INTERNATIONAL COMMISSION ON NON‐IONIZING RADIATION PROTECTIONICNIRP STATEMENTON MEDICAL MAGNETIC RESONANCE (MR)
PROCEDURES: PROTECTION OF PATIENTSPUBLISHED IN: HEALTH PHYSICS 87(2):197‐216; 2004

ICNIRP PUBLICATION – 2004

ICNIRP Statement
MEDICAL MAGNETIC RESONANCE (MR) PROCEDURES:PROTECTION OF PATIENTSThe International Commission on Non-Ionizing Radiation Protection*
INTRODUCTION

magnetic and radiofrequency electromagnetic fields associated with the use of MR diagnostic devices, and on
precautions to be taken to minimize health hazards andrisks to patients and volunteers undergoing MR examinations.
The document is intended for use by international or
national medical device regulatory authorities, MR users
and health professionals, and those involved in the design
and manufacture of MR equipment for clinical applications. Contraindications, precautions, and safety considerations for patients are given.
Guidelines on occupational and general public exposure limits to all ranges of electromagnetic fields(static, time-varying gradients, radiofrequency) havebeen published (ICNIRP 1994, 1998). However, theseguidelines were written many years ago, and they are
now under review.

MAGNETIC RESONANCE imaging (MRI) has become anestablished diagnostic imaging modality. The clinicalusefulness of in-vivo magnetic resonance spectroscopy(MRS) has been demonstrated in several clinical applications and is being explored further. These techniques
involve exposure of the patient to static and time-varying
magnetic fields, radiofrequency electromagnetic fields,
and acoustic noise. In particular exposure conditions,these fields may pose a health hazard or increased risk.
The International Non-Ionizing Radiation Committee of the International Radiation Protection Association(IRPA/INIRC) has published a guideline on protection of
the patient undergoing a magnetic resonance examination (IRPA/INIRC 1991). In recent years, the successorof IRPA/INIRC, the International Commission on NonIonizing Radiation Protection (ICNIRP) has performed
reviews of the biological effects from exposure to staticand time-varying electromagnetic fields (ICNIRP 1997a,
1997b, 2001). Guidelines on limits of exposure to static
magnetic fields and guidelines for limiting exposure totime-varying, electric, magnetic, and electromagnetic
fields (up to 300 GHz) have been published by ICNIRP
respectively in 1994 and 1998 (ICNIRP 1994, 1998).
Recently, several reviews concerning safety aspectsof MR procedures have been published (e.g., Ordidge etal. 2000; Shellock 2001a; Shellock 2003). These publications, in conjunction with other reviews and recent
literature, form the basis for the review of research dataon MRI procedures in this document.

RATIONALE
A review of the biological effects from exposure tomagnetic fields is contained in UNEP/WHO/IRPA(1987) and ICNIRP (1997a and b, 1999). Additional data
and references can be found in Magin et al. (1992), IEC(2001) and Shellock (2001a). Recommendations forradiofrequency exposure levels are based on the datacontained in reports by the NCRP (1986), UNEP/WHO/
IRPA (1993), ICNIRP (2001), WHO (2003) and on therationale appended to ICNIRP (1998).
The following is a brief summary of conclusions
drawn from the review of scientific literature.

PURPOSE AND SCOPE

Static magnetic fields
The possible health effects that might result fromacute exposure to intense static magnetic fields havebeen recently reviewed by Schenk (2000, 2001). The
basic actions of static magnetic fields are physical effects(translation and orientation), electrodynamic forces onmoving electrolytes, and effects on electron spin states ofchemical reaction intermediates. At a higher level oforganization, biological effects in cells, tissues and livingorganisms have been studied.

The purpose of this document is to provide information on levels of exposure and health effects from* ICNIRP, c/o BfS—R. Matthes, Ingolstaedter Landstr. 1, 85764
Oberschleissheim, Germany.
For correspondence or reprints contact: R. Matthes at the above
address or email at info@icnirp.org.
(Manuscript received 5 February 2004; accepted 28 April 2004)
0017-9078/04/0
Copyright © 2004 Health Physics Society
197

198

Health Physics

Translation and orientation of molecular and cellular
substances such as retinal rods, and some living cells
have been observed in vitro in experiments with fields at
and above 1 tesla (T). This type of effect occurs on diaand paramagnetic materials such as hemoglobin, collagen, fibrin, and also on ferrimagnetic particles such as
magnetite. It is also now known that water distribution
can be affected by high-intensity high-gradient magnetic
fields (e.g., 8 T, 50 T m⫺1; Ueno and Iwasaka 1994).
Ueno and Iwasaka showed that in an 8 T magnetic fieldwater can experience a force up to 30% the force ofgravity. However, this force depends directly on thesquare of the magnetic field strength and inversely on theradius of the magnet. Consequently, in a whole-body 4 T
magnet, the force is only about 1% of gravity.
Static magnetic fields exert electrodynamic forceson moving ions in blood vessels. These forces lead to thegeneration of electric potential across the blood vessels(Hall effect) and theoretically a reduction of blood flow
velocity (Tenforde 1992). Such flow potentials caused byfields of more than 0.1 T can be detected during T-wave
recording in the electrocardiogram (ECG) (Togawa et al.
1967; Tenforde et al. 1983). Kinouchi et al. (1996)
calculated that the maximum flow potential induced by afield of 5 T would cause a current density of 100 mA m⫺2
at the location of the sinuatrial pacemaker mode (0.5 Vm⫺1, assuming a tissue conductivity of 0.2 S m⫺1). Thisis below the estimated threshold for cardiac stimulation(Reilly 1998). The T-wave indicates the repolarization ofventricular heart muscle when electrical excitability
gradually recovers following contraction (Antoni 1998).
Fibrillation of the heart, potentially fatal asynchronousand irregular cardiac muscle contraction, can only be
induced during this “vulnerable” period. However, thefibrillation threshold is about 10 –20 times higher thanthat for cardiac muscle stimulation per se. Reilly (1998)
estimates the 1- and 50-percentile ranks for cardiac
stimulation in humans due to induced electric fields to be
5 and 10 V m⫺1, respectively. In addition, a 5 and 10%
reduction in blood flow in the aorta was predicted tooccur in static fields of 10 and 15 T, respectively, due tomagneto-hydrodynamic interactions (Kinouchi et al.
1996). However, Kangarlu et al. (1999) found thatvolunteers exposed to an 8 T field for 1 h showed nochange in heart rate or diastolic or systolic blood pressure
either during or after exposure. The ECG recorded duringexposure was regarded as uninterpretable due to thesuperposition of the potential generated by aortic bloodflow and smaller potentials generated by blood flow inother vessels. In addition, vertigo and other sensations
were recorded during movement in this field.
More detailed studies by the same group haverecently been published (Chakeres et al. 2003a, 2003b);

August 2004, Volume 87, Number 2

both were approved by the U.S. FDA Investigational
Review Board. The authors report that exposure of 25
healthy volunteers, aged between 18 – 65 y, to 8 T static
fields (duration unspecified) had no effect on cognitive
function, assessed during exposure using seven standard
neuropsychological tests (Chakeres et al. 2003a). The
second paper reports a lack of clinically significant
effects of exposure to fields of up to 8 T on heart rate,
respiratory rate, systolic and diastolic blood pressure,
finger pulse oxygenation levels, and core body temperature (Chakeres et al. 2003b). There was a statisticallysignificant trend for systolic pressure to increase withflux density, but at 8 T this was only 3.6 mm Hg,
approximately half of the difference seen moving from asitting to a supine body position. Interestingly, thevolunteers were moved very slowly (one or two feet overa few seconds, followed by a 15–30 s pause, taking
overall about 3– 4 min) into the magnet bore in order toavoid the transient, movement-induced sensations described above (Schenck et al. 1992). Nevertheless, ninesubjects reported sensations of dizziness, and two reported a metallic taste, assumed to be due to electrolysisof metallic chemicals in the subjects’ teeth fillings.
The literature does not indicate any serious adverse
health effects from the whole-body exposure of healthy
human subjects up to 8 T. However, it should be noted
that, to date, there have been no epidemiological studies
performed to assess possible long term health effects inpatients, workers, or volunteers. It is important that suchresearch be carried out, particularly on individuals suchas workers and volunteers with high levels of exposure.

Time-varying magnetic fields
The rapidly switched magnetic gradients used inmagnetic resonance imaging produce time-varying magnetic fields in three orthogonal directions: normally, thez-axis corresponds to the longitudinal axis of the patient,
the y-axis to the transverse one, and the x-axis to theantero-posterior one. These fields are characterized by
the magnitude and duration of the change in magnetic
field and are often described by the parameter, dB/dt, thetime rate of change of the magnetic field. It induces
electric fields in a subject in accordance with Faraday’slaw, which, if of sufficient magnitude, can produce nerve
and muscle stimulation. The induced electric field is
proportional to dB/dt and the size of the subject. In
theory, the threshold for stimulation in terms of dB/dt
should be lower in larger subjects. However, otherfactors also influence the nerve stimulation threshold of
an individual and no correlation with subject size has
been observed in volunteer studies.

Medical magnetic resonance procedures ● ICNIRP

From a safety standpoint, the primary concern withregard to time-varying magnetic fields is cardiac fibrillation, because it is a life-threatening condition. The
threshold for cardiac stimulation is lower than that for
fibrillation (Reilly 1998). Peripheral nerve stimulation isa practical concern because, if sufficiently intense, it canbe intolerable and result in termination of the examination.
Recommendations on limiting patient and volunteer
exposure to time-varying magnetic fields are based
primarily on stimulation of peripheral nerves and muscles, including the heart, by the induced electric fields.
Peripheral nerve stimulation. The most extensive
work on peripheral nerve stimulation in humans was a
study of 84 volunteers at Purdue University (Bourland etal. 1999; Schaefer et al. 2000; Nyenhuis et al. 2001).
Data were obtained for the stimulation threshold, thethreshold for uncomfortable stimulation, and the threshold for intolerable stimulation during exposure to ramped
x- or y-gradient fields that induced pulsed electric fields
and associated currents.
The thresholds varied with ramped gradient pulse
width, increasing as pulse widths decreased in a mannerthat can be approximated by a hyperbolic relationship.
Conventionally, the term rheobase describes the electrical stimulation threshold for long (infinite) pulse duration, and the term chronaxie is used to describe the pulse
duration at which the stimulation threshold is twice therheobase. Bourland et al. (1999) reported an average

199(dB/dt) rheobase and chronaxie for the sensation threshold of 15 T s⫺1 and 365 ␮s for the y-coil and 26 T s⫺1 and378 ␮s for the z-coil. Significant motor contraction ofeither abdominal or thoracic skeletal muscles was observed for gradient field strengths approximately 50%
greater than the sensation threshold. A dB/dt magnetic
field rate of approximately twice the sensation threshold
was described by the subjects in this study as intolerable.
The dB/dt values for the z gradient are about 50% higher
than the y gradient due to the fact that the induced
electric field depends on the cross-sectional area, and thearea of the coronal cross section of the human body islarger than that of the axial cross section.
The results were standardized by assuming achronaxie for the responses of each volunteer of 380 ␮s;
population median rheobases for perception without
discomfort, uncomfortable sensation and intolerable sensation were calculated to be 18.8, 28.3, and 36.9 T s⫺1 forthe y-axis, and 28.8, 44.0, and 59.8 T s⫺1 for the z-axis.
These results for the y-axis are illustrated in Fig. 1,
where, for comparison with the results of other authors,
the values have been scaled by a factor of 1.48 for the ygradient to convert to maximum dB/dt values at 0.2 m off
axis; the corresponding value for the z-gradient is 1.21
(Schaefer et al. 2000; Nyenhuis et al. 2001). Results
consistent with these values have been reported in earlier
studies (Ham et al. 1997; Abart et al. 1997).
A finite element calculation (Nyenhuis et al. 2001)
indicated that the calculated rheobase electric fields forthe y and z coils were about 2 V m⫺1, somewhat lower

Fig. 1. Comparison of dB/dt values for threshold, uncomfortable and intolerable stimulation—Y gradient (from
Bourland et al. 1999).

200

Health Physics⫺1

than previous theoretical estimates of 6 V m for theexcitation threshold of 20 ␮m diameter nerve fibers(Reilly 1990, 1992). The authors suggested local enhancement of electric field in the body might be due tothe concentration of eddy currents by relatively highimpedance structures such as bone, and that physiologic
summation and local anatomical differences aroundnerves might also play a role.
When shapes of gradient pulses deviate from trapezoidal or sinusoidal, the biological effectiveness in causing stimulation changes. In such cases, the thresholdmust be evaluated on the basis of the shape and frequency of the pulse (Jokela 1997, 2000).
Fig. 2 shows stimulation data by Bourland’s group(Nyenhuis et al. 2001) in terms of the number of subjects
responding vs. dB/dt. Bourland found that the dB/dt
needed for the lowest percentile for uncomfortable stimulation is approximately equal to the median thresholdfor perception. The lowest percentile for intolerable
stimulation occurs at a dB/dt approximately 20% above
the median perception threshold.
Cardiac stimulation data. Early in the development of fast-scan MR techniques, there was concern that

August 2004, Volume 87, Number 2

the heart could be at risk with respect to stimulation bygradient pulses. Reilly (1990) noted that, for long durations of induced current, thresholds for peripheral nerveand cardiac stimulation are similar. However, Nyenhuis
et al. (2001) observe that, with gradient pulse sequences
presently used in MRI, cardiac stimulation thresholds aremuch higher than peripheral nerve stimulation thresholds.
Bourland et al. (1999) observed the mean thresholdfor magnetically stimulated respiration in dogs at approximately three times the mean peripheral nerve stimulation threshold. For cardiac stimulation (the induction ofectopic beats) in these animals, the threshold was reported to be nine times the peripheral nerve stimulationthreshold at a pulse duration of 530 ␮s. The mediancardiac stimulation threshold at this pulse width was
found to be 2,700 T s⫺1. When the threshold varies in a
hyperbolic manner with stimulus pulse width, the threshold conditions can also be expressed by a constant equal
to the product of the ramp time and the effective dB/dt,
in this case equal to 1.43 T. By scaling the numbers tohuman geometry and assuming a chronaxie of 3 ms, a
gradient field ramp of 0.43 T (a factor 0.3 lower) wasFig. 2. Percentage thresholds for perception (1), uncomfortable stimulation (5) and intolerable stimulation (10) in agroup of 84 volunteers. Data are presented as normalized values to the median threshold of perception (from Nyenhuis
et al. 2001).

Medical magnetic resonance procedures ● ICNIRP

estimated to be the cardiac stimulation threshold forhumans. Using Reilly’s observation that the 1-percentile
level in the population is half the median stimulationthreshold, Bourland estimated a value of 0.215 T for this
1-percentile level, which is in good agreement withReilly’s estimate of 0.19 T. The corresponding value ofdB/dt would be 405 T s⫺1 at a current pulse width of 530
␮s, which is a factor of ten above the threshold for
peripheral nerve stimulation and a factor of five above
the median for intolerable peripheral nerve stimulation.
Nyenhuis et al. (2001) conclude that because there isessentially “zero risk” of cardiac stimulation in present
day MRI gradient fields, the practical physiological limitof exposure to such fields can be based on avoiding
uncomfortable or intolerable sensations caused by thevery strong perception of the field.
Brain stimulation data. Neural tissue of the brain
can be stimulated by time-varying magnetic fields, as
demonstrated by transcranial magnetic stimulation(TMS) studies. Reilly (1998) calculated induced electricfield rheobase thresholds to be of the order of 20 V m⫺1,
about an order of magnitude greater than the thresholds
cited above for peripheral nerve stimulation. Similarvalues derived from volunteer studies have been recently
reported by Kowalski et al. (2002). These TMS studies
show some people to be more susceptible. Such people
include those with epilepsy, a family history of seizure,and users of tricyclic anti-depressants, neuroleptic agentsand other drugs that lower seizure threshold (Wassermann 1998). Serious heart disease and increased intracranial pressure have also been suggested as contraindications due to the potential complications that would be
introduced by seizure.
Such potential complications may be borne in mind
when contemplating experimental investigations of very
high rates of change of gradient field. However, studies
of volunteers following MRI exposure have found noeffects on cognitive function (Sweetland et al. 1987;
Brockway and Bream 1992; Chakeres et al. 2003a).
For long durations of dB/dt pulses, stimulationthresholds may be lower. From the work of Jefferys et al.
(2003) and Durand (2003), it looks as though fields of theorder of 1 to a few V m⫺1 can have effects on the
excitability of neurons in the brain. However, theseeffects have time constants that mean that induced
currents will need to be maintained for several ms,
probably ⬎10 ms.
Radiofrequency fields
Radiofrequency energy at frequencies above 10
MHz deposited in the body during an MR examination
will be converted to heat, which will be distributed

201

largely by convective heat transfer through blood flow.
Since temperature changes in the various organs andtissues of the body during an MR procedure are difficult
to measure in clinical routine, RF exposure of patients is
usually characterized by means of the “specific energy
absorption rate” (SAR), which is defined as the averageenergy dissipated in the body per unit of mass and time.
In general, only parts of the patient’s body are exposed
simultaneously during an MR procedure. Therefore, notonly the whole-body SAR but also partial-body SARs forthe head, the trunk, and the extremities should beconsidered.
It is important to restrict the rise of tissue and bodytemperature to levels below which local thermal injury or
systemic thermal overload may occur. These can bederived from experimental, clinical, and dosimetric modeling studies of the thermal effects of MR exposure(Adair and Berglund 1986, 1989; Athey and Czerski
1988; Shellock et al. 1989; Grandolfo et al. 1990;
Shellock et al. 1994; Hand et al. 1999, 2000; Gandhi andChen 1999; Shellock 2000; Gandhi 2001; Shellock
2001a; Shellock and Schaefer 2001) and from experimental and clinical studies of the effects of hyperthermia(WHO 2003).
The prime consideration in limiting radiofrequency
exposure is elevation of temperature in the skin, bodycore, or in spatially limited volumes of tissues where
local temperature increases (“hot spots”) may occur
under conditions of MR exposure. Non-uniformity in RF
energy deposition, especially in tissues particularly sensitive to heat such as the hypothalamus or poorly perfused such as the lens of the eye were considered in therecommendations that follow. In addition, heat loss fromthe embryo and fetus across the placental barrier is lessefficient than heat dissipation in other well-vascularizedtissues.
Whole-body physiological responses in healthy
adults. Cardiovascular responses to heat are central tobody temperature regulation in humans (Donaldson et al.
2003). Except in various pathological conditions or
overwhelming heat stress, the body “core” temperature ismaintained under a wide range of environmental conditions at a “set-point” value showing a circadian fluctuation of about ⫾0.5°C around a mean value of about
37°C. Heat gained during exposure to RF radiation has tobe compensated by heat loss and is often accompanied bya small increase in heat storage.
Humans possess comparatively effective heat loss
mechanisms, the principal effectors are the sweat glands
and cutaneous blood vessels (Rowell 1983; Nadel 1984).
In addition to a well developed ability to sweat, which inhumans can be produced over most of the body surface,202

Health Physics

the dynamic range of blood flow rates in the skin is muchhigher than in other species. Skin blood flow can increase
from approximately 0.2– 0.5 L min⫺1 in thermally neutral
conditions, to values exceeding 7– 8 L min⫺1 during
severe hyperthermia, an increase of about twenty-fold(Fig. 3).
Prolonged rates of increase in heat storage, however,will lead to unacceptable rises in body temperature.
Generally, organizations responsible for safety during
work in hot environments advise restricting body temperature rise to 38°C in order to protect individuals fromheat-related disorders such as heat exhaustion (e.g.,
NIOSH 1986).
Similar, less marked, physiological responses havebeen seen in resting, healthy volunteers exposed to MR
under controlled environmental conditions. Two suchstudies by Shellock et al. (1989, 1994) of the effects ofacute abdominal exposure at 64 MHz electromagnetic
fields indicate that volunteers will accommodate wholebody RF heat loads of between approximately 1 W kg⫺1
for 45 min at environmental temperatures of up to 31°C
to 6 W kg⫺1 for at least 15 min at ambient temperatureswith increased skin blood flow and profuse localized
sweating but with minimal changes in core temperature.
Generally, these studies are supported by mathematical modeling of human thermoregulatory responses to
MR exposure. For example, Adair and Berglund (1986)
calculated that, during exposure at a whole-body SAR of
4 W kg⫺1, the body temperature of a lightly clothedpatient whose thermoregulatory ability was unimpaired
would rise by up to a maximum of 0.6°C, depending on
environmental conditions. Skin blood flow was estimatedto rise to maximum values at higher SARs, requiring
increased cardiac output. In these circumstances, however, they noted that the health of patients with impaired

Fig. 3. Cardiovascular adjustment in supine resting volunteers
heated through the skin and peripheral tissues via a “hot water
jacket” to the limits of thermal tolerance of around 39°C bodytemperature (from Rowell 1983).

August 2004, Volume 87, Number 2

cardiac function could be compromised. Further calculation (Adair and Berglund 1989) predicted that patientswith reduced skin blood flow (up to about 70% normal)
would still experience body temperature increases of lessthan 1°C at 4 W kg⫺1 (for 40 min or less) under normal
ambient conditions but noted that patients with greater
degrees of impairment might be compromised underthese circumstances.
Possible complications resulting from disease andmedication. Cardiovascular diseases that compromise
the circulation, such as peripheral vascular disease,which may be caused by atherosclerosis or heart failure,are highly prevalent in the elderly (Corti et al. 2001; De
Sanctis 2001; Lakatta 2002). In addition, dehydration
makes the blood more prone to clot, and this increases
the incidence of coronary and cerebral thrombosis inpeople with pre-existing roughening of the arteries due toatheroma. This is commonly present in middle-aged andolder people and accounts for the fact that most of theheat-related mortality from arterial thrombosis occurs in
people of this age group (Donaldson et al. 2003). Older
people also appear less effective at maintaining normal
body temperature compared to the young due to declines
in sweating and blood flow responses as well as from adecline in the neural control of these responses (Rooke etal. 1994; Anderson et al. 1996).
Otherwise, heat-related disorders in sedentary people are usually associated with impairment of sweating
and vasodilatation, either by drugs or disease. They are
most commonly seen in psychiatric patients receiving
drugs such as barbiturates or phenothiazines which depress reflex regulation of body temperature generally, or
anticholinergic drugs which specifically suppress sweating and vasodilatation (Donaldson et al. 2003; Shellock
2001a). General autonomic hypofunction due to diabetes
also increases susceptibility in sedentary people. Generally, however, the interactions between drugs and temperature regulating mechanisms are complex and depend
on a number of factors.
Fever is also generally assumed to increase susceptibility to exogenous sources of heat such as RF radiation(e.g., NCRP 1986). Adair et al. (1997) have investigatedthe effect of exposure to RF radiation on experimentallyinduced fever in a non-human primate (the squirrel
monkey). Fevers are generated by an increased metabolic
rate and by peripheral vasoconstriction, the opposite
response to hyperthermia induced by RF radiation or
exercise. The authors reported that RF energy absorbed
during a febrile episode may have a sparing effect on
endogenous energy production but may augment the
fever if deposited deep within the body, as occurred
during exposure at a resonant frequency (450 MHz in theMedical magnetic resonance procedures ● ICNIRP

