Magnetic resonance (MR) imaging has advanced at an astonishing speed since its inception in the early 1970s. Although the overwhelming majority of examinations are safe and uneventful, injuries and rare fatalities continue to occur. It is important to recognize and understand that there are three distinct fields inherent in MR imaging to be evaluated for bioeffects and potential safety issues: the static magnetic field, the radiofrequency electromagnetic field, and the time-varying gradient magnetic field. The static magnetic field has no known upper limit for safe exposure, although increasing use of high-field imaging spotlights the need for continued research. The radiofrequency electromagnetic field is limited by induced currents in electrically conductive material and can lead to burns. The time-varying gradient magnetic field can cause nerve and muscle tissue stimulation by induction of electric fields. Effective screening includes both written questionnaires and verbal interviews. Intensive screening of biomedical implants and devices as well as for foreign metallic objects is a critical part of MR imaging safety.
is currently Chief Resident in Diagnostic Radiology at Vanderbilt
University Medical Center, Nashville, TN. She earned a double major
in Biomedical Engineering and Mathematics, graduating cum laude,
from Vanderbilt University in 1994. She subsequently received her
MD from the University of Kentucky, Lexington, KY, and graduated
with distinction. Dr. Draud will begin a fellowship in
musculoskeletal imaging at the University of California, San Diego,
Magnetic resonance (MR) imaging has advanced at an
astonishing speed since its inception in the early 1970s.
Although the overwhelming majority of examinations are safe and
uneventful, injuries and rare fatalities continue to occur. It is
important to recognize and understand that there are three
distinct fields inherent in MR imaging to be evaluated for
bioeffects and potential safety issues: the static magnetic
field, the radiofrequency electromagnetic field, and the
time-varying gradient magnetic field. The static magnetic field
has no known upper limit for safe exposure, although increasing
use of high-field imaging spotlights the need for continued
research. The radiofrequency electromagnetic field is limited by
induced currents in electrically conductive material and can lead
to burns. The time-varying gradient magnetic field can cause
nerve and muscle tissue stimulation by induction of electric
fields. Effective screening includes both written questionnaires
and verbal interviews. Intensive screening of biomedical implants
and devices as well as for foreign metallic objects is a critical
part of MR imaging safety.
Although the safety and bioeffects of magnetic resonance (MR)
imaging have long been of interest to those in radiology, recent
events and research have brought this topic to the forefront of
medical imaging. MR safety and bioeffects are currently receiving
significant attention from the research community as new
technologies are being developed and brought to the patient care
arena. Equally as important is the spotlight on patient safety from
the general medical community and the public, which manifested most
significantly after a 6-year-old boy received a fatal head injury
from a projectile metal oxygen tank in an MR imaging suite in July
This tragic occurrence was reported both by
The New York Times
The New England Journal of Medicine
and garnered international attention.
Since the yearly estimate for MR imaging is more than 8 million
studies in the United States and more than 20 million worldwide, it
is clear that the procedure is uneventful and safe in the
overwhelming majority of cases.
However, there are hazards associated with MR imaging that can
threaten safety. In addition to the previously mentioned death, a
fatality resulted from the torquing of an aneurysm clip, and
implantable medical devices, such as cardiac pacemakers, have been
implicated in 5 more deaths.
Other injuries include unilateral blindness because of magnetic
force on an intraocular metallic foreign body and burns from
external monitoring devices, such as pulse oximeters.
The numerous examples of objects being drawn into the magnet bore
are perhaps most disturbing and can also be quite hazardous, even
fatal, as in the instance of the oxygen tank.
These occurrences--along with frontier research involving
high-field imaging, rapidly switching magnetic fields,
radiofrequency (RF) electromagnetic fields, and new advances in MR
imaging intervention--contribute to the importance of the topic of
MR imaging safety and bioeffects. Efforts such as the recent
American College of Radiology White Paper on MR Safety,
an educational course at the International Society for Magnetic
Resonance in Medicine annual meeting, a categorical course at the
2001 annual Radiological Society of North America meeting, and
development of Web sites (eg, MRIsafety.com), are attempts to
highlight the significance of the topic.
This review is intended to serve as an aid in practical day-to-day
patient care for physicians at different points in their training
There are three distinct fields inherent in MR imaging that have
been evaluated for bioeffects and potential safety issues (Table
1). The static magnetic field, B
, is currently limited only by technology and cost because no upper
limit for safe exposure to the intense static field has been shown.
The RF electromagnetic field, B
, is limited by thermogenic qualities of RF radiation.
Finally, the time-varying gradient magnetic field can affect
peripheral nerve stimulation (PNS).
These three fields along with acoustic considerations are regulated
by the U.S. Food and Drug Administration (FDA) Center for Devices
and Radiological Health guidelines, which are summarized in Table
The International Electrotechnical Commission describes a
three-tiered safety system of increasing restriction that consists
of a normal operating mode, first controlled operating mode, and
second controlled operating mode, as described in Table 3.
Bioeffects of static magnetic fields
The static magnetic field produces a net magnetization of the
protons in a patient's tissue, which causes the protons to be
aligned with the field. The strength of the field may vary and is
determined by the specific type of magnet used. In a comprehensive
review article on the safety of strong, static magnetic fields,
summarizes the investigative work performed to date as lacking "a
single example of a scientifically sound and rigorously verified
pathologic effect of such [static] fields." He attributes this lack
of detrimental effects to the absence of in vivo ferromagnetic
components in healthy subjects and to the extremely weak
diamagnetic susceptibility of human tissue. It is important to
note, however, that technology is advancing rapidly and safety
concerns need to be revisited at higher static magnetic fields.
High static magnetic fields are loosely defined as >=1.0 T.
Imaging of humans has already been performed at 8 T for research
A 9.4 T magnet is now being built for human studies, and field
strengths of 12 T and higher may be possible with the development
of flux pumped magnets.
Well-known nonpathologic biological effects of the MR imaging
static magnetic field include elevation of the T wave in
electrocardiographic (ECG) tracings.
The movement of blood, a conductive medium, through the static
magnetic field, B
, causes a magnetohydrodynamic effect that produces a voltage
across the vessel. Because the maximal flow rate of blood occurs
during ventricular contraction, or the T wave interval, this
electromotive force (EMF), or added voltage, is seen as an
elevation of the T wave (Figure 1). This effect was an important
consideration in the original evaluation of MR safety as well as in
the initial work with higher static magnetic fields. However, no
detrimental outcomes from the expected T wave alterations were
observed at exposure to a static magnetic field of 8 T in human or
This induced EMF, although shown to be safe at 8 T, should continue
to be evaluated for potential biologic effects.
Other reversible, nonpathologic effects occurring at high field
imaging include vertigo, metallic taste, and magnetophosphenes or
These effects can be reduced with slow movement across magnetic
field gradients to minimize induced electromotive forces.
Bioeffects of RF electromagnetic fields
The main bioeffect of exposure to RF radiation is related to the
thermogenic qualities of the electromagnetic field.
Deposition of energy in the body because of RF radiation is
quantified by determining the specific absorption rate (SAR). The
SAR is the mass normalized rate at which RF power is coupled to
biological tissue and is expressed in units of watts per kilogram
of body weight (W/kg). The FDA guidelines for SAR limits are shown
in Table 2.
Because of resistive losses, most of the transmitted RF power is
transformed into heat within the patient's tissue.
Studies have been performed up to a whole-body SAR of 6 W/kg and
showing no deleterious effects in an individual with normal
Recent research has suggested that there may be an in vitro
RF-induced cell membrane target effect with an increase in
cytosolic calcium concentration.
The significance of this is unclear and further work is necessary
to support or refute this data, as well as apply it clinically. The
use of more "RF-intensive" MR imaging protocols using
fast-spin-echo (FSE) and magnetization transfer contrast pulse
sequences, especially with high-field strength systems, will
require continuing investigation into their safety. Also, patients
who have an underlying health condition or are taking medication
that might impair their thermoregulatory system will require
In addition to direct RF heating of the body, more than 150
incidents have been reported involving burn injuries to patients
undergoing MR imaging examination resulting from inductive heating
of electrically conductive devices and materials placed in contact
with the body.
First-, second-, and third-degree burns have been associated with
ECG leads, pulse oximetry, and imaging coils.
Less frequently, ferromagnetically active pigment in permanent
eyeliner, a breast implant metal-backed expander injection portal,
and other devices have been reported to show heating.
Heating mechanisms include direct electromagnetic induction,
resonant circuit, and antenna effect.
Direct electromagnetic induction can be minimized by preventing the
looping of conductive cables and placing electrodes as close
together as possible. Other precautions include placing the lead
and cable away from the RF coil and using fiberoptic or
high-resistance graphite electrodes and cables.
An inadvertent conductive loop can be formed on a patient between
two points by touching the arms or legs. All skin-to-skin contact
points should be eliminated, and cables should be insulated from
the patient's skin (Figure 2). An additional degree of safety can
be achieved by using foam padding to insulate the patient from all
electrical components. Maximum current induction will occur when a
circuit is in a resonant condition.
The "resonant frequency" occurs when the tissue is approximately
half the size of the incident wavelength.
Burns can also occur with straight wires, as with pulse oximetry
leads from the antenna effect. The antenna effect results when
lengths of conductive cable are used and an additional electric
field is induced with a maximum point at the tip.
Bioeffects of time-varying gradient magnetic
Pulsed gradient magnetic fields are used in MR imaging to
produce resonant frequencies that encode the data spatially. These
time-varying magnetic fields, dB/dt, can induce electric fields in
which, in turn, can lead to nerve and muscle stimulation.
The mean threshold levels are 60 T/s for the peripheral nerve
90 T/s for peripheral nerve pain; 900 T/s for the respiratory
system; and 3600 T/s for the heart.
Currently, the FDA does not mandate a numerical upper limit value
for dB/dt. Rather, dB/dt is limited by "severe discomfort" or
painful nerve stimulation.
When gradient ramp times are less than 1 to 2 millisecond, the
probability of cardiac stimulation is estimated to be near zero,
providing dB/dt levels are low enough that PNS is rare.
However, the widespread use of echoplanar imaging (EPI), allowing
faster acquisition of images with rapid switching of magnetic
gradient fields at higher amplitude, has increased the incidence of
Recently, another frequently used fast sequence, steady-state free
precession, was found to have threshold parameters similar to EPI.
Peripheral nerve stimulation can be uncomfortable for patients,
limiting their cooperation or the quality of MR imaging. With
continued advances in rapidly switching gradient magnetic fields,
this muscle and nerve stimulatory effect will remain an area of
Various types of acoustic noise are produced in an MR
examination. The frequently described tapping or knocking noises of
the MR scanner are mainly due to the pulsed gradients. The rapid
switching of currents within the large static magnetic field
produces strong Lorentz forces that physically deform the gradient
These coils impact against their mountings, which also flex and
vibrate, causing acoustic noise that is enhanced with decreased
field of view, smaller section thickness, and shorter repetition
Echoplanar imaging and FSE sequences use high gradient amplitudes
and very fast gradient switching times that, in turn, generate
higher levels of acoustic noise.
The FDA mandates that acoustic noise levels of MR systems not
exceed a peak acoustic noise of 140 dB.
Even before EPI and FSE sequences, temporary shifts in hearing
thresholds were reported in 43% of patients scanned with improper
or no hearing protection.
The routine use of earplugs or headphones for patients or staff who
might be in the MR system for long periods of time is recommended.
Unfortunately, these can impede communication with the patient, so
"antinoise" or active noise cancellation systems are being
Noise levels have also been reduced by a number of different
sound-"damping" methods such as the installation of acoustic foam
around the gradient coils.
Imaging of pregnant women has long been fraught with concern.
Although MR imaging lacks ionizing radiation, safety considerations
of the static, gradient, and RF electromagnetic fields warrant
caution. Currently, there is insufficient data supporting or
refuting the use of MR imaging during pregnancy. The MR safety
committee of the Society for Magnetic Resonance Imaging indicates
that MR can be performed on pregnant women in certain
circumstances: when other nonionizing forms of imaging such as
sonography are inadequate or when the examination provides
important information that would otherwise require exposure to
Patients should be informed that while no data have shown
deleterious effects, MR safety has not been proven.
Written consent should be obtained.
Pregnant health-care workers present a different set of
concerns. Exposure to RF electromagnetic or gradient fields is very
limited in this population because these fields are present
basically only within the system bore. However, the exposure to the
static field may be chronic and low level. An epidemiological study
of imaging technologists failed to show any statistically
significant elevation in the rate of spontaneous abortion,
infertility, or premature delivery.
Shellock and Kanal
recommend a policy to "permit healthcare workers to perform MR
procedures, as well as to enter the MR system room, and to attend
to the patient" during pregnancy, regardless of trimester. They
also recommend, as a precautionary measure, that the pregnant
worker exit the MR system room during operation of the system.
Ensuring a safe workplace environment should be the top priority
for all involved with MR imaging. With this goal in mind, the task
of providing and maintaining a secure setting can be a daily
challenge. The first step involves having a set of safety
guidelines in place as a framework in which to operate. Second, the
screening of both patients and healthcare workers is extremely
important. For patients, a combination of written and verbal
screening is common (a sample screening form is presented in Figure
Notably, a history of uneventful MR imaging does not preclude the
screening process at the time of subsequent examinations.
Differences in magnetic field strength, patient positioning, and
gradient-switching fields in each examination can change the
Safety of biomedical implants and devices
Evaluation of implants and devices is critical to patient safety
in the imaging milieu. The FDA defines two terms for items being
considered for the MR environment. "MR safe" indicates that the
device presents no additional risk to the patient in the MR
surroundings but can affect the image quality. "MR compatible"
indicates that the device presents no additional risk to the
patient in the MR environment and does not affect image quality.
It is important to note the magnetic field strength of the system
used during testing, particularly with the increasing use of higher
field strength magnets. For example, a device shown to be "MR safe"
at 1.5 T may not necessarily be safe at 3.0 T.
Metallic biomedical implants and devices with ferromagnetic
properties are subject to torque and translational force. Torque is
a rotational force and results in a ferromagnetic object aligning
parallel to the static magnetic field. Torque is greatest in the
center of the magnet, but also exists outside the magnet bore.
Translational force is a linear force that attracts an object into
the magnet bore. This force is greater the closer it is to the
magnet bore where a higher spatial gradient exists. As an object
gets closer to the magnet bore, the spatial gradient goes higher,
and greater translational force is exerted on the object. Once the
object is in the center of the magnet, there is no longer a spatial
gradient, and thus no translational force.
For metallic implants with significant ferromagnetic properties,
these forces can result in tissue damage surrounding the apparatus,
including rupture of blood vessels. Examples of potentially
injurious implants include cerebral aneurysm clips and intraorbital
metal fragments. The fatality involving the ferromagnetic aneurysm
clip occurred with the development of intracerebral hemorrhage and
tearing of the middle cerebral artery while in the MR unit.
Imaging suggested that torquing of the clip was the cause. The
reported case of unilateral blindness resulted from an occult 2.0 *
3.5 mm intraocular metal fragment dislodging in a 0.35 T scanner
causing vitreous hemorrhage.
Radiofrequency electromagnetic fields can cause the heating of
an implant, its lead wires, or surrounding tissues.
An example is RF energy interacting with a pacemaker output
potentially causing tachycardia or other arrhythmias.
Imaging with an implantable device requires thorough
investigation regarding the MR safety or compatibility of the item
with attention to the testing environment, ie, field strength.
Detailed information for the specific devices appears in the
Guide to MR Procedures and Metallic Objects: Update 2001
or at MRIsafety.com.
The projectile effect
Torque and translational force are also responsible for the
projectile effect or missile effect in which unrestrained magnetic
objects can literally fly through the air and crash into the
Even a very small object can accelerate and cause injury to
patients or others within the area, as well as to the MR system
A wide variety of objects, both medical and otherwise, have been
drawn into magnets. Such items include oxygen or nitrous oxide
tanks, a defibrillator, a wheelchair, a respirator, ankle weights,
an IV pole, a toolbox, sandbags containing metal filings, a vacuum
cleaner, and mop buckets.
Other reported objects include parts of a forklift, a laundry cart,
a chair, a ladder, a light fixture, scissors, traction weights, a
pillow with ferromagnetic springs, and more.
Recently, there has even been a report of the spontaneous firing of
a gun into the wall of the imaging room despite the firearm's thumb
safety being engaged.
The projectiles can do tremendous damage to patients and to the MR
systems. Figure 4 presents an example of the IV pole that was
inadvertently drawn into the bore of a 1.5 T magnet.
These reports of metallic objects leading to adverse events are
particularly worrisome because they are largely preventable with a
combination of education and vigilance. A healthcare risk
management article published after the projectile oxygen tank
caused the child's death suggested that at least every 2 weeks, MR
imaging manufacturers across the country remove metal objects from
The Emergency Care Research Institute has issued several
recommendations to help prevent these incidents, including
appointing a safety officer to enforce procedures, providing formal
safety training to those working in the MR environment, and
restricting the equipment within the 5-gauss line to that labeled
American College of Radiology (ACR) White Paper on MR
The ACR Blue Ribbon Panel on MR Safety recently issued a report
after the MR imaging incident and subsequent media attention
spotlighted the need for review.
The committee was chaired by Emanuel Kanal, MD, FACR, a leader in
MR safety. In the editorial, Rogers states that "safety in the MR
suite must be a collaborative effort."
Guidelines include establishing safety policies and designating
a person responsible for ensuring their implementation. A plan for
site access restriction using four zones is also outlined. Zone I
is the general public access area. Zone II is an interface to the
restricted areas for the initial screening and checking-in of
patients. Zone III is restricted and is controlled by MR personnel,
usually technologists, under the authority of the MR medical
director. This zone should be physically restricted with access
keys/passkeys needed for entry. The 5-gauss line is within this
zone and all persons entering Zone III must have passed the
screening process. Zone IV is the room in which the MR system is
The ACR article describes levels of MR personnel, which are
based on the amount of safety education and training they have
received. It states that when scanning in a nonemergency coverage
situation, there should be at least two MR technologists (or one MR
technologist and one other individual from MR personnel) in the MR
In regard to pregnancy, the committee stresses that to scan a
pregnant woman, the data needed should "affect the care of that
patient and/or fetus DURING the pregnancy." Other guidelines
address contrast agent safety, cryogen-related issues, the
bioeffects of magnetic fields, and other topics.
Particular attention is paid to screening in the ACR White
Paper. The guidelines suggest that nonemergent patients be screened
safely by a minimum of two separate individuals, with one screening
being verbal. Any non-MR personnel also must pass the screening
process to enter Zone III and, by definition, Zone IV, as it is
contained within Zone III. Other specific guidelines address the
screening of devices and review questions of clinical concern, such
as when to obtain orbital plain films.
In a commentary following the ACR White Paper, Shellock and
described the concept of zones as interesting but without empiric
support for its use. Additionally, they felt it was probably
unnecessary to mandate screening by two different individuals for
nonemergent patients. Other areas of disagreement included the use
of a strong, handheld magnet to test devices or implants and the
use of color-coded labeling designating products as "MR safe" or
"MR compatible." A formal response to this commentary will be
published in an upcoming issue of the
American Journal of Roentgenology
The last 2 decades have provided an explosion of exciting
technology and applications for MR imaging. Unfortunately, there
have also been injuries and fatalities associated with MR
examinations. Safety in the MR environment has long merited the
attention it is now receiving. The radiology community has
highlighted MR safety and bioeffects with journal articles, meeting
courses, Web sites, and the new ACR White Paper on MR Safety. With
technological advances providing higher static magnetic fields and
more rapidly switching gradient magnetic fields, research to
delineate the bioeffects of MR imaging is of paramount importance.
Ensuring a safe environment is part of the fundamental commitment
we make to our patients. Awareness, vigilance, and resolve can help
make this challenge a reality.
The author thanks Ron R. Price, PhD, for reviewing the
manuscript and for providing invaluable insight. The author is also
thankful for thoughtful assistance from the "MR King," Ric Andal,
RT(R)(MR), and for technical support from John Bobbitt. Special
thanks are extended to Michael Rohmiller, MD for his generous
assistance in obtaining the images that appear in this article.