IN BRIEF, WHAT ARE NMR AND MRI? 
HOW MANY MR MACHINES ARE THERE? 
ARE THERE OTHER THAN MEDICAL APPLICATIONS OF MR?
IS MR IMAGING A SAFE PROCEDURE OR ARE THERE SIDE EFFECTS? 
HOW DID MAGNETIC RESONANCE IMAGING DEVELOP?
I DO NOT UNDERSTAND ALL THE ACRONYMS USED FOR 
CAN YOU PROVIDE AN OVERVIEW OF MR CONTRAST AGENTS?
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 | July
30, 2001 The child, whom the medical center would not identify by name, sex or age,died Sunday. The hospital said the child was sedated following an operation Friday and was inside the machine when the 10-ton electromagnet drew the canister. Edward
Stolzenberg, president and chief executive of the medical center, said the hospital
assumes full responsibility and "will do anything it can to ease the family's
grief.'' |
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Introduction Any
new method in medicine, be it diagnostic or therapeutic, must be thoroughly checked
for possible adverse side effects. More than 100 years ago, x-rays represented
a major step forward, but then sobered radiologists and the public after the hazards
of ionizing radiation were detected. During
the last century, several hundred papers focusing on the effects or side effects
of magnetic or radiofrequency fields have been published. This overview cannot cover all potential sources of hazards. Several reviews of the literature have been published recently, e.g., by Shellock and Kanal [41], by Persson and Ståhlberg [31], and by Magin, Liburdy, and Persson [25]. Several of the side effects associated with MR are unique to this kind of medical diagnostic tool; others are similar to hazards of other diagnostic methods. Possible hazards can arise from or be connected to: •
static magnetic fields; and
specifically: •
devices necessary to operate the imager (such as cooling gases) or to ensure the
quality of life of the patients (such as intracorporal implants and extracorporal
monitors); These hazards can affect patients, personnel, and other persons within the field of the magnet. They can be categorized as acute and subacute. Acute
Hazards Danger and prohibition symbols and signs used in MR installations
External Objects and Devices Projectiles.
The most imminent danger for both patients and personnel in the magnetic field
of an imaging system may result from ferromagnetic objects such as scalpels, scissors,
pens, and even sand bags (not filled with sand but with iron shot) and gas dewars,
which can be attracted by the magnet and thus behave like projectiles. To prevent such
accidents, the installation of a metal detector through which everybody has to
pass before entering the MR suite has been recommended, but is rather cumbersome.
Every person
working or entering the magnet room or adjacent rooms with a magnetic field has
to be instructed about the dangers. This should include the intensive-care staff,
and maintenance, service and cleaning personnel, as well as the crew at the local
fire station. The
best protection against this danger is not to allow personnel other than those
directly involved in patient examinations, i.e., the operator and the radiologist,
into the magnet room. Constant education of everybody involved is also vital.
Monitors
and respirators. The dependence on physiological monitoring, on mechanical
respiration, and electric infusion pumps during MR examinations renders difficulties,
and in certain instances does not allow such an examination. However,
with the development of appropriate monitoring and life-support equipment during
the last few years, dependence is no longer a contraindication of MR imaging.
Details on monitoring can be found in an article by Kanal and Shellock [20]. Contrast agents. Paramagnetic, superparamagnetic or ferromagnetic magnetic resonance contrast agents or other substances which have to be injected or applied in another way may present risks similar to those in any other invasive technique such as x-rays, particularly in patients with kidney diseases. The clinical experience of administering gadolinium-based or other agents intravenously to patients has shown that these agents are generally safe and well-tolerated. Only very few severe anaphylactoid reactions and cases of glottis edema have been reported. Still, all necessary precautions for intensive-care treatment have to be considered when injecting such contrast agents, particularly in patients with a history of allergy or drug reaction [37]. With at least two contrast agents a rare, but serious syndrome (Nephrogenic systemic fibrosis – NSF) has been observed in patients with severe kidney failure. It involves fibrosis of skin, joints, eyes, and internal organs, and can be fatal [27a]. A precautionary 24-hour suspension of breast-feeding was generally recommended following the administration of gadolinium-containing contrast agents. However, it has been proposed that this suspension be reduced to 12 hours [18]. MR Equipment Noise.
The noise created by the switching of the gradients is an additional source of
inconvenience and, possibly, ear damage for the patient and, occasionally, personnel.
This noise is comparable to very heavy traffic. Noise levels increase with field
strength. Disposable
earplugs for the patient are recommended in high-field systems. Noise-cancelling
systems and special earphones are available, and active acoustic control systems
are being developed [26]. Cooling
gases. In superconductive magnet systems, helium and nitrogen are used as
cooling gases. In the case of a quench, gases are released to the outside. Under
normal circumstances, the gases should escape through a pipe system and not reach
the magnet-room atmosphere. However, accidentally some gas could be released into
the magnet room. In this case, there are two potential dangers. Frostbite can be induced because the gases are extremely cold. Secondly, nitrogen is to be considered hazardous, in particular under pressure (whereas there is no danger of direct intoxication from helium). All personnel and patients must evacuate the area immediately and return only after proper ventilation of the magnet room. Oxygen monitors with an audible alarm, situated at an appropriate height within the magnet room are recommended safety devices [41]. Patient-Related Devices Implants.
A particular danger is presented by small metallic surgical implants. Hemostatic
or other clips in the CNS can move in their position. Dislocation by magnetic
attraction or torque presents a risk of hemorrhage. In other parts of the body,
we consider this to be a minimal risk, because after the healing phase of six
to eight weeks, fibrosis and encasement of the clip help to keep it in a stable
position. The label stainless steel is not a guarantee for non-ferromagnetic steel. Implants that
involve magnets such as magnetic sphincters, stoma plugs, dental implants, etc.,
can be demagnetized by the MR imager. They should be removed prior to the examination.
An extensive
overview of the behavior of implants is given by Shellock 40, including a list
of several hundred devices which are not prone to dislocation. It also lists a
selection of those metallic implants, materials, and foreign bodies that are potential
risks for patients undergoing MR imaging examinations. Foreign
bodies. Occult ferromagnetic foreign bodies incorporated in accidents are
dangerous, in particular those close to the eyes. The patient's history may help
to rule out such foreign bodies. Many patients, however, do not remember such
accidents. In case of doubt, x-rays should be taken prior to MR imaging. Ferromagnetic
makeup and tattoos cannot only distort MR images, but also can be irritated and
makeup can even be pulled into the eye by magnetic forces. Makeup should be removed
before the examination, if possible. Pacemakers.
Research on the influence of magnetic and radiofrequency fields upon cardiac
pacemakers reported that the RF radiation of the MR imager might disturb the function
of demand pacemakers by closing the reed relay and switching to the asynchronous
mode; varying magnetic fields may mimic cardiac activity. Magnetic attraction
can provoke motion of the pacemaker in its pocket and thus move the conducting
lead. Therefore, pacemaker patients or other persons bearing pacemakers should
not be examined in, or come close to, an MRI or MRS system, although recently
some exceptions have been described for new-generation pacemakers [43]. Pavlicek
et al. reported a threshold for initiating the asynchronous mode of a pacemaker
at 17 Gauss [30]. The national regulatory boards decided to limit the threshold
for access to MRI areas to 5 Gauss. It seems advisable to mark this area by signs
or lines on the floor. It
is of special interest for the observer of bureaucratic procedures that the 5-Gauss
limit is ten times higher than the average earth magnetic field, but lower than
the magnetic field in electric trains such as subways (up to 7 Gauss). The fields
measured on the surface of the receiver of a telephone are 35 Gauss and of an
audio headset 100 Gauss. Similar
considerations hold for pacemakers used for stimulation of the carotid sinus or
intracorporal insulin pumps, for instance. Here, no adverse effects have been
observed [38]. However, interference in electronic cochlear implants and ferromagnetic
mechanical stapedial replacements has been reported [17]. Prosthetic heart valves are not considered to be dangerous in low fields. Patients should not undergo MR imaging in high fields if valve dehiscence is clinically suspected [44]. Wires,
other metallic objects, and skin contact. Wire configurations such as pacemaker
lead wires, ECG and plethysmographic cables, and surface-coil connections can
act as antennae. Gradient and RF fields may induce current into these wires and
thus cause fibrillations and burns. This presents a risk to the patient and must
be eliminated prior to the examination. This
holds in a similar way for all clothing containing metallic threads or components,
as well as all metallic objects such as eye glasses, jewelry, hairpins, buttons,
watches, bracelets, prostheses, etc. All of these objects must be removed prior
to the examination. The
patient’s skin should not be in contact with the inner bore of the magnet.
Large-radius wire loops should not be formed by leads or wires that are used in
the magnet bore during imaging procedures. If
the patient’s arms and legs are not completely covered with clothing, insulating
material must be placed between the legs and between legs and magnet. Leg-to-leg
and leg-to-arm skin contact must be prevented in order to avoid the risk of burning
due to the generation of high current loops if the legs or arms are allowed to
touch. IUDs.
Most of the commonly used intrauterine contraceptive devices (IUD) do not move
under the influence of the magnetic field, do not heat up during sequences usually
applied for pelvic imaging, and do not produce major artifacts in vitro or in
vivo. Thus, patients with either all plastic or copper IUDs can be safely imaged
with magnetic resonance [27]. Joint
and limb prostheses. Generally, such prostheses present no risk. However, they
can introduce image artifacts. If possible, they should be removed prior to the
MR examination. Skin patches. Pharmaceutical products in transdermal skin patches may cause burns due to the absorption of RF energy. Such patches must be removed prior to MR examinations. Other Considerations Sedation.
MR has become an important tool in pediatric imaging. Since some infants and
children are unable to cooperate with the examiners, there is an increased demand
for sedation. Some
infants sleep soundly through an MR examination, particularly if they have eaten;
however, many infants and children up to eight years require sedation, even if
they are accompanied by their parents into the scanner room. In most instances,
teenagers can be treated like adults. Details
on sedation and procedures can be found in the literature [3, 4, 20]. Claustrophobia.
This is a very real psychological danger for some patients. Claustrophobia and
other psychological stress situations have been reported severe enough to interrupt
the examination in about 1-4% of cases. In this respect, small and wide-bore MR
imagers are advantageous because the percentage of claustrophobic incidents drops
significantly. Explanation of the imaging procedure and the equipment prior to
the examination helps to reduce claustrophobia significantly. The
possibility of the patient falling from the examination couch and hypotonic syndrome
(due to heat, motionless horizontal lying for a certain time, and psychological
agitation) are additional hazards. Pregnancy. There is no evidence that MR can harm the fetus or embryo — MR imaging is used for fetography, particularly for imaging the brain. An epidemiological study by Kanal, et al. concluded that data collected from MR imaging technologists were negative with respect to any statistically significant elevations in the rates of spontaneous abortion, infertility, and premature delivery [21]. As a safety precaution, MR scanning should be avoided in the first three months of pregnancy. MR imaging is indicated for use in pregnant women if other nonionizing forms of diagnostic imaging are inadequate, or if the examination provides important information which would otherwise require exposure to ionizing radiation such as x-ray or CT. Similar considerations hold for pregnant staff of a magnetic resonance department. Mainly for psychological reasons, it might be a wise precaution that pregnant staff members do not remain in the scan room during actual scanning; however, they are allowed to prepare and position the patient, administer contrast agents, and scan and film. Contraindications for MR imaging and spectroscopy. Never forget that the magnetic memory of credit and similar cards, as well as magnetic devices such as tapes, will be erased by MR magnets. Leave home without them or leave them outside the magnet room. Absolute
Contraindications electronically,
magnetically, and mechanically activated implants: Relative
Contraindications electronically,
magnetically, and mechanically activated implants: Legal Requirements In
the early 1980s, a number of national health and radiation protection boards first
established recommendations concerning magnetic resonance imagers and spectroscopic
units [15, 19, 33, 34]. All limits set by them were recommended levels, not mandatory
ones. Legal
requirements in some European and Asian countries exist; some of them are without
any scientific background, imposed by economic lobbies rather than learned societies.
Others have no connection to reality or clinical routine [19, 29]. In
the meantime, however, some manufacturers started using field strengths beyond
2.0 T, different pulse sequences, and gradient-switching procedures without any
reported ill effects. Thus, these recommendations are partly outdated. Adjustments made do not cover all possible medical applications of MR imaging, although the US-American Food and Drug Administration (FDA) extended the designation of ‘nonsignificant risk’ to MR systems with field strengths of up to 4.0 T in 1997 [5b] and 8.0 T in 2003 (4.0 T in neonates younger than one month) [5c]. However,
there is no sufficient evidence that imaging at fields higher than 2.0 Tesla is
risk-free. Until reliable scientific studies about
the effects of ultrahigh fields are available, MR equipment operating at fields
higher than 2.0 Tesla should be purchased with caution. For legal reasons,
the owner of MR equipment has to ensure that the equipment does fulfil the local
requirements. In some countries, the regulations are more stringent than in others;
in other countries, they are nonexistent. These requirements must be guaranteed
by the manufacturer because the user in general is unable to check power output,
gradient strength, or even field strength. This guarantee must cover authorized
hardware and software updates after the initial installation. Specially designed
computer programs usually supervise the power output of MR systems and will not
allow or will interrupt any imaging or spectroscopy procedure exceeding those
limits considered safe. The
subacute risks of magnetic and RF fields have been intensively examined for a
long time. There
are some publications associating an increase in the incidence of leukemia with
the location of buildings close to high-current power lines with extremely low-frequency
(ELF) electromagnetic radiation of 50-60 Hz 46, and industrial exposure to electric
and magnetic fields [28]. However, a transposition of such effects to MRI or MRS
seems unlikely. According to the National Radiological Protection Board of the United Kingdom [35], the available experimental evidence weighs against electromagnetic fields acting directly to damage cellular DNA, implying that these fields may not be capable of initiating cancer in a manner that parallels that of ionizing radiation and many chemical agents. The results of some animal and cellular studies suggest the possibility that electromagnetic fields may act as co-carcinogens or tumor promotors, but taken overall, the data are inconclusive. In the following paragraphs, we shall discuss some possible subacute hazards. Static Magnetic Fields In every MR examination, a large static magnetic field is applied. Field strengths for clinical equipment can vary between 0.2 and 3.0 T (2,000 and 30,000 Gauss); experimental imaging units have a field strength of up to 17.5 T, depending on the equipment used. In MRS, field strengths up to 12 T (120,000 Gauss) are currently used. No permanent hazardous effects of static magnetic fields upon human beings have yet been demonstrated [8]. However,
there have been no long-term studies following persons who have been exposed to
a static magnetic field. Budinger calls the following five biophysical mechanisms into question whereby static magnetic fields might influence biological processes or an organism’s behavior [14]:
Orientation
changes of macromolecules and living-cell subcellular components. The result
of replicable experiments on the orientation effects of retinal rods in fields
of 1 Tesla, the alignment of sickle cells at 0.35 T, and the orientation of certain
bacteria and animals might be explained by the physical torque rather than the
sensing of the turning torque by nervous tissue. Nerve
conductivity. As early as 1893, the first results of experiments about a possible
influence of static magnetic fields upon nerve tissue were obtained [6]. These
and all later experiments showed negative results. There are apparently no effects
on the conduction of impulses in the nerve fiber up to a field strength of 0.1
T generated by either changing the electrical resistance or the potential of the
excitation [1, 2]. Theoretical examinations argue that fields of 24 T are required
to produce a 10% reduction of nerve impulse conduction velocity [23]. A
preliminary study has indicated neurological effects in subjects exposed to a
whole-body imager at 4.0 T [41]. Here, additional research is necessary. Cardiac changes. A field-strength-dependent increase in the amplitude of the ECG in rats has been observed during exposure to homogeneous stationary magnetic fields. The minimum level at which augmentation could be observed was 0.3 T; at 2.0 T, the increase was by an average of 400%. The
augmentation in T-wave amplitude occurred instantaneously and was immediately
reversible after exposure to the magnetic field ceased. There have been no abnormalities
in the ECG in the later follow-up [16]. The authors suggest that augmentation
of the signal amplitude in the T-wave segment may result from a superimposed electrical
potential. At field strengths of between 7 and 10 T, no arrhythmia could be proven
[10]. According
to the national radiation protection and health agencies, it is unlikely that
cardiac fibrillation would occur as a result of induced flow potential in the
major blood vessels or heart chambers at this level of field intensity. No
circulatory alterations coincide with the ECG changes. Therefore, no biological
risks are believed to be associated with them. Magnetohydrodynamic effects. A blood pressure increase of 28% is predicted theoretically for a field of 10 T. This is claimed to be caused by interaction of induced electrical potentials and currents within a solution, e.g. blood, and an electrical volume force causing a retardation in the direction opposite to the fluid flow. This decrease in flow velocity must be compensated for by an elevation in pressure. At 1.5 T, no significant changes are expected; at 6.0 T a 10% pressure change is expected [14, 45}. In
addition to Budinger's reflections, the following points are valid for discussion: Genetic effects.
There have been several reports that static magnetic fields may provoke genetic
mutations, changes in growth rate and leukocyte count and other effects [36].
The results of these experiments could not be reproduced [39]. Inhibition of growth
rate of Eschericha coli induced by low-frequency magnetic field could be shown.
Nevertheless, some authors claim it be unlikely that mutagenic effects are introduced
by fields lower than 1.0 T [24]. No reports have been published that persons exposed
to magnetic fields, including personnel at MR departments, have a higher incidence
of genetic damage to their children than found in the average population. We believe, however,
that this research needs further investigation and that pregnancy should be considered
a relative contraindication for MRI and MRS. Taking into account that clinical
MR imaging devices operate at field strengths of between 0.2 and 2.0 T, caution
demands further experiments at higher field strengths. Membrane transportation and blood sedimentation. Other potential hazards from static fields include, for instance, membrane transportation and blood sedimentation induced by the field. As Mansfield and Morris pointed out, static magnetic field gradients of 0.01 T/cm (100 G/cm) make no significant difference in the membrane transport processes. The influence of a static magnetic field upon erythrocytes is not sufficient to provoke sedimentation, as long as there is a normal blood circulation [24]. As discussed above, many results presented in publications about effects of static magnetic fields are contradictory and cannot be explained by biophysical or biochemical mechanisms. In some cases, the effects observed must be attributed to other causes which had not been considered by the researchers in the setup of the experimental protocol. Critical considerations of such experiments can be found in a number of reviews [14, 31]. However, the data available are not comprehensive enough to assume MR imaging and spectroscopy are absolutely safe. Varying Magnetic Fields Varying
magnetic fields are necessary for the localization of nuclei with magnetic properties
within the sample. A
well described effect of varying magnetic fields is the so-called magnetic phosphenes
[16], which were first observed some 90 years ago [7]. They are attributed to
magnetic-field variations and may occur in a threshold field change of between
2 and 5 T/s. Phosphenes are stimulations of the optic nerve or the retina, producing
a flashing sensation in the eyes. They seem not to cause any damage in the eye
or the nerve. Varying
magnetic fields are also used to stimulate bone-healing in non-unions and pseudarthroses.
The reasons why pulsed magnetic fields support bone-healing are not completely
understood [9]. Rapid echo-planar imaging and high-performance gradient systems create fast-switching magnetic fields that can stimulate muscle and nerve tissues. The mean threshold levels for various stimulations are 3,600 T/s for the heart, 900 T/s for the respiratory system, and 60 T/s for the peripheral nerves. Guidelines in the United States limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation. Radiofrequency Fields Radiofrequency
pulses are used in MR imaging for the excitation of the nuclei. RF fields may
interact with both tissues and foreign bodies, such as metallic implants, in the
patient. The main result of this type of interaction is heat. The
higher the frequency, the larger will be the amount of heat developed; and the
more ionic the biochemical environment in the tissue, the more energy that will
be deposited as heat [22, 35]. This effect is well-known for homogeneous model
systems, but the complex structure of various human tissues makes detailed theoretical
calculations very difficult, if not impossible. The
specific absorption rate, SAR, helps to estimate RF heating effects. It increases
with field strength, radiofrequency power and duty cycle, transmitter-coil type
and body size. In high and ultrahigh fields, some of the multiple echo, multiple-slice
pulse sequences may create a higher SAR than recommended by the agencies. Hot spots may
occur in the exposed tissue. At present, it seems unlikely that such hot spots
in the body exist, but to avoid or at least minimize effects of such theoretical
complications, the frequency and the power of the RF irradiation should be kept
at the lowest possible level. In
several in vitro and in vivo experiments, no threatening increase in temperature
could be shown [13, 23]. Even in high magnetic fields, no local temperature increase
greater than 1°C occurred. The highest skin temperature increase described
reached 2.1°C [41]. Eddy currents may heat up implants and thus may cause
local heating. In vitro worst-case experiments performed with a large and very
thin thermally insulated aluminium sheet at 1.5 T after 15 minutes of exposure
showed a temperature rise of only 0.08°C. According
to the specific FDA criteria for SAR limits, the SAR must not be greater than: • 4 W/kg
averaged over the whole body for any 15-minute period; Some European countries have issued SAR restriction too. No common denominator has been found. References 1.
Abashin VM, Yevtushenko GI. Influence of a permanent magnetic field on biological
systems. Biofizika 1975; 20: 276-280. | |
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danger — strong magnetic field
danger — high-frequency electromagnetic field
active
implants and metallic implants, such as pacemakers, prohibited
loose ferromagnetic objects prohibited
metal body implants prohibited
magnetic media such as credit cards, diskettes, magnetic tapes prohibited
mechanical watches, cameras and similar devices prohibited