squirrel monkey). In addition, the fever may also be
exacerbated if exposure occurs later in the febrile episode, during defervescence when active heat loss mechanisms are required.
Clearly, it is important to assess the responses ofpatients of varying degrees of susceptibility to imposedheat loads. Information regarding patient responses has
been collected and is summarized in Shellock andSchaefer (2001). Unfortunately, few records have beenmade; the data are highly variable and mostly clusteredaround responses at whole-body SARs of 0.3– 0.6 W
kg⫺1 with no indication of duration of exposure. Further,
there was no indication as to the disease status of thepatient or their potential susceptibility to hyperthermia.
Exposure during childhood. The main physicaldifference between children and adults affecting thermoregulation is the much higher surface-area-to-mass index
of children. In a warm environment this allows them torely more on increased skin blood flow and heat loss
through convection and radiation, and less on evaporative cooling (Falk 1998). Nevertheless, during exercise inthermally neutral or warm environments, children thermoregulate as effectively as adults. However, dehydration from excessive sweating may have a more detrimental effect on children because of their greater reliance on
elevated skin blood flow to dissipate heat.
Localized heating. The extent to which RF absorption in tissues or organs of the body results in localized
peaks of temperature rise compared to the average rise incore body temperature depends not only on the local
SAR but also on the vascularity and flow of blood
through the tissue or organ in question. Whilst the former
is an intrinsic property of the tissue, that latter can bevaried considerably, particularly through the skin andmusculature, by metabolic, endocrine and neural control
mechanisms (e.g., Sukkar et al. 2000). The distribution ofblood flow through organs and tissues is also likely to becompromised in the older people. Whilst cardiac output
is maintained in healthy older people, total peripheral
resistance is increased (Ferrari 2002); cardiovasculardiseases which will further compromise the circulation,such as atherosclerosis, peripheral vascular disease or
chronic heart failure, are also highly prevalent (Corti etal. 2001; De Sanctis 2001; Lakatta 2002). In addition,
people taking medications such as beta-blockers that
affect the peripheral distribution of blood flow may also
be compromised in this respect.
Hyperthermia is being used increasingly as an adjunct to radio- or chemotherapy in the treatment oftumors (Dahl et al. 1999; Falk and Issels 2001). A
considerable number of acute studies have been carried203

out both in vitro and in vivo, investigating “dose”response relationships for tissue damage resulting fromlocalized tissue or whole-body heating. Temperatures
have usually ranged between 40 – 45°C, sometimes up to
50°C or more, for periods lasting from a few minutes toseveral hours. The results of such studies have beensummarized recently by Dewhirst et al. (2003).
In animal studies and in a very small number ofhuman studies (mostly of skin damage), cell loss and/or
tissue lesions have been induced in a variety of tissues
following whole-body or localized heating. The resultsfrom different studies are variable but in many cases
lesions occurred when temperatures exceeded 42°C or sofor periods of more than about 1 h (Dewhirst et al. 2003).
This occurred with increasing rapidity as temperaturesrose further so that at around 45°C, lesions could occur
within 10 –30 min in many tissues. With regard to thesusceptibility of different animal tissues, the central
nervous system, including the blood-brain barrier, andthe testes seemed the most sensitive to heat; significant
changes were reported occurring after exposure to temperatures of only 40 – 41°C for periods of around 1 h(Dewhirst et al. 2003). However, human and pig skinseemed less susceptible to raised temperature than theskin of some other animals such as mice.
It can generally be assumed that adverse effects will
be avoided with a margin of safety if temperatures in thehead are less than 38°C, temperatures in the trunk lessthan 39°C and temperatures in the limbs less than 40°C
(Athey and Czerski 1988).
Simple calculation (Athey 1989) relating localized
heating in the eye to SAR in the head suggests that
exposure resulting in an SAR to the head of 3 W kg⫺1 isunlikely to raise the temperature of the eye by more than
1.6°C; brain temperatures are unlikely to rise by morethan 1°C under these conditions.
Gandhi and Chen (1999) and Gandhi (2001) haveapplied the finite-difference time-domain method to amillimeter resolution model of the human body developed from MRI scans to estimate SAR distribution in the
body. To this end, the electrical properties of 30 different
tissue types were prescribed in the model. Their calculations, which were performed for different RF coils
operated at 64, 128, and 170 MHz, indicate that the
maximum SAR for 100 g of tissue can be ten times
greater than the whole-body average when exposed in a
64 MHz body coil.
For RF decoupling coils operated at frequencies
between 64 and 213 MHz, electromagnetic and thermal
modeling of SAR and temperature fields in the humanleg have been performed by Hand et al. based on athree-dimensional bio-heat transfer model (Hand et al.
1999, 2000). The predicted SAR distributions were204

Health Physics

strongly dependent on the spatial distribution of muscle
and fat/bone within the leg. Since currents induced in thetissue tend to pass through the more conducting muscle,
it was in this tissue that the higher SARs occurred.
As an alternative to these time-consuming computer
simulations, the temperature rise in a partial-body region
during RF exposure can roughly be assessed by means ofanalytical solutions to an appropriate bio-heat transfer
model. This approach has been employed to assess
temperature changes in a homogeneous tissue region due
to localized RF heating from interventional MR devices(Yeung and Atalar 2001) and to roughly predict the
maximum temperature rise during an MR procedure withvarying SAR levels at the center of a larger homogeneous
tissue region (Brix et al. 2002).
Summary. For whole-body exposures, no adverse
health effects are expected if the increase in body coretemperature does not exceed 1°C. In the case of infants
and persons with cardiocirculatory impairment, the temperature increase should not exceed 0.5°C. With regard
to localized heating, it seems reasonable to assume thatadverse effects will be avoided with a reasonable certainty if temperatures in localized regions of the head areless than 38°C, of the trunk less than 39°C, and in thelimbs less than 40°C.
MR procedures and pregnant patients
In pregnancy, there is both the safety of the motherand that of the unborn child to consider. In particular, thedeveloping embryo and fetus is susceptible to a variety ofteratogens, including heat. Potentially susceptible processes include the highly ordered sequences of cellproliferation and differentiation, cellular migration andprogrammed cell death (apoptosis). Sensitivity varies
during gestation and is usually greatest during organogenesis when the main structures of the body are formed.
This period occurs during the first trimester of humanpregnancy. The fetal stage is largely characterized by thegrowth of these structures and is generally consideredless sensitive to the effect of teratogens, although thecentral nervous system continues to develop during thisperiod and postnatally and can be adversely affected bysome agents.
General health and safety issues relating to MR
procedures and pregnancy have been discussed by, forexample, Kanal (1994) and Colletti (2001). The possible
developmental effects of exposure to static and extremely low frequency magnetic fields, often in an
occupational or environmental context, have been reviewed recently by IARC (2002) and ICNIRP (2003).
The effects of maternal hyperthermia, including RF-

August 2004, Volume 87, Number 2

induced hyperthermia, have been reviewed recently, forexample by Edwards et al. (1995, 2003).
Static magnetic fields. Generally, no consistenteffects of static magnetic field exposure on reproduction
and development have been seen using mammalian
species (IARC 2002; ICNIRP 2003); however, few
comprehensive studies have been carried out. Several
studies indicate that implantation and prenatal and postnatal development of the embryo and fetus are notaffected by exposure for varying periods during gestation
to fields of between 1 and 9.4 T (Sikov et al. 1979;
Konermann and Monig 1986; Murakami et al. 1992;
Okazaki et al. 2001). However, Mevissen et al. (1994)
reported continuous exposure to a 30 mT field slightly
decreased the numbers of viable fetuses per litter. As aprobable consequence of these smaller litter sizes, prenatal development as measured by skeletal ossification
was accelerated, and postnatal growth up to day 50 wasincreased. IARC (2002) note that two somewhat dated
studies (Neurath 1968; Ueno et al. 1984) report that
exposure to 1 T fields with high spatial gradients (10 –1,000 T m⫺1) can adversely effect the development offrogs and toads, but note that most studies indicate a lackof effect of static field exposure.
Low frequency magnetic fields. The possibility
that exposure to low frequency electromagnetic fields,
mostly at levels encountered occupationally or in theenvironment, may affect fertility, reproduction, or preand postnatal growth and development has been widely
investigated (AGNIR 1994; NIEHS 1988; IARC 2002;
ICNIRP 2003). This issue has been addressed using bothmammals and birds, although the results of studies using
mammals are most relevant to possible effects on humans (Brent 1999). However, it should be noted that the
coupling of low frequency magnetic fields to small
mammals will be different to that in humans because of
the differences in body size and shape.
In summary, several well designed studies noted anincrease in the incidence of minor skeletal variants
following in utero exposure to power frequency (50 – 60Hz) fields up to 30 mT (e.g., Kowalczuk et al. 1994;
Mevissen et al. 1994; Juutilainen et al. 1997; Huuskonen
et al. 1998a) or VLF fields (mostly 10 –20 kHz) up to 15
␮T (peak-to-peak) (e.g., Juutilainen et al. 1997; Huuskonen et al. 1998a, 1998b) although other studies have
not reported this effect (e.g., Rommereim et al. 1996;
Ryan et al. 1996; Dawson et al. 1998). The effects of
exposure of three generations of rats to 60 Hz magnetic
fields of up to 1 mT had no effect on fertility andreproductive performance in rats (Ryan et al. 1999).
Generally, the prolonged exposure of rodents to lowMedical magnetic resonance procedures ● ICNIRP

frequency magnetic fields of up to 30 mT has had no
adverse effect on reproductive outcome.
Only one very preliminary study has investigatedthe effect of pulsed gradient fields on pregnancy outcome
in mice (McRobbie and Foster 1985); rates of change offlux density (several kT s⫺1) were sufficient to causemuscle contraction in some experimental runs. No adverse effects were seen, but the results are not informative. Variable but small numbers of mice were used, few
teratological end-points were examined (litter size andweight) and the animal husbandry was poor (in at least
two of the four experimental runs, the mice had parasitic
infections resulting in post-natal deaths).
RF-induced hyperthermia. Generally, pregnant
women maintain their heat balance as well as nonpregnant women during exercise in warm environments(Donaldson et al. 2003). Fetal temperature is “clamped”
to the temperature of the mother, but is usually about
0.5°C higher because of counter-current heat exchange inthe vessels of the umbilical cord (Schröder and Power
1997) and may be affected not only by changes tomaternal body temperature per se but also by changes toplacental blood flow, during heat stress for example.
However, excessive hyperthermia has a direct effect on
the growth of the embryo and fetus in utero. Neural tube
and facial defects have been found in children whose
mothers had experienced prolonged or repeated hyperthermia (39°C or more for longer than 1 d) during thefirst trimester of pregnancy (Milunsky et al. 1992;
Chambers et al. 1998). Similar effects have been seen inexperimental studies; the threshold in many species
occurs when maternal body temperature is raised byabout 1.5–2.0°C above normal core body temperature fortens of minutes up to an hour or so (reviewed by, e.g.,
Graham et al. 1998; Edwards et al. 1995, 2003). Higher
elevations, up to about 5°C, were required for shorter
durations. RF-induced hyperthermia has similar effects
to those induced by more conventional heating (e.g.,
Verschaeve and Maes 1998; O’Connor 1999). However,
the effects of hyperthermia on the developing nervous
system, and by implication RF-induced heating, have notbeen fully characterized and should be further investigated.
MRI exposure—animal and human studies. Several studies have examined the effects of exposure ofanimals to the fields generated by MRI systems. Whilst
such exposure is clearly realistic, the coupling of MRI
fields to small mammals will differ from the way inwhich they couple to humans.
Tyndall (1993) reported that the exposure of pregnant mice to MRI significantly increased the percentage

205

of affected fetuses with regard to both cranio-facial
perimeter and crown-rump length. Heinrichs et al. (1988)
had previously reported decreased crown-rump length
following prolonged (16 h) exposure during gestation.
These latter authors suggested that noise-induced stress
may have contributed to the outcome. In addition, bothstudies based their analyses on individual fetuses which
ignores litter effects resulting from interactions with themother and tends to overestimate statistical significance(AGNIR 1994). Further, many experimental parameters,such as SAR, were unspecified. More recently, Carnes
and Magin (1996) and Magin et al. (2000) report decreased post-natal survival, fetal weight and crown-rumplength in some groups of mice exposed to MRI fields but
not in other groups. The authors note a general lack ofconsensus concerning possible MRI effects on reproduction and development and suggest that exposure thresholds and mechanisms of such effects be further evaluatedbefore the associated safety risks can be determined.
There are two studies of pregnancy outcome inhumans following exposure in utero to echo-planar MRI,
discussed by Coletti (2001). Baker et al. (1994) carriedout a 3-y follow-up of 20 children examined in utero withecho planar MRI during the second or third trimester. All
the fetuses imaged were from pregnancies deemed to beabnormal, with congenital abnormalities diagnosed on
ultrasonography, suspected growth retardation, or problems with the placental site. Pregnancy outcome wasbriefly summarized, no unexpected abnormalities were
found that the authors considered could reasonably be
attributed to the MRI exposure. With such an uncommon
number of subjects, however, it would be difficult toassess any comparatively small effects of MRI exposure.
Subsequently, the same group (Myers et al. 1998)
carried out a prospective study of pregnancy outcome inwhich 74 pregnant women were exposed to up to fiveserial scans of echo planar imaging (0.5 T static field and500 Hz gradient fields) between 20 and 40 weeks
gestation compared with a control group of 148 pregnant
women receiving only ultrasound scans. The controls
were matched closely by maternal age, parity, ethnic
origin, smoking history and postcode; however, no information was given on alcohol consumption, diet, socioeconomic status, etc. The results were again briefly
summarized: infant birth weight was significantly lower
in the MRI group compared to the control group, although this disappeared if corrected for gestational age.
The authors note the lack of “blinding” in this study; the
MRI group was closely monitored, which may haveresulted in the significantly higher induction rate seen in
the MRI group and in a lower gestational age.

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Summary. Excessive hyperthermia, including hyperthermia induced by exposure to RF radiation, is
teratogenic in animals and would possibly be so inhumans if the temperature excursions were sufficiently
large. It seems reasonable to assume that adverse developmental effects will be avoided with a margin of safety
if the body temperature of pregnant women does not riseby more than 0.5°C and the temperature of the fetus isless than 38°C.
There is no clear evidence that exposure to static or
low frequency magnetic fields can adversely affect pregnancy outcome. Few studies of static field effects havebeen carried out. A greater number of low frequency
studies have been conducted but the interpretation andrelevance of some of these studies is obscured by thedifferences in exposure level and duration, and in the
coupling of such fields to small mammals compared tohumans.
Whilst there is no evidence of any adverse effect, thetwo studies of pregnancy outcome in women exposed to
MRI during pregnancy were small and lacked rigor.
Delayed effects of acute and repeated MRI exposure
There is virtually no information about any delayedor long-term effects of single or repeated exposure to theelectromagnetic fields used for MRI procedures, as longas excess heating is avoided, and no guidance can bederived in this area other than that concerning thebenefits to the patients.
Acoustic noise
The gradient magnetic field is the primary source of
acoustic noise associated with MR procedures (McJury
and Shellock 2000; McJury 2001; Shellock et al. 1998;
Shellock 2001a). This noise occurs during the rapid
alterations of currents within the gradient coils. The
currents, in the strong static magnetic field of the MR
system, produce significant Lorentz forces at the wires of
the gradient coils that induce vibrational modes in thecoil structures generating substantial noise. The situation
is exacerbated in ultra-high speed imaging because of thevery high switching rates used in these techniques. Noiselevels can be as high as 140 dB, which is well above
generally accepted safety level permitted in the workplace (Mansfield et al. 2001).
Acoustic noise is characterized in terms of frequency spectrum (in Hz), intensity (in dB), and duration.
Since the ear is not equally sensitive to all frequencies,
data are normally weighted using the dB(A) measurement scale, which responds similarly to the human ear.
Quantitative data about physiological effects of noise canbe found in the review of Jansen (1991). Temporary
noise levels of 60 –70 dB(A) start activation processes inAugust 2004, Volume 87, Number 2

the central nervous system. Transient hearing loss may
occur following loud noise exceeding 100 dB(A), resulting in a temporary threshold shift in audible threshold.
Noise levels above 100 dB(A) may result in disturbancesof the micro-circulation in the cortical organ and shortterm impulse levels above 120 –130 dB(A) produce
primary mechanical damage in this organ. If the noise
insult is severe, full recovery can take up to several
weeks. If the noise is sufficiently injurious, this may
result in a permanent hearing loss. 85 dB(A) is thethreshold for permanent hearing loss following long termnoise.
The problems associated with acoustic noise forpatients and health care workers include simple annoyance, difficulties in verbal communication, heightened
anxiety, temporary hearing loss, and potential permanent
hearing impairment (McJury and Shellock 2000; Shellock 2001a). Acoustic noise may pose a particular hazard
to specific patient groups who may be at increased risk(e.g., patients with psychiatric disorders, elderly, pediatric and sedated patients, newly born children) (McJury
and Shellock 2000). Noise exposure for the fetus could
be of concern for both patients and interventional MRI
staff (Academy of Pediatrics 1997). For dangerously illpatients a maximum level of 67 dB(A) can be consideredas the hazardous threshold (Jansen 1991).
Occupational exposure
Routine MR procedures. The exposure of medical
staff to static magnetic fields while positioning thepatient can be as high as the main field in the system,
although locally and for limited periods of time. The
trend towards the routine use of high field MR systems(3– 4 T) peak static magnetic field exposures is anincreasing concern that people are exceeding the existing
recommended guidelines. Static field exposure levels
must be quantified and controlled for medical staff
safety. In the near future, ICNIRP is going to review its
guidelines on limiting exposure to static magnetic fields.
Medical or paramedical staff may be exposed at low
magnetic and electromagnetic fields around the device oroutside the room for hours daily.
An analysis of the RF fields outside of a cylindricalbore type MR scanner with a birdcage body coil indicatesthat they are more than a hundred times smaller at theentrance to the magnet than at isocenter. The power
deposition is more than a factor of 10,000 smaller at theentrance to the magnet than that received by the patient.
Thus, the exposure of the operator to RF fields is well
below existing recommended limits (Elder et al. 1989).
Open MR devices. Concerning open or interventional MR devices with magnetic field strength below 1.0

Medical magnetic resonance procedures ● ICNIRP

T, staff operating such devices are not exposed at levels
higher than the currently recommended limits for occupational exposure. However, there are only limited dataon the exposure of surgeons at open MR devices.
Pregnant operators. Kanal et al. (1993) conducted
a comprehensive postal questionnaire survey of the
reproductive outcomes in pregnant female workers inclinical MR facilities in the U.S. Almost 2,000 responses
were received from MR workers (technologists or
nurses) and various controls groups, principally “other”
workers, from which data concerning about 1,400 pregnancies could be analyzed. The authors found no large or
statistically significant difference in pregnancy outcome
between 280 MR workers compared to ‘other’ workers
for a variety of outcomes such as premature delivery,
infertility, low birth weight or spontaneous abortion.
Whilst acknowledging weaknesses in the data, namely alow response rate (20 –50%) and a subjective assessmentof pregnancy outcome, the authors note that bias introduced by these factors typically tends to elevate rather
than decrease any positive outcome. They concluded thatthe data do not suggest a substantial increase in common
adverse reproductive outcomes in women working in the
MR environment. However, as the response rates were
below 50%, it is difficult to draw any meaningfulconclusion from the study.
The key issue with regard to occupational exposure
and pregnancy is to obtain far more complete data thanwere obtained in the study by Kanal et al. (1993). Further
study is required with much higher participation rates
based on an objective assessment of pregnancy outcome.
For outcomes such as birth weight, a cohort of moderate
size would probably give reasonable information,
whereas for individual congenital malformations, which
are comparatively uncommon in the general population,
the size of cohort needed would be much larger. A
case-control approach might be suitable if it was in a
population in which a reasonably high proportion ofpregnancies had MRI exposure.
RECOMMENDATIONSGeneralThere are many gaps in knowledge of biologicaleffects and interaction mechanisms of MRI-related electromagnetic fields with tissues. ICNIRP emphasizes thefollowing in the medical application of MR:
1. The need for a MRI examination and the safety of thepatient undergoing such an examination is the responsibility of the medical practitioner;
2. Where MR examinations form part of a research
project, the project shall be guided by rules of human207

ethics; informed consent of the patient shall be obtained;
3. MR equipment users must be adequately trained in theprinciples and operation of the equipment, indications
and contraindications for use, record keeping requirements, safety aspects, and precautions; and4. MR system manufacturers should supply complete
documentation about patient and staff exposure levels
for their equipment, and these safety guidelines
should be considered in the design of equipment andfacility layout so that exposures to magnetic andradiofrequency fields are within the levels recommended for patients and staff.
Exposure levels for patients
Clinical experience currently indicates that adequate
diagnostic information can be obtained while examining
a patient for periods ranging from 5 min to over 1 hourand may be repeated several times over the course of thedisease or abnormal condition. In some cases, due to theuse of MR spectroscopy or interventional MR procedures, examinations can last several hours.
Because of the uncertainty over identified deleterious effects, it is recommended that exposure limits bedivided into three tiers:● routine

MR examinations for all patients (normal
operating mode);● specific MR examinations outside the normal operating range where discomfort and/or adverse effects forsome patients may occur. A clinical decision must be
taken to balance such effects against foreseen benefits;
exposure must be carried out under medical supervision (controlled operating mode);● experimental MR procedures, at levels outside thecontrolled operating range, for which special ethicalapproval is required in view of the potential risks(experimental operating mode).
Static magnetic fields. Until now, most MRI or
MRS examinations have been made using static magnetic fields up to and including 3 T, although devices
with static magnetic fields up to 7 T are already used inclinical tests. Higher magnetic flux densities offer potential diagnostic advantages, particularly for MRS. Because exposure to magnetic fields above 2 T can produce
nausea and vertigo due to static field gradients, it isrecommended that examinations above flux density be
conducted under especially careful medical supervision.
The recommended upper limit for clinical routine wholebody exposure to static magnetic fields is 4 T, due to thelimited information concerning possible effects above
this static field strength (U.S. FDA 1997; IEC 2001). At
higher field strengths applications should be made based

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Health Physics

on critical risk/benefit analyses and informed consent of
the subject. Additionally, more careful monitoring of
subjects is required, although physiological monitoringin an MR scanner does itself increase the risk of RF
burns, which is however an avoidable risk. Recently, the
FDA deems magnetic resonance devices to be a significant risk when the main static magnetic field is greaterthan 8 T for adults, children and infants aged ⬎1 month,and greater than 4 T for neonates, i.e., infants aged 1
month or less (U.S. FDA 2003).
Time-varying magnetic fields. The threshold for
cardiac stimulation is well above the level for intolerable
stimulation, except at very long pulse durations which
are not clinically useful. The trend in clinical MRI is
toward shorter gradient pulses to reduce imaging time.
The study on nerve stimulation (Nyenhuis et al.
2001) is the largest study to date, and because it distinguishes between varying levels of stimulation (i.e.,threshold, uncomfortable, intolerable) it provides a basis
for recommended exposure levels for time-varying magnetic fields. Many medical procedures are uncomfortableor painful, but are tolerated by patients for the medical
benefit. However, intolerable stimulation would interferewith an examination and the patient would receive nobenefit. Consequently, this is to be avoided. Bourland’sdata indicate that the lowest percentile for intolerable
stimulation is approximately 20% above the medianthreshold for peripheral nerve stimulation (Fig. 2 above).
Consequently, it is recommended that the maximum
exposure level for time-varying magnetic fields be set
equal to a dB/dt of 80% of the median perception
threshold for normal operation, and 100% of the medianfor controlled operation. This median perception threshold is described by the following equation, which is anempirical description:⫺1dB/dt ⫽ 20共1 ⫹ 0.36/␶兲T s ,(1)where ␶ is the effective stimulus duration in ms. The
effective stimulus duration is the duration of the period ofmonotonic increasing or decreasing gradient. It is definedas the ratio of the peak-to-peak variation in B and the
maximum value of the time derivative of B during that
period.
Radiofrequency fields
For whole-body exposures, no adverse health effects
are expected if the increase in body core temperaturedoes not exceed 1°C. In the case of infants, pregnant
women, and persons with cardiocirculatory impairment,
it is desirable to limit body core temperature increases to
0.5°C. Similarly, local temperature under exposure to thehead, trunk, and/or extremities should also be limited toAugust 2004, Volume 87, Number 2

the values given in Table 1. It is important whensignificant whole-body heating occurs, particularly in theelderly, to ensure that patients drink adequate amounts ofliquid prior to investigations.
As only parts of the patient’s body are exposed
simultaneously during an MR procedure, not only thewhole-body SAR but also partial-body SARs for thehead, the trunk, and the extremities should be estimatedon the basis of suitable patient models (e.g., Brix et al.
2001). It should be noted that there are no dosimetricmodels of pregnant women. Based on the publishedexperimental studies concerning temperature rise andtheoretical simulations, the SAR levels summarized inTable 2 should not be exceeded in order to limit
temperature rise to the values given in Table 1.
With respect to the application of the SAR levels
defined in Table 2, the following points should be taken
into account:● Partial-body SARs scale dynamically with the ratio r

between the patient mass exposed and the total patientmass. For r 3 1 they converge against the corresponding whole-body values, for r 3 0 against the localized
SAR level of 10 W kg⫺1 defined by ICNIRP for
occupational exposure of head and trunk (ICNIRP
1998);● The recommended SAR restrictions do not relate to anindividual MR sequence, but rather to running SAR
averages computed over each 6-min-period, which isassumed to be a typical thermal equilibration time ofsmaller masses of tissue (Brix et al. 2002);● Whole-body SARs are valid at environmental temperatures below 24 °C. At higher temperatures, theyshould be reduced depending on actual environmental
temperature and humidity.
Users of diagnostic MR devices usually do not haveadequate resources to determine energy deposition
within the patient’s body. Such information should be
supplied by the manufacturer, and it is recommended thatthe user requests detailed data from the manufacturer. In
addition, real-time temperature monitoring may be performed during MR procedures in the controlled operating

Table 1. Basic restrictions for body temperature rise and partialbody temperatures.
Spatially localized
temperature limits
Operating mode
NormalControlled
Restricted

Rise of bodycore temperature
(°C)

Head
(°C)

Trunk
(°C)

Extremities
(°C)

0.5
1
⬎1

38
38
⬎38

39
39
⬎39

40
40
⬎40

Medical magnetic resonance procedures ● ICNIRP

209

Table 2. SAR levels valid at environmental temperatures below 24°C.
Averaging time: 6 min
Wholebody SAR(W kg⫺1)
Body region 3
Operating mode 2
NormalControlled
Restricted
Short term SAR

Partial-body SAR

Local SAR
(averaged over 10 g tissue)(W kg⫺1)(W kg⫺1)

Wholebody

Any,
Head
Head
Trunk
Extremities
except
head
2
2−10a
3
10b
10
20
4
4−10a
3
10b
10
20
a
b
⬎4
⬎(4−10)
⬎3
10
⬎10
⬎20
The SAR limit over any 10 s period should not exceed 3 times the corresponding average
SAR limit.

aPartial-body SARs scale dynamically with the ratio r between the patient mass exposed and the total patient mass:– normal operating mode: SAR ⫽ (10 – 8 䡠 r) W kg⫺1
– controlled operating mode: SAR ⫽ (10 – 6 䡠 r) W kg⫺1
The exposed patient mass and the actual SAR levels are calculated by the SAR monitor implemented in the MR system for eachsequence and compared to the SAR limits.
b
In cases where the eye is in the field of a small local coil used for RF transmission, care should be taken to ensure that the temperaturerise is limited to 1°C.

mode for patients at risk and should be performed in allcases in the experimental operating mode.
Acoustic noise. Generally, the acoustic noise produced during the MR procedures represents a potential
risk to patients undergoing examinations on MR systems
operating above 0.5 Tesla (McJury and Shellock 2000;
Shellock 2001a). Internationally recommended limits foracoustic noise produced during MR procedures are basedon recommendations for occupational exposures that are
inherently chronic; comparable recommendations do notexist for non-occupational exposure to relatively shortterm noise produced by medical devices. For the safe useof medical equipment the acoustic noise must be restricted. Technical standards recommend that hearing
protection shall be required for the safety of the patient if
the maximum A-weighted r.m.s. sound pressure level ofthe MR equipment can exceed 99 dB A (IEC 2001).
However, this noise level may not be appropriate forindividuals with underlying health problems, who may
have problems with noise at certain levels or at particularfrequencies. Other guidelines recommend to limit thenoise level at the ear of the patient for exposure times up
to 2 h to 91 dB A (SSK 2003).
It is recommended to offer hearing protection to thepatients, when a noise level of 80 dB(A) is exceeded;hearing protection should always be worn by patients
undergoing MR procedures at levels exceeding 85 dB A,
at best by headphones allowing verbal communication.
Other devices such as earplugs hamper verbal communication with patients during the operation of the MR
system and offer non-uniform noise attenuation over the
hearing range; however, earplugs are often used toprevent problems from acoustic noise associated with

MR procedures. For adolescents and infants, smaller
earplugs are required to attenuate acoustic noise associated with MR procedures.
The exposure of staff and other health workers near
the MR system is also a concern. This includes particularly those persons who are involved in interventional
procedures or who remain in the room for patientmanagement reasons (McJury and Shellock 2000; Shellock et al. 1998). Several national guidelines recommendthat hearing protection be worn by staff exposed to an
average of 85 dB A over an 8-h day (SSK 2003). In viewof the inherently chronic nature of occupational exposureand the comparably small inconvenience, it is recommended that staff follow the same rules for wearing
hearing protection that is recommended for patients.
Because of the different problems mentioned above,
ICNIRP strongly recommends the design of “quiet”
gradient coils. Gradient coils windings can be designed
such that all Lorentz forces generated by the pulsing ofcurrent are balanced (Mansfield et al. 2001). Greater coil
stiffness and damping the coil should reduce mechanicalvibration and associated noise.
Pregnant patients
There is at present insufficient knowledge to establish unequivocal guidance for the use of MRI procedureson pregnant patients. In these circumstances, it is advisedthat MR procedures may be used for pregnant patients
only after critical risk/benefit analysis, in particular in thefirst trimester, to investigate important clinical problems
or to manage potential complications for the patient or
fetus. The procedure should be conducted using a verbal
and written informed consent procedure. The pregnant
patient should be informed on the potential risks, also

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Health Physics

compared with those of other alternatives. Excessive
heating is a potential teratogen; because of uncertainties
in the RF dosimetry during pregnancy, it is recommended that exposure duration should be reduced to theminimum and that only the normal operation level isused. In addition, large doses of MRI gadolinium-based
contrast agents have been shown to cause postimplantation fetal loss, retarded development, increased
locomotive activity, and skeletal and visceral abnormalities in experimental animals. Such agents should only be
used during pregnancy if the potential benefit justifies
the risk to the fetus.
The few studies on pregnancy outcome in humans
following MRI have not revealed any adverse effects, but
are very limited because of the small numbers of patients
involved and difficulties in the interpretation of the study
outcomes.
In 1991, the Safety Committee of the Society for
Magnetic Resonance Imaging (Shellock and Kanal 1991)
recommended that “MR imaging may be used in pregnant women if other non-ionizing forms of diagnostic
imaging are inadequate or if the examination provides
important information that would be otherwise require
exposure to ionizing radiation (e.g., fluoroscopy, CT,etc.).”
Recommendations on research with volunteers
Where MR examinations are performed within theframework of a research project, the project should be
guided by rules of human ethics. According to the“Declaration of Helsinki for ethical principles on medical
research on human beings,” the leading principles should
include that● the expected scientific result justify the performance ofstudies on volunteers;schedule of the study contain the aim of theresearch, the methods of examination including technical details;● criteria of inclusion and exclusion of volunteers are
justified.
● the

The research project should be reviewed and approved by a local Ethics Committee according to local
requirements or national regulations.
Prior to the study, the volunteer should be instructed
about type and significance of the planned examinationand should give his or her written consent. The parameters of the examination and the results should be kept in
the minutes.
Special care must be taken in experimental MR
procedures at levels outside the controlled operating
mode, for which special ethical approval is required inview of the potential risks. Depending on national

August 2004, Volume 87, Number 2

regulations, a special insurance for investigations withvolunteers may be required.
Special requirements for the experimental operating
mode should include that● specific security measures are provided to prevent

unauthorized operation in the experimental operating
mode (IEC 2001);● the specific security measures involve a key-lock, asoftware password or other protective devices (IEC
2001);● the experimental operating mode may be assessed only
under the authorization of the responsible medical
person;● appropriate supervision is guaranteed to avoid possible
detriments caused by the well known interaction
mechanisms. In addition, a permanent supervision of
the state of health of the volunteers in general (subjective state of health, circulation, respiration, etc.)
must be put in safe keeping;● a permanent optical and acoustical contact is provided
between investigator and volunteer;● specific emergency medical procedures are defined inorder to provide medical treatment if necessary.
Contraindications and further considerations
Contraindications. Examinations of patients who
have electrically, magnetically, or mechanically activated implants (e.g., cardiac pacemakers), or who rely onelectrically, magnetically, or mechanically activated lifesupport systems, may be contraindicated for certain
devices. Examinations of patients with ferromagnetic
aneurysm clips or certain metallic implants are also
contraindicated. Lists of implants and materials tested forsafety or compatibility in association with MR systems
have been established and identified. Objects in patientsshould be checked against those lists (Shellock 2001b;
Shellock 2002a; Shellock 2003). This information isreadily available on-line at www.MRIsafety.com.
Considerations related to patient’s condition.
Communication with the patient and/or monitoring (e.g.,of the anesthetized patient) must be assured throughout
the examination.
Certain patients may experience claustrophobia. The
possibility of claustrophobic reactions should be explored before an examination is undertaken.
Because some individuals may exhibit an increased
sensitivity to heating, it is advisable to ascertain if thepatient’s history comprises incidents indicative of sensitivity to heat and, if necessary, limit the duration of theexamination.
Special requirements apply to the equipment andmethods used for the monitoring of the patient under MR

Medical magnetic resonance procedures ● ICNIRP

exposure conditions. Cardiorespiratory function may bemonitored using non-ferromagnetic transducers to register the heart beat rate, blood pressure, and respiratory
rate. The ECG may be subject to distortion because ofelectrohydrodynamic interactions and may not yielduseful information. Non-perturbing fiber-optic probes formeasurement of body temperature are available. Under
MR exposure conditions, oral temperature and the temperature of the skin of exposed body parts are suitable forpatient monitoring. Detailed information on body temperature measurements may be found in a review by
Shellock (2001c).
Ferromagnetic objects are attracted by magnetic
fields. Depending on size, composition, and location of
metallic implants or inclusions, serious injuries may
result because of motions and displacement of suchobjects. Moreover, the presence of such objects results in
artifacts in diagnostic information. The presence orabsence of such objects in the body has to be ascertained
and the consequences evaluated before an examination isundertaken. Additionally, adjustments with regard to theMR safety information may be required in considerationof the field strength of the MR system that is being used
for the patient. For example, magnetic field interactions
are different using a 0.2-T vs. a 3.0-T MR system(Shellock 2002a; 2002b; Shellock et al. 2003a; 2003b).
Projectile/missile effects. The magnetic fringe fieldnear the MR system’s magnet may be strong enough toattract ferromagnetic objects and to cause them to fly
towards the magnet. Thus, metallic objects, particularly
with sharp edges, may become dangerous projectiles. Allsuch objects have to be eliminated from the examination
room and proper danger or warning signs must be posted.
Details are presented in the IEC standard (IEC 2001).
Guidelines to prevent projectile or missile effect accidents have been presented (Shellock 2001b; 2003).
The manufacturer should provide information regarding the extent of the zone in which collision hazards
and danger of uncontrolled movements of objects exist(e.g., uncontrolled movement of hospital carts, trolleys,loose tools, medical instruments, etc.).
Record keeping and patient follow-up. Examination records should be kept and patients should bemonitored according to standard requirements of good
medical practice. Observations on adverse reactions
should be collected, reported according to national requirements, and published in the medical literature.
Electromagnetic interference. There are numerous
reports of failures of electronic and mechanical medical

211

devices due to electromagnetic interference (EMI) generated by the intense pulsed and static fields emitted by
MRI systems. EMI involves the induction of deleterious
voltages and currents into the circuitry of medical electronics, causing temporary or permanent failures ofcritical components. Damage of mechanical components
due to forces induced by static magnetic fields also canbe considered EMI. Non-fatal problems were experienced by several patients during MRI treatment involving each of the following devices: totally implanted andexternal infusion pumps delivering large drug overdoses,
pulse oximeters for cardiac monitoring failing or providing erroneous data, and a ventilator malfunctioning
during an MRI examination.
In addition, more than several dozen reports of burns
exist for patients who were connected to external monitoring devices while receiving MRI imaging procedures.
Most of these problems occurred when using ECG
electrodes mounted on the chest, or pulse oximeter
sensors on the finger tip. Heating is caused by RF powerthat is coupled into the metallic leads that connect anexternal monitoring instrument to objects mounted on thesurface of the body.
The use of “MRI compatible” devices can prevent
associated patient burns and the electronic and mechanical failure of such devices. MRI compatible devices arebeing designed using appropriate materials and wires orfiber optics to minimize coupling of the RF fields to
medical devices.
Research needs
While the data cited herein provide a good foundation for safety guidelines, there are several areas in which
additional knowledge could be used. ICNIRP therefore
recommends:● Monitoring of occupational exposure;● Epidemiological studies of possible long-term healtheffects in patients, volunteers, and staff with occupational exposure, particularly those with high levels of
cumulative exposure, and carefully conducted epidemiological studies on pregnancy outcome. Especially,
there is a need for monitoring workers above 4 T,
suitable to allow epidemiological studies in the future.
Vibration can also be an issue for long term studies;● More studies should be performed on biological effects of strong static magnetic fields, especially ofchronic exposure on reproduction and development;● Further investigation to define more precisely thespatial deposition of RF energy during an MR procedure and the corresponding temperature fields in thehuman body using a three-dimensional bio-heat transfer model, including modeling of the pregnant woman

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and fetus. Based on these studies, more detailed andrealistic patient models for SAR monitoring should be
developed; and● Body core temperature should be investigated during MR
examinations using high SARs, particularly in patients
suffering from impaired thermoregulatory ability.

Acknowledgments—The support of the German Federal Environment
Ministry (BMU), the French Ministry of Health, of the French National
Center for Scientific Research (CNRS) and of the French Radio Protection
Society (SFRP) is gratefully acknowledged.

The Commission gratefully acknowledges the comments of J. de Wilde, P.
Gowland, J. Jefferys, N. Leitgeb, E. van Rongen, F. Shellock, and E. Vogel
during the review of this document.

The Commission expresses its thanks to CRC press, Lippincott Williams
and Wilkins, and Elsevier Press for their permission to use the figures
reproduced here.

During the preparation of this document, the ICNIRP was composed of thefollowing:
A.F. McKinlay, Chairman, UK
J.H. Bernhardt, Vice-Chairman, Germany
A. Ahlbom, Chair SCI, Sweden
U. Bergqvist, Sweden
J.-P. Césarini, France
F. De Gruijl, Netherlands
M. Hietanen, Finland
R.D. Owen, USA
D.H. Sliney, Chair SC IV, USA
A.J. Swerdlow, UK
M. Taki, Chair SC III, Japan
T.S. Tenforde, USA
P. Vecchia, Italy
B. Veyret, Chair SC II (acting), France
M.H. Repacholi, Chairman Emeritus, Switzerland

The working group for this document was composed of the following:
H. Bassen (USA)J.H. Bernhardt, Chairman (Germany)
G. Brix (Germany)
J.-J. Lejeune (France)
R.D. Owen (USA)R. de Seze (France)
R. Saunders (United Kingdom)
S. Ueno (Japan)
B. Veyret (France)
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