa te the s ignal
f rom background noise (21).
A convenient method to eliminate ferromagnetic wire is to use a f iber-opt ic cable
(79,83). If a s tandard ferromagnetic cable is used, i t should be wrapped wi th
aluminum or copper foi l to shield the metal and decrease RF interference
(22,80,84).
A varie ty of MRI-compatible sensors , cables, and consoles are available. The
sensor can be shielded to prevent noise from reaching the MR system. The digi ts
can be protec ted wi th c lear plast ic coverings (27).
The sensor should be placed as fa r from the scan si te as possible. This may
require that i t be placed on the foo t or head, depend ing on the area to be imaged.
The cable should be extended in a s traight l ine away from the patien t. The length of
the cable should a llow the oximeter to be far enough f rom the magnet to prevent
image degradation .
Respiratory Gas Monitoring There are a number of di fferent technologies to measure respiratory gases. They
are discussed in Chapter 22 . Mainstream capnometry sys tems cause image
interference. The sensor requires shielding f rom RF rad iat ion. Sidestream moni tors
may be used during imaging as long as the moni toring device is ou ts ide the
magnetic f ie ld (19,58,85). Mult iple lengths of tubing may be connected to a llow the
moni tor to be outs ide the magnetic f ie ld. The extra length of tubing between the
patient and the monitor may degrade the capnogram and decrease the accuracy of
the end-tidal reading, especially with smal l pa tients (22). However, even if end-tidal
measurements are no t accura te, the trends and resp iratory pa ttern may s ti l l g ive
useful info rmation. If enough tubings are connected together, the resistance can
become so high that the alarm for blocked tubing is activated (83).
MRI-compatible mul tigas monitors are avai lable . This techno logy can be combined
wi th other modal it ies such as pulse oximetry, electrocardiography, and non invasive
blood pressure monitoring (52). Such equipment can be posi tioned either ins ide or
outside the magnet room.
Chemica l detectors (Chapter 22) tha t change color in the presence of carbon
dioxide are made of plas tic and contain no metal parts. They mus t be placed c lose
to the patient and may be diff icul t to see. They are qual itat ive devices and are on ly
generally quanti tative . This makes them more sui ted for checking tracheal tube
placement than for long cases. Electrochemical oxygen analyzers are usua lly safe
for use in the MRI sui te as long as they are outside the magnetic f ie ld (20,22).
Noninvasive Blood Pressure Monitors Automated blood pressure monitors are discussed in Chapter 27. The MRI does not
af fect monitors that use the osci l lometric method as long as the elec tronic uni t is
ei ther sh ielded or wel l away f rom the magnet core (18 ,19,20,26,27,36,85,86,87,88).
I t may be necessary to use extended tubings. Ferrous connections on the cuff
should be replaced wi th plast ic ones (83).
Manual sphygmomanometers have been adapted for use during MRI by rep lacing all
ferromagnetic components wi th brass, aluminum, or plast ic pieces (36). A plast ic
s tethoscope can be used to moni tor Koro tkoff sounds. The magnetic f ie ld may
af fect the accuracy of analog gauges since the movement mechanism in some
gauges uses a smal l magnet and a co il for operation . Blood pressure can be
moni tored outside the scanner by lengthen ing the tubing connected to the cuff (89).
Ul trasonic Doppler units have been used to moni to r sys tolic blood pressure (89,90).
Invasive Blood Pressure Monitors Most intravascular l ines are nonmagnetic and are not affec ted by the magnetic f ie ld
(36). Direct pressure readings can be obtained if the lead f rom the pressure
P.879
transducer is passed th rough a RF f il ter or posi tioned away from the magnet
(30,58). Fiber-op tic MRI-compatible transducers are avai lable (18,91).
Electrocardiographic Monitors The ECG can cause patient burns and image degradation as we ll as funct ion
inaccurate ly in the MRI envi ronment (15,17,21,27,35,52,58). ST-segment
moni toring is impossible in the presence of the magnetic f ie ld using current
technology (92).
The amount o f RF interfe rence tha t is coupled into the leads can be controlled by
appropriate design measures (e.g., by se lecting appropriate materials for the ECG
leads and by fi l tering the amplifier c ircui try input) (10). Special MRI-compatible
electrocardiographic leads may be constructed by using high-resistance conduc tors
that minimize induced RF currents (23).
A number of measures wil l minimize effects (7,21 ,27 ,35 ,58,93). Non-ferromagnetic
fasteners, e lectrodes, leads , and cables are avai lable. A f i l ter can reduce artifacts
in most leads (68,94). Good skin prepara tion is important to opt imize the ECG
signa l. The skin should be shaved and cleansed wi th alcohol before the e lec trodes
are placed (15). For best resul ts , the sk in should be dried or l ightly abraded (27).
Electrodes should be placed as c lose to the center of the magnet as poss ible
because the RF power is chang ing least at this point. The l imb electrodes should
be as c lose together as possible and in the same plane. Using chest leads, V5 and
V6 minimizes art ifac ts (95). Liquid crystal sc reens do not undergo distortion as do
ca thode ray tubes (58). The relat ive length of ECG leads may be cri t ical in
preventing gradient-induced art ifact; therefore, many manufac turers advise against
modifying ECG leads supp lied wi th MR sys tems (58).
Telemetry may sound appea ling because i t reduces the wiring and cables needed,
but it may interfere wi th the RF used for imaging. Th is interference is not seen wi th
ul trahigh f requency uni ts . These telemetry uni ts are l imited to QRS complex
detec tion (27,88).
Temperature Monitors Standard thermistor temperature probes can distort the magnetic f ie ld, causing
localized image distort ion if placed near the area of in terest (96). They could also
cause a localized concentration of RF power that would result in increased heat ing.
Smal l wire probes have been used in the MRI without excessive heating (96).
Al though temperature probes with f i l tered cables are availab le, they carry the same
risks of s ignal inte rference and burns as other electronic moni tors (97,98). A burn
associated wi th temperature monitoring us ing a probe specially designed for use
during MRI has been reported (98). A burn may be caused by inadvertent formation
of a cable loop during patient movement.
Fluoropt ic temperature probes that contain material that has temperature-
dependent op tical properties are not affected by high magnetic field strength and
RF pulses and are therefore more sui table for use in the MRI env ironment (96).
These probes are more expensive and more f ragi le than wire thermistors.
Liquid crystal temperature s tr ips work we ll in the MRI room but may not be easy to
observe (15).
Laryngoscopes The s tandard laryngoscope (Chapter 18) is no t fe rromagnetic but can undergo a
degree of to rque in a magnetic f ie ld (58). Plast ic laryngoscopes are available, but
most batteries are magnetic . One solution is to modify the hand le to operate f rom a
non-battery-based power source (36 ,52). A remote f iberopt ic l ight source can be
used. MRI-compatible laryngoscopes are avai lable. Most are powered by l i th ium
batteries . A paper- or p las tic-covered l i th ium battery can be used wi th a plas tic
laryngoscope (17,18,19,58,99).
I f an MRI-compatible la ryngoscope is not avai lable , anesthesia can be induced and
the ai rway intubated outs ide the magnet room using a conventional laryngoscope
before the patient is moved in to the scanner (19). If inadvertent ex tubat ion occurs
whi le the patient is in the scanner, an MRI-safe laryngoscope wil l be needed.
Tracheal Tubes Tracheal tubes (Chapter 19) are most commonly made of polyv inyl chloride and are
safe to use in the MRI env ironment. There are reports of signal interfe rence from
the spring in the cuff inf lat ion check valve (60 ,100,101). A tracheal tube wi th a wire
sp iral cannot be used in the MRI room (35,102). Connectors shou ld be plast ic .
Supraglottic Airway Devices The laryngeal mask (Chapter 17) is sui table for the MRI envi ronment. It a llows the
l ight level of anesthesia that is possib le, as there are no painful s t imuli involved in
the MRI process. Because some la ryngeal inf lation valves masks contain metall ic
material , i t may be necessary to remove the valve and knot the pilot tube (103).
Special laryngeal mask a irways (LMAs) wi th valves that do not conta in ferrous
material are availab le (104). I f the LMA-Flex ible or LMA-ProSeal is used, the metal
co il p roduces a large black hole in the image in the area surrounding the airway as
wel l as deteriora tion of the image fu rther out (105).
P.880
The LMA may not be sui table if magnetic resonance spec troscopy is performed
because the resonance of some s i l icone-containing materials compromises
inte rpretat ion of the scans (106).
Other supraglott ic devices are for the most part made of polyv iny l chloride or
s il icone, wh ich does not cause problems with the MRI. There are no reports of
problems at the t ime of this wri ting .
Other MRI-safe and -compatible suc tion regulators and nonmetall ic precordial
s tethoscopes are availab le and should be used.
Some infusion pumps need to be located away from the magnet o r shielded or they
may perform inaccura tely (17,36). MRI-safe infus ion pumps are avai lable.
Extension tubing may be required. There is a report of a pat ient-control led
analgesia device that malfunc tioned a fter exposure to the magnetic f ie ld (107,108).
Some infusion pumps func tion correct ly in the MRI envi ronment (52,109,110,111).
I t is possible to moni tor intrac ran ial pressure in this setting by using a
nonferromagnetic subdural bo lt o r plast ic catheter and a f iberoptic cable
(21,53,112,113).
Electroencephalography and evoked potential monitoring, both of wh ich use long
conduc ting wires , wil l requ ire technologica l advances before they can be used in
magnetic f ie lds (92).
Personnel Hazards I t is no t possible a t th is time to determine wi thout question if there are hazards to
personnel who work in the MRI envi ronment. Th is is largely due to the relatively
short t ime for wh ich this method of imaging has been in exis tence and the fact that
this work is occasional for most pract i tioners. Future ep idemiologic ev idence may
or may not demonstra te harmfu l effects of long-term exposure . Female staff should
consider avoiding th is area during the f irs t 3 months of a pregnancy.
Noise Noise is due to v ibrat ion of the swi tched gradient coils and is enhanced wi th h igher
strength gradient coils , shorter gradient duty cycles, and fas ter pulse repetit ion
f requenc ies in the magnet (114). The trend toward upgraded imaging systems and
stronger magnets would suggest tha t noise levels are likely to increase (31). No ise
levels up to 106 ±4 decibe ls wi th an average around 92 decibels have been
demonstra ted (31). These are potentially damaging levels, and care must be taken
to reduce these levels of exposure as much as possible . Ear plugs shou ld be
considered, a lthough they may interfe re wi th hearing aud itory ala rm signals (43,52).
Exposure to the Magnetic Field
With the inc reased use o f MRI and magnets of increasing strength, anesthesia
providers are experiencing increased exposure to stat ic magnetic f ie lds wi thin MRI
units . The s ta tic magnetic f ie ld extends beyond the conf ines of the magnet. The
strength of this f ringe f ield depends on the magnet conf igura tion and sh ie lding . It
decreases rapidly as the dis tance from the magnet increases. The biologic effect of
s tat ic magnetic f ie lds is a controvers ial topic (38). There are no demonstra ted
oncogenic or teratogenic e ffects (52). I t wou ld seem prudent to take reasonable
precautions against repeti tive and avoidable exposure to intense magnetic f ie lds.
Risks include danger from unrestrained ferromagnetic objects and from movement
or malfunct ion of MR-incompatible implants wi thin their bodies (31).
Trace Anesthetic Gases Trace anesthet ic gases and the ir r isks to anesthesia personnel are discussed in
Chapter 13. Scavenging sys tems and good venti lat ion wi th f requent ai r exchanges
are not always available in the MRI uni t (31). Some breathing systems such as the
Mapleson sys tems (Chapter 8 ) are often used for pediatrics, and many of these are
not scavenged.
Hypoxia I f l iquid helium that surrounds the superconducting so lenoid in the magnet escapes ,
i t is possible that a hypoxic envi ronment may be created (31). Oxygen sensors can
be placed in the cei ling to warn of this problem. 1Most physicians refer to this type of imaging as MRI, wh ile most physic is ts refer to
i t as nuclear magnetic resonance (NMR) (7). Both are accepted terms and refer to
the same technology.
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Questions For the fol lowing quest ion, select the correct answer
1. The distance from the center of the magnet that is usually considered to be safe for ferromagnetic objects is A. The 2-gauss l ine
B. The 3-gauss l ine
C. The 4-gauss l ine
D. The 5-gauss l ine
E. Beyond the 5-gauss l ine
View AnswerFor the following quest ions, answer
• i f A, B, and C are correct
• i f A and C are correct
• i f B and D are correct
• i f D is correct
• i f A, B, C, and D are correct.
2. Which factors determine the attractive force on a ferromagnetic object in the MRI unit? A. The strength of the magnet
B. The mass and geometry of the ob jec t
C. Magnetic shie ld ing
D. The d is tance of the objec t from the outer f ie ld of the magnet
View Answer3. Which of the following are considered to be safe in the MRI scanner?
A. Aluminum
B. Beryll ium
C. Gold and s ilver
D. All types of s tainless steel
View Answer4. Which metals can cause a negative magnetic screen and absorb radio frequency energy that can result in an MRI artifact or thermal injury to the patient?
A. Aluminum
B. Gold
C. Copper
D. Silver
View Answer5. Sources of radio frequency noise that can degrade the MRI image include
A. Commercial rad io stat ions
B. Beepers
C. Motors
D. Liqu id crystal d isplays
View Answer6. Which items may cause a burn from radio frequency fields? A. ECG electrodes
B. Epidural catheters
C. Pu lse oximetry cables
D. Swan-Ganz catheters
View Answer7. Methods to keep pediatric patients from developing hypothermia in the MRI unit include A. Warm f luids
B. Radiant l ights
C. Covering the pat ient
D. Electrical heating pads
View Answer
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