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MR Procedures: Biologic Effects, Safety, and Patient Care¹

¹ From the Keck School of Medicine, University of Southern California; and Institute for Magnetic Resonance Safety, Education, and Research, 7511 McConnell Ave, Los Angeles, CA 90045 (F.G.S.); and Radnet Management, Los Angeles, Calif (J.V.C.). Received May 21, 2003; revision requested July 18; revision received August 8; accepted October 8. Address correspondence to F.G.S. (e-mail: frank.shellock@gte.net).

	ABSTRACT

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
The technology used for magnetic resonance (MR) procedures has evolved continuously during the past 20 years, yielding MR systems with stronger static magnetic fields, faster and stronger gradient magnetic fields, and more powerful radiofrequency transmission coils. Most reported cases of MR-related injuries and the few fatalities that have occurred have apparently been the result of failure to follow safety guidelines or of use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices. To prevent accidents in the MR environment, therefore, it is necessary to revise information on biologic effects and safety according to changes that have occurred in MR technology and with regard to current guidelines for biomedical implants and devices. This review provides an overview of and update on MR biologic effects, discusses new or controversial MR safety topics and issues, presents evidence-based guidelines to ensure safety for patients and staff, and describes safety information for various implants and devices that have recently undergone evaluation.

© RSNA, 2004

Index terms: Magnetic resonance (MR), biological effects, _**.1214² • Magnetic resonance (MR), safety, _**.1214 • Reviews

	INTRODUCTION

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
Magnetic resonance (MR) procedures have been utilized in the clinical setting for approximately 20 years. During this time the technology has evolved continuously, yielding MR systems with stronger static magnetic fields, faster and stronger gradient magnetic fields, and more powerful radiofrequency (RF) transmission coils. Short-term exposures to the electromagnetic fields used for MR procedures at the levels currently recommended by the U.S. Food and Drug Administration (FDA) and with adherence to proper safety guidelines have yielded relatively few serious injuries for the more than 150 million MR examinations performed to date, with the exception of several second- and third-degree burns that have occurred (1–4).

Many of the MR-related injuries and the few fatalities that have occurred were the apparent result of failure to follow safety guidelines or of the use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices (1–7). The preservation of a safe MR environment requires constant attention to the care of patients and individuals with metallic implants and devices, because the variety and complexity of these objects constantly changes (5–7). Therefore, to guard against accidents in the MR environment, it is necessary to revise information on biologic effects and safety according to changes that have occurred in MR technology and with regard to the use of current guidelines for biomedical implants and devices (1,2,5–17).

In consideration of the above, this review will (a) provide an overview of and update on MR biologic effects, (b) discuss new or controversial MR safety topics and issues, (c) present evidence-based guidelines to ensure safety for patients and staff members, and (d) describe MR safety information for various implants and devices that have recently undergone evaluation.

While a comprehensive discussion of MR biologic effects, safety, and patient care is not within the scope of this review, these topics have been addressed in recently published review articles (8–12,16,17) and textbooks (5–7). In addition, there are at least two Web sites devoted to MR safety that are updated with content on a frequent basis (18,19).

	BIOLOGIC EFFECTS OF STATIC MAGNETIC FIELDS

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
The introduction of MR technology as a clinical imaging modality in the early 1980s is responsible for a substantial increase in human exposure to strong static magnetic fields (1,9,16). Most MR systems in use today operate with magnetic fields ranging from 0.2 to 3.0 T. In the research setting, an exceptionally powerful MR system operating at 8.0 T is located at Ohio State University (Columbus). According to the latest guidelines from the FDA, clinical MR systems that use a static magnetic field up to 8.0 T are considered a "nonsignificant risk" for patients. The exposure of research subjects to fields stronger than 8.0 T requires approval of the research protocol by an institutional review board and the informed consent of the subjects.

Schenck (1,9) conducted comprehensive reviews of biologic effects associated with exposure to static magnetic fields. With regard to short-term exposures (eg, limited exposures or those associated with the clinical use of MR systems), the available information for effects of static magnetic fields on biologic tissues is extensive (1,9,20–38). Investigations include studies on alterations in cell growth and morphology, cell reproduction and teratogenicity, DNA structure and gene expression, pre- and postnatal reproduction and development, blood-brain barrier permeability, nerve activity, cognitive function and behavior, cardiovascular dynamics, hematologic indexes, temperature regulation, circadian rhythms, immune responsiveness, and other biologic processes (20–38). In the majority of these studies, the authors concluded that exposures to static magnetic fields produce no substantial harmful biologic effects. Although there have been some reports of potentially injurious effects of static magnetic fields on isolated cells or organisms, none of these effects have been verified or firmly established as a scientific fact (1,9). The relatively few documented injuries that have occurred in association with MR system magnets were attributed to the inadvertent presence or introduction of ferromagnetic objects (eg, oxygen tanks, aneurysm clips) into the MR environment (1,5–7,9).

With regard to the effects of long-term exposure to static magnetic fields, there are interactions between tissues and static magnetic fields that could theoretically lead to pathologic changes in human subjects (1,9,16). However, quantitative analysis of these mechanisms indicates that they are below the threshold of importance with respect to long-term adverse biologic effects (1,9,16).

At present, the pertinent literature does not contain carefully controlled studies that demonstrate the absolute safety of chronic exposure to powerful magnetic fields. With the increased clinical use of interventional MR procedures, there is a critical need for such investigations. However, it may be virtually impossible to demonstrate "absolute safety," given the various difficulties in conducting such a study. In addition, although there is no evidence for a cumulative effect of magnetic field exposures on health, further studies of the exposed populations (eg, MR health care workers, patients who undergo repeated studies) will be helpful in establishing rational guidelines for occupational and patient exposures to static magnetic fields (1,9,16).

	BIOLOGIC EFFECTS OF GRADIENT MAGNETIC FIELDS

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
During MR procedures, gradient magnetic fields may stimulate nerves or muscles by inducing electric fields in patients. This topic has been thoroughly reviewed by Schaefer et al (8), Nyenhuis et al (39), and Bourland et al (40). The potential for interactions between gradient magnetic fields and biologic tissue is dependent on a variety of factors, including the fundamental field frequency, the maximum flux density, the average flux density, the presence of harmonic frequencies, the waveform characteristics of the signal, the polarity of the signal, the distribution of current in the body, the electric properties, and the sensitivity of the particular cell membrane (8,39–48).

Several investigations have been conducted to characterize MR system–related gradient magnetic field–induced stimulation in human subjects (41–48). At sufficient exposure levels, peripheral nerve stimulation is perceptible as a "tingling" or "tapping" sensation. At gradient magnetic field exposure levels of 50%–100% above perception thresholds, patients may become uncomfortable or experience pain (8). At extremely high levels, cardiac stimulation is of concern. However, the induction of cardiac stimulation requires exceedingly strong and/or rapid gradient magnetic fields—more than an order of magnitude greater than those used in commercially available MR systems (8,39,40). Fortunately, current safety standards for gradient magnetic fields associated with present-day MR systems appear to provide adequate protection from potential hazards or injuries in patients (2,8,16,39).

Of interest, results of studies performed in human subjects indicate that anatomic sites of peripheral nerve stimulation vary depending on the activation of a specific gradient (ie, x, y, or z gradient) (8). Stimulation sites for x gradients included the bridge of the nose, the left side of the thorax, the iliac crest, the left thigh, the buttocks, and the lower back. Stimulation sites for y gradients included the scapula, the upper arms, the shoulder, the right side of the thorax, the iliac crest, the hip, the hands, and the upper back. Stimulation sites for z gradients included the scapula, the thorax, the xyphoid, the abdomen, the iliac crest, and the upper and lower back (8). Typically, peripheral nerve stimulation sites were at bony prominences. According to Schaefer et al (8), because bone is less conductive than the surrounding tissue it may increase current densities in narrow regions of tissue between bone and skin, resulting in lower nerve stimulation thresholds than expected.

