The AAPM/RSNA Physics Tutorial for Residents

Transcription

1 IMAGING & THERAPEUTIC TECHNOLOGY The AAPM/RSNA Physics Tutorial for Residents MR Imaging Safety Considerations 1 Ronald R. Price, PhD LEARNING OBJECTIVES After reading this article and taking the test, the reader will be able to: List current FDA guidelines and recommendations about MR imaging safety. Make recommendations for patient screening before MR imaging procedures. Make recommendations for maintaining a safe environment for both patients and workers. Experience and research over the past decade have demonstrated that diagnostic magnetic resonance (MR) imaging is a biologically safe imaging modality. Specifically, there is currently no convincing evidence that there is any longterm or irreversible biologic effects associated with the radiation and magnetic fields used in MR imaging, specifically radio-frequency (RF) radiation, static magnetic fields, and time-varying gradient fields. However, numerous hazards of MR imaging do exist that can cause severe injuries or even death. These hazards are primarily the result of (a) strong magnetic fields and the strong force that they exert on ferromagnetic objects brought into their influence, including interference with electronic devices such as pacemakers and other implanted electronic devices, and (b) RF burns resulting from inadvertently induced currents in conductive loops placed on the patient s skin surface (eg, electrocardiographic leads and other monitoring devices). Other potential concerns are peripheral nerve stimulation resulting from rapidly switched gradients and auditory noise levels. Establishing a complete and coordinated educational program for all MR imaging facility personnel and conducting effective screening and preparation of patients scheduled for MR imaging procedures are essential to avoid accidents and RF burns and to maintain a safe MR imaging facility. Abbreviations: FDA = Food and Drug Administration, RF = radio frequency, SAR = specific absorption rate Index terms: Magnetic resonance (MR), biological effects Magnetic resonance (MR), safety RadioGraphics 1999; 19: From the Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, R-1308 Medical Center North, st Ave South, Nashville, TN From the AAPM/RSNA Physics Tutorial at the 1998 RSNA scientific assembly. Received July 13, 1999; revisions requested August 23 and received September 23; accepted September 24. Address reprint requests to the author. RSNA,

2 n INTRODUCTION Magnetic resonance (MR) imaging has been widely available for over a decade. During this time, it has become an essential component of diagnostic medicine. Concerns about the biologic effects of static magnetic fields produced by clinical systems have largely been put to rest. The Food and Drug Administration (FDA) recently (October 3, 1997) extended the designation of non-significant risk to systems with static field strengths up to 4.0 T. (Copies of the document entitled Guidance for Magnetic Resonance Diagnostic Devices Criteria for Significant Risk Investigations can be viewed on the FDA web site Concerns about static magnetic fields now relate primarily to risks associated with metallic objects becoming projectiles in the influence of the static field and with interactions with implants and other electrical devices. In addition, burns resulting from radio-frequency (RF) induced currents in conducting loops, either electrical leads or from inadvertent body geometries, continue to be reported. With the introduction of high-performance gradients for echoplanar imaging, the potential now exists for peripheral nerve stimulation from rapid gradient switching. The purpose of this article is to review the current level of knowledge about the safety of MR imaging, including the biologic effects from static magnetic fields, RF fields, time-varying gradient fields, and acoustic noise; the hazards associated with each of these; and the safety recommendations. Guidelines for maintaining a safe MR imaging environment and screening patients are also discussed, with references to safety resources, relevant research, and current FDA guidelines and recommendations. Current guidelines on RF specific absorption rates (SAR), the rate of change of gradient fields with time (db/dt), and acoustic noise levels can all be found at the FDA web site ( The reader is also referred to the book by Shellock and Kanal for a general review of all aspects of MR imaging safety (1). Other general reviews are also available in the literature (2,3). A wide spectrum of questions and answers on MR imaging safety can be found on the Shellock and Kanal web site ( n BIOLOGIC EFFECTS l Static Magnetic Fields Experiments designed to study biologic effects of static magnetic fields have generally resulted in either negative or inconclusive findings (4). Several reviews of these findings are available (1 3). The recent increase in interest and use of ultra-high-magnetic-field systems (3.0 T and above) for functional MR imaging and other areas of MR imaging research has rekindled concern about the biologic effects of static magnetic fields. Currently, there are over 30 systems in operation with field strengths in excess of 3.0 T. Large numbers of individuals have now received substantial exposure at these field strengths with no reported long-term adverse effects (5). Initial reports of MR imaging of humans at 8.0 T have also indicated no adverse effects (6). A known reversible biologic effect associated with exposure to a strong static magnetic field is the elevation of the T wave in electrocardiographic tracings. Because blood is a conductive medium, the movement of blood in the magnetic field of the MR imaging unit causes a magnetohydrodynamic effect that produces a voltage across the vessel. The mechanism responsible for the induced voltage depends on the velocity of the blood flow, the strength of the external magnetic field, the diameter of the vessel, and the angle of the blood flow with respect to the direction of the magnetic field (Fig 1). Because the peak flow rate occurs during the T-wave interval of the electrocardiographic tracing, the added voltage from the flowing blood manifests as an elevation of the T wave (Fig 2). Currently, no mechanism has been proposed that suggests a hazard would be associated with the phenomenon. Budinger (7) has estimated that at magnetic fields of ultra-high strength (10 T), a potential on the order of 64 mv could possibly be produced across an aorta of about 16 mm in diameter. l RF Fields No convincing evidence exists for direct nonthermal biologic effects of RF radiation resulting from diagnostic MR imaging (7 9). There is, however, clear evidence that significant RF burns can and do occur (8 12). Precautions to avoid RF burns in patients are an essential component of MR imaging safety and are discussed in a later section n Imaging & Therapeutic Technology Volume 19 Number 6

3 Figure 1. Diagram illustrates how flowing blood behaves as a moving conductor within a magnetic field. Blood flowing with velocity (V) within a vessel oriented at an angle (q) with respect to the magnetic field (B) will produce a force (F) on the charge carriers in the blood. This force in turn produces a voltage difference across the vessel in a direction perpendicular to the blood flow. The magnitude of this electromotive force (emf) is equal to the product of the vessel diameter (d), the velocity (V), the field strength (B), and the sin of the angle between the direction of the blood flow and the magnetic field. l Time-varying Gradient Magnetic Fields Time-varying gradient magnetic fields have recently been reconsidered for biologic effects with the introduction of rapid echo-planar imaging and the use of high-performance gradient systems, since it is known that rapidly switching magnetic fields can stimulate muscle and nerve tissue. The specific concern has been peripheral nerve stimulation (13,14). In response, the FDA has established guidelines that limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation. Incidentally, this level is a factor of 30 below the threshold for cardiac stimulation. At present, there is no known mechanism that would suggest an irreversible biologic effect caused by rapidly switching magnetic fields. l Acoustic Noise Guidelines for acceptable acoustic noise levels associated with MR imaging are based on Occupational Safety and Health Administration guidelines for industrial workers and are set at maintaining root mean square (rms) levels below 100 db with hearing protection in place. With certain pulse sequences, it is possible to exceed these guidelines (15,16). It is always prudent to ensure that patients are supplied with hearing protection regardless of what particular pulse sequence is used. Figure 2. Diagram illustrates the magnetohydrodynamic effect of blood on an electrocardiographic tracing. The largest electromotive force (voltage) induced in a vessel will occur during that part of the cardiac cycle that produces the highest velocity blood flow. Because this flow occurs during the T-wave portion of the cycle, the effect causes the largest additive voltage to occur during the T wave, which results as an enhanced T wave in the electrocardiographic tracing. n OPERATIONAL HAZARDS The primary concerns for handling the hazards associated with operating an MR imaging facility are risk management of the fields that exceed 5 G ( T) and education of facility personnel and patients. Because it is widely accepted that MR imaging is a medical procedure with insignificant risk, the burden of effort to maintain a safe facility has shifted to education. It is generally accepted that patients should be educated about the risks associated with the MR imaging procedure during the screening interview (17,18). However, it is sometimes overlooked that it is equally necessary to educate all facility employees who may have even a remote chance of entering the high-magnetic-field area of the MR imaging unit. It is obvious that all medical personnel (technologists, physicians, nurses, escorts, etc) must be trained, but it may not be obvious that maintenance, security, and custodial personnel who may be present after normal working hours must also be educated about risks associated with the MR imaging facility. A complete and coordinated educational program must therefore be a part of a safe MR imaging facility. The educational program should include information about the behavior of metallic objects in static magnetic fields and the hazards associated with those fields, the hazards accompanying RF fields, and those associated with timevarying magnetic fields, with substantial emphasis on patient screening and preparation to avoid accidents and RF burns. November-December 1999 Price n RadioGraphics n 1643

4 l Review of the Effects of Magnetic Fields on Ferromagnetic Objects The field from a conventional MR imaging unit can be characterized in terms of two spatial regions. Region 1 is the area surrounding the isocenter of the magnetic field that is contained within the bore of the magnet. The magnetic field in this region is relatively constant and uniform in strength. Region 2 is the field external to the physical magnet and is a gradient field; that is, the strength of the field varies with spatial position. Metallic objects within region 1 experience only a rotational force called a torque. In region 1, a nonspherical metallic object either inside or outside the body will experience a torque, depending on the size and shape of the object. The induced rotational motion of an object inside the body can cause the object to tear surrounding tissue. The field strength in region 2 decreases with distance from the magnet. A metallic object located in the gradient field of region 2 may experience both rotational and translational forces. The direction of the translational force will be in the direction of the higher field strength (toward the center of the magnet) (Fig 3). In region 2, a metallic object within the body will experience a translational force, the strength of which depends on the size of the object and the strength of the gradient field. (The magnitude of the gradient field is usually greatest in the regions immediately adjacent to the magnet). Objects within the body will be caused to accelerate toward the magnetic isocenter and can thus tear the surrounding tissue. The effects of the field in region 2 will be experienced by the patient as he or she approaches the magnet before actually being placed within the magnet bore. The pull on a ferromagnetic object in the magnetic field depends on the net unbalanced forces on the object. The force exerted on the object depends on the interaction of the induced magnetic moment in the object and the strength of the external field. The magnitude of the induced magnetic moment depends on the size of the object and its proximity to the magnet. Objects will be accelerated along the direction of the spatial gradients in the static field (Fig 3). Once the object enters the uniform field at the isocenter of the magnet, it ceases to be accelerated and comes to rest. Magnets with large fringe fields and consequently fringe field spatial gradients create the greatest hazard. Figure 3. Diagram illustrates the induced magnetic moment of a metallic object placed in a magnetic field. The strength of the magnetic moment (m) will depend on the strength of the magnetic field in which the object is located and the susceptibility of the object. The induced magnetic moment of the object will be attracted by the magnetic field of the MR imager. The force of the attraction will depend on the magnitudes of the induced magnetic moment and of the field gradient in which the object is located. The object will be attracted in the direction of the higher field strength region of the MR imaging unit. In general, the magnitude of the field gradients increases as the distance from the imaging unit isocenter decreases. Thus, the force of attraction is greater as the object comes closer to the magnet. The number of magnetic field lines per unit area is used to indicate the region of stronger fields. l Hazards Associated with Static Magnetic Fields Implanted or Foreign Metallic Objects. One of the most substantial hazards encountered by those working around strong magnets is the attraction of the magnet to ferromagnetic objects. It is a common misconception that all stainless steel is nonferromagnetic. This is definitely not true, and clips or other implants of stainless steel construction should not be assumed to be nonmagnetic. Any ferromagnetic object close to the magnet even objects in the soft tissues of a patient who enters the magnetic field of an MR imaging unit will experience a magnetic pull and, depending on the shape and orientation of the object, a rotating torque. The object can thus be dislodged from its site and moved, resulting in cuts and tears in the surrounding tissue. Two specific metallic objects that may be encountered in patients and that can be adversely affected by magnetic fields have been identified: (a) metallic fragments in the eye and (b) intracerebral aneurysm clips. It is unfortunate that these objects were identified after disastrous accidents, one of which resulted in the death of a patient. Magnetic movement of metallic fragments in the eye has led to blindness (19,20) n Imaging & Therapeutic Technology Volume 19 Number 6

5 Movement of a metallic aneurysm clip in the brain has led to at least one fatality when the clip tore the middle cerebral artery (21). Older aneurysm clips were made with a magnetic steel spring. Newer clips have been tested and declared safe for MR imaging (22,23). A patient presenting for an MR imaging examination and having an aneurysm clip should not be imaged unless definitive proof supporting the safety of the clip can be produced. Certain intravascular implants are known to become firmly incorporated into the vessel wall about 6 weeks after introduction. It is therefore considered safe to image patients with most intravascular coils, filters, and stents after a suitable time has elapsed (1). As always, it is essential to carefully document the characteristics of any implanted device before imaging is attempted and to specifically refrain from imaging if there is any reason to believe that the device is not held firmly in place. In general, the presence of any electrically active implant should be considered a contraindication for MR imaging until proof of its safety is demonstrated. Literature references are available specifically for cochlear implants (24), implanted magnetic eye sockets (25,26), neurostimulators (27), and cardiac pacemakers (2,28 30). At least one new neurostimulator is now being evaluated for its MR imaging safety. The FDA maintains a database on adverse incidents that have occurred with medical devices. The Medical Device Report database is available for review on the FDA web site ( and contains the complete description of specific events. The database specifically contains accounts of instances in which individuals with metallic objects in their bodies were injured when they came into the gradient fringe field of the MR imaging unit. Metallic Projectiles. One of the most serious risks associated with MR imaging is the hazard of ferromagnetic objects inadvertently being brought into the influence of the strong magnetic field of the MR imaging unit. Depending on the size and closeness of the object to the magnet, the pull on the object can often exceed the strength of an individual to restrain the movement of the object. There have been documented cases of oxygen cylinders, intravenous fluid poles, so-called sand bags (which actually contain iron shot), mop buckets, vacuum cleaners, and numerous medical devices becoming projectiles and being accelerated into the magnet. Anyone, staff or patient, in the path between the metallic object and the center of the magnet could be seriously injured or killed. Numerous incidents of ferromagnetic projectiles causing injury have been reported (17, Because accidents can happen at anytime, awaiting only a lapse in security, identifying all metallic objects is a high-priority component of patient and staff screening. The 5-Gauss Line. The most commonly recognized safety policy is the so-called 5-G ( T) line. The criterion of a 5-G ( T) line was established very early to limit access of individuals with cardiac pacemakers into high-magnetic-field areas that could potentially disable the pacemaker. The safety policy requires that physical barriers and appropriate signage be employed to exclude access by members of the general public to areas in which the static magnetic field strength is greater than 5 G ( T). Additional information is available at It is important to stress that the magnetic fringe fields extend in three dimensions and that the 5-G ( T) field restrictions must be applied in both the horizontal and vertical planes. Specifically, one must consider the fringe field extent into the floors above and below the magnet. The extent of the 5-G ( T) line depends on the location of the magnet isocenter and the strength of the magnet. The locations of the specific fringe field line should be supplied to the MR imaging site by the magnet vendor. The signage used to indicate the 5-G ( T) line should not only clearly identify the existence of the high-magnetic-field area but also describe potential consequences. These signs may contain comments that point out the potentially life-threatening circumstances that could result from bringing ferromagnetic objects into the high-magnetic-field region (ie, the projectile effect). The signs should also include warnings about the potential for disabling cardiac pacemakers and electronic implants and about the damages that could occur to magnetic data storage devices (computer disks and November-December 1999 Price n RadioGraphics n 1645

6 credit cards), cameras, watches, and other electrical and electronic devices. Operational Safety Rules. MR imaging facilities should consider implementing the following specific operational rules to maintain security and safety in high-magnetic-field areas: 1. Access to the high-magnetic-field area should be limited to trained personnel or to screened patients and visitors who are accompanied by trained personnel. 2. Entrance to the high-magnetic-field area should be controlled by a lockable door, and the keys to the area should be issued only to trained personnel. 3. All entrances to the high-magnetic-field area should be visible to the system operator. 4. All visitors must be screened by the operator before entry is allowed. 5. Appropriate warning signs must be posted. l Hazards Associated with RF Fields A substantial hazard of MR imaging that may often be overlooked is that of the associated RF effects. The scientific evidence for biologic effects of RF suggests that direct tissue heating leading to burns is the most important hazard of RF fields. Currently, RF burns that result from the inductive heating of conductors and leads for patient monitoring constitute the majority of MR imaging related injury reports (8 12,31 33). Direct tissue heating by RF is a function of many parameters, including the mass, conductivity, and heat capacity of the body part under study; its level of blood perfusion; ambient temperature; air circulation; the specific pulse sequence; and RF frequency. The FDA has used the tissue RF SAR as the parameter of heating to establish guidelines for allowable RF energy deposition. Specific Absorption Rate. The SAR is expressed in units of watts per kilogram of body weight (W/kg). The general guideline used by the FDA to establish allowable RF energy deposition is based on levels that produce a maximum change in tissue temperature of 1 C. According to the specific FDA criteria for SAR limits, the SAR must be no greater than (a) 4 W/kg averaged over whole body for any 15-minute Figure 4. Diagram illustrates the inductive coupling of the RF field to a wire loop. The quantity B 1 represents the strength and direction of the RF field with frequency (w) in the presence of a conductive loop with area A. The time-varying magnetic field produced by the RF coils will induce an electromotive force (emf) in the coil, which will in turn cause a current flow. The magnitude of the force is given by: emf = wb 1 A sin q, where q is the angle between the direction of the B 1 field and the plane of the coil. The maximum induced electromotive force occurs when the B 1 field is perpendicular to the plane of the coil. For maximum patient safety, nonconducting fiberoptic cables should be used instead of conductors whenever possible. period, (b) 3 W/kg averaged over the head for any 10-minute period, or (c) 8 W/kg in any gram of tissue in the head or torso or 12 W/kg in any gram of tissue in the extremities for any period of 5 minutes. RF Burns. Typical RF-related injuries recorded in the Medical Device Report database include burns from a conductive lead placed against the patient s bare skin during an MR imaging procedure, blisters on a patient s finger from a non MR imaging compatible pulse oximeter, and a burn to the hand of an anesthetized child from an electrocardiographic cable. Care must be taken to ensure that all conductive leads are placed such that they do not form loops and that they are insulated from contact with bare skin through the use of sheets or other thermal and electrical insulating materials. Loops of conductors within the magnet bore can present substantial risk for RF burns as a result of inductive heating of the conductor from the time-varying RF field. The rapidly changing magnetic field will induce an electromotive force or voltage in the conductor that causes a flow 1646 n Imaging & Therapeutic Technology Volume 19 Number 6

7 of current. The flowing current in a conductor with electrical resistance will result in heating the conductor; thus, the conductor in contact with the skin can cause a burn. The maximum induction occurs when the plane of the conducting loop is perpendicular to that of the changing magnetic field (Fig 4). Inductive heating can also occur with receive-only surface coils when the active-decoupling circuitry fails. For this reason, surface coils should also be insulated from the patient s skin. Inadvertent conducting loops can also occur within the body without any external metallic conductors. When a patient lying within the magnet bore touches his or her hands together, a conductive loop can occur, with heating occurring at the high-resistance skin-to-skin contact point (Fig 5). There have also been literature reports of inadvertent loops in the lower body, with the skin-to-skin contact point occurring where the patient s calves were touching (10). The result was blistering on both calves at the point of contact. The lesson from these chance observations is to eliminate all skin-toskin contact points while the patient is within the imaging unit through the use of sheets or other insulating materials. Burns can also occur with straight wires. Induction, causing substantial current flow to ground, has been hypothesized as the cause for recently reported RF burns with pulse oximeter leads (32). It has also been reported that conductive materials applied directly to the skin such as in tattoos, tattooed eyeliners, and some cosmetics can result in burns (33). Figure 5. Diagram shows an inadvertent body loop, which can cause RF induction and result in burns. Inadvertent loops can be created in the upper body (when the skin of the two hands touch) and the lower body (when the skin of the two calves touch). Inadvertent loops can be avoided by eliminating bare skin contact points. l Hazards Associated with Time-varying Gradient Magnetic Fields Rapidly switching magnetic fields can stimulate muscle and nerve tissues. The mean threshold levels (measured in tesla per second) for various stimulations are 3,600 T/sec for the heart, 900 T/sec for the respiratory system, 90 T/sec for pain, and 60 T/sec for the peripheral nerves. It should be stressed that these are mean threshold levels; that is, the stimulation thresholds in a single individual may be either higher or lower than these values. The general FDA guidelines for switching magnetic fields have been established to limit gradient-switching rates to a factor of three below the mean peripheral nerve stimulation level. Gradient waveforms may have either a sinusoidal or square wave switching pattern. A switching time (t) is defined for a square wave pattern as the time needed to go from zero gradient to the maximum gradient strength. For sinusoidal switching, t is generally understood to represent the time for one-half of a total switching cycle. Stimulation threshold values vary not only from person to person but also depending on gradient direction. Gradients along the long axis of the body (z) have a lower stimulation threshold than do gradients oriented transverse (x, y) or perpendicular to the long axis of the body (13,14). The FDA guidelines take gradient orientation into account and have specified limits that allow a factor of three higher levels for transverse gradients relative to axial gradients. November-December 1999 Price n RadioGraphics n 1647

8 The FDA guidelines are also specified as a function of the switching rate (t). For an axial gradient (z) specifically, db/dt < 20 T/sec for t > 120 msec, or db/dt < 2,400/t (msec) in T/sec for 12 msec < t < 120 msec, or db/dt < 200 T/sec for t > 12 msec. A graphical illustration of the stimulation thresholds and FDA guidelines is shown in Figure 6. n GUIDELINES FOR MR IMAGING FA- CILITY MANAGEMENT Figure 6. Graph demonstrates thresholds of stimulation of cardiac and peripheral nerves from rapid gradient switching in terms of db/dt as a function gradient ramp time. The graph also shows the current FDA guidelines for allowable gradient switching rates. The FDA guideline values are the lowest of the three curves. The middle curve is the mean peripheral nerve stimulation threshold, which is approximately three times higher (3x) than the FDA levels. The top curve is the mean threshold for cardiac stimulation. The cardiac stimulation threshold is approximately 30 times (30x) higher than the FDA guideline. The vertical axis is a log scale rather than linear. l Patient Questionnaire Each MR imaging facility should adopt a standard questionnaire to be used to screen patients before they undergo an MR imaging examination (18). A questionnaire similar to the one shown in Figure 7 should be completed by the patient or guardian before the MR imaging procedure. The questionnaire should be reviewed by the appropriate health care provider before the patient enters the examination room. The patient questionnaire must establish the nature of any previous surgical procedure, presence of any implants (either active or passive), presence of any foreign metallic bodies or prostheses, and any special medical conditions, including pregnancy, which may be relevant to the appropriateness of the requested MR imaging procedure. The questionnaire should include a diagram of the human body to allow patients with minimal written and verbal skills to easily communicate locations of prior surgical procedures or locations of any metal inside the body. It is the responsibility of the specific health care provider interviewing the patient to determine if the requested MR imaging procedure would be safe for that particular patient. If a patient has been occupationally exposed to metal fragments, the screening should include either radiography or computed tomography of the orbits to rule out the presence of metal fragments in the eyes (17). If there is any question, the MR imaging procedure should be postponed pending consultation with the radiologist in charge or until additional information is gathered. In some cases, the decision must be to cancel the study because of a contraindicating condition. In general, each of the following conditions must be considered as an absolute contraindication to MR imaging until information to the contrary is obtained: (a) the presence of an active electronic device (cardiac pacemaker, cochlear implant, nerve or bone stimulator) in the body, (b) the presence of a cerebral aneurysm clip, (c) the presence of intraocular metal fragments, (d) the presence of ferromagnetic foreign bodies, (e) magnetic eye sockets, or (f) the presence of any unfamiliar device. Patient safety is of the utmost concern, and thus one must be very conservative about any safety decision. On the other hand, a thorough knowledge of MR imaging safety and a familiarity with the literature may prevent the unwarranted cancellation of a needed study. In that regard, not all metallic implants should be considered an absolute contraindication to MR imaging. In general, internal orthopedic hardware; extracranial surgical clips; staples and wires in the body (specifically excluding cerebral aneurysm clips); intravascular stents, coils, and filters; and essentially all dental devices are generally safe and pose no substantial hazard to the patient. However, even though these objects are safe, they may produce susceptibility artifacts that render the images near the object nondiagnostic. All of these devices, although generally safe, must be confirmed as verified safe in each individual circumstance by using medical documentation from either the device vendor or references such as Shellock and Kanal (1). Once the patient has completed the screening questionnaire, all responses must be read carefully and additional questions should be 1648 n Imaging & Therapeutic Technology Volume 19 Number 6

9 a. b. Figure 7. Sample patient screening questionnaire for MR imaging procedures. (Reprinted, with permission, from reference 1.) asked if some responses raise concern. After an acceptable screening interview, the patient should then be prepared to enter the MR imaging examination room. The most desirable patient preparation from a purely safety perspective is to have the patient remove his or her shoes, outer clothing, all metallic items (hair clips, pins, belts, and jewelry), and any underclothing with metal or wire and to don a hospital gown or surgical scrubs. If the use of gowns or scrubs is not possible, the patient must be carefully screened for any metal before entering the examination room. Any metallic items in question should be checked with a small magnet before the patient enters the room to ensure that no ferromagnetic items are taken into the room. l Pregnant Patients The safety of MR imaging procedures for pregnant patients was one of the first concerns raised when MR imaging became an approved medical imaging procedure. In 1997, the American College of Radiology issued a statement on the safety of MR imaging in pregnant patients. The full text of this statement can be found in the ACR s 1997 Radiology Practice Standards. The essence of the statement is that in light of the lack of data demonstrating deleterious effects of MR on the developing human fetus, MR imaging should be recommended for evaluating pregnant patients when any alternative imaging procedure involves ionizing radiation. The statement further indicates that each case should be considered on an individual basis and that MR imaging should be performed only after the approval by the attending radiologist. l Pregnant Workers A different but related question is the safety of pregnant MR imaging workers. To evaluate the potential hazard that might be posed to the pregnant worker from the MR imaging environment (static magnetic fields, switched gradient fields, and RF fields), Kanal, Shellock and Savitz (34) conducted an epidemiologic study of MR imaging technologists in the United States. They concluded that their data were negative with respect to any statistically significant elevations in the rates of spontaneous abortion, infertility, and premature delivery (34). November-December 1999 Price n RadioGraphics n 1649

10 n CONCLUSIONS The material presented in this article is clearly not an exhaustive review of all MR imaging related safety issues. Other patient safety concerns include use of sedatives and reactions to MR imaging contrast materials. Other MR imaging safety issues include proper procedures for dealing with magnet quenches, proper handling of cryogens, and the procedures for determining the MR compatibility of new devices. A quotation from the FDA draft document published February 7, 1997, can serve as a summarizing statement: For a properly operating system, the hazards associated with direct interactions of these fields [static magnetic, pulsed gradient, and RF] and the body are negligible. It is the interactions of these fields with medical devices that create concerns for safety. n REFERENCES 1. Shellock FG, Kanal E. Magnetic resonance, bioeffects, safety, and patient management. Philadelphia, Pa: Lippincott-Raven, Baker KA, DeVor D. Safety considerations with high field MRI. Radiol Technol 1996; 67: Shellock FG. Biological effects and safety aspects of magnetic resonance imaging. Magn Reson Q 1989; 5: Weiss J, Herrick RC, Taber KH, Contant C, Plishker GA. Bio-effects of high magnetic fields: a study using a simple animal model. Magn Reson Imaging 1992; 10: Schenck JF, Dumoulin CL, Redington RWQ, Kressel HY, Elliott RT, McDougall IL. Human exposure to 4.0 Tesla magnetic fields in a wholebody scanner. Med Phys 1992; 5: Robitaille PML, Abduljalil AM, Kangarlu A, et al. Human magnetic resonance imaging at 8 T. NMR Biomed 1998; 11: Budinger TF. Current facts about induced current densities and E-fields. In: Young IR, Crues JV III, Felmlee JP, et al. Workshop on new insights into safety and compatibility issues affecting in vivo MR. Berkeley, Calif: International Society for Magnetic Resonance in Medicine, 1998; Grandolfo M, Polichetti A, Vecchia P, Gandhi OP. Spatial distribution of RF power in critical organs during magnetic resonance imaging. Ann N Y Acad Sci 1992; 649: Shellock FG, Crues JV. Corneal temperature changes induced by high-field-strength MR imaging with a head coil. Radiology 1988; 167: Knopp MV, Essig M. Debus J, Zabel HJ, van Kaick G. Unusual burns of the lower extremities caused by a closed conducting loop in a patient at MR imaging. Radiology 1996; 200: Brown TR, Goldstein B, Little J. Severe burns resulting from magnetic resonance imaging with cardiopulmonary monitoring: risks and relevant safety precautions. Am J Phys Med Rehab 1993; 72: Buchli R, Saner M, Meier D, Boskamp EB, Boesiger P. Increased RF power absorption in MR imaging due to RF coupling between body coil and surface coil. Magn Reson Med 1989; 9: Schaefer DJ (SDW), Schenck JF. Effects of landmark, gradient axis, and ramp time on gradient-induced nerve stimulation (abstr). Proceedings of the Sixth Meeting of the International Society for Magnetic Resonance in Medicine. Berkeley, Calif: International Society for Magnetic Resonance in Medicine, 1998; Cohen MS, Weisskoff RM, Rzedzian RR, Kantor HL. Sensory stimulation by time-varying magnetic fields. Magn Reson Med 1990; 14: Prieto TE, Bennett K, Weyers D. Acoustic noise levels in a head gradient coil during echo planar imaging at 3 T (abstr). Proceedings of the Sixth Meeting of the International Society for 1650 n Imaging & Therapeutic Technology Volume 19 Number 6

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