	ACOUSTIC NOISE

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
Various forms of acoustic noise are produced in association with the operation of an MR system (49). The primary source of acoustic noise, however, is the gradient magnetic field activated during the MR procedure. This noise occurs during rapid alterations of current within the gradient coils that, in the presence of the powerful static magnetic field of the MR system, produce substantial (lorentzian) forces. Acoustic noise, manifested as loud tapping, knocking, or chirping sounds, is generated when these forces cause motion or vibration of the gradient coils as they impact their mountings.

Problems associated with acoustic noise for patients and health care workers include simple annoyance, difficulties in verbal communication, heightened anxiety, temporary hearing loss, and, potentially, permanent hearing impairment (49–61). Acoustic noise may pose a particular hazard to specific patient groups who are at increased risk. Patients with psychiatric disorders, the elderly, and pediatric patients may be confused or experience heightened anxiety (49,51). Sedated patients may experience discomfort due to high noise levels. Certain drugs are known to increase hearing sensitivity (52). Neonates with immature anatomic development may have an increased reaction to acoustic noise, as has been reported by Philbin et al (53).

Characteristics of MR-related Acoustic Noise
Variations in MR-related acoustic noise occur with alterations in the gradient output (rise time or amplitude) associated with different MR parameters (49,54–64). Noise levels, pitch, and frequency characteristics are predominantly increased when section thickness, field of view, repetition time, and echo time are decreased. The physical features of the MR system, especially the presence or absence of special sound insulation, and the material and construction of gradient coils and support structures also affect the transmission of acoustic noise and its perception by the patient.

The patient’s presence and the patient’s size also affect the level of acoustic noise. An increase in acoustic noise has been reported with a patient or volunteer present in the bore of the MR system (63); this may be due to pressure doubling (ie, an increase in sound pressure) close to an object, as sound waves reflect and undergo in-phase enhancement. Noise characteristics also have a spatial dependence. For example, noise levels have been found to vary by as much as 10 dB as a function of patient position along the magnet bore (63).

MR-related acoustic noise levels have been measured during a variety of pulse sequences for MR systems with static magnetic field strengths ranging from 0.2 to 4.7 T (54–56,61–64). Recent studies performed with MR parameters that included "worst-case" pulse sequences showed that, not surprisingly, fast gradient-echo, fast spin-echo, and echo-planar pulse sequences produced the greatest acoustic noise levels (49,55,56).

MR-related Acoustic Noise and Permissible Limits
The FDA indicates that MR-related acoustic noise levels must be below the level of concern established by pertinent federal regulatory or other recognized standards-setting organizations (2). If the acoustic noise is not below this level, the sponsor (ie, the manufacturer of the MR system) must recommend steps to reduce or alleviate the noise perceived by the patient. A single upper limit of 140 dB is applied to peak acoustic noise (2). However, the instructions for use of MR systems must advise the MR system operator to provide hearing protection to patients for operation above an acoustic noise level of 99 dB (2).

In general, acoustic noise levels recorded by various researchers in association with conventional or routine MR procedures have been below the maximum limit permissible by the U.S. Occupational Safety and Health Administration (2). Notably, when one considers that the duration of exposure is one of the more important physical factors that determine the effect of noise on hearing, then acoustic noise levels associated with MR procedures do not tend to be problematic because of the relative short periods of exposure (65,66).

Prevention of Acoustic Noise Problems
Various techniques have been described to attenuate noise and, thus, prevent problems or hazards associated with exposure to MR-related acoustic noise (49,64). The simplest and least expensive means is to use disposable earplugs or commercially available noise-abatement headphones (49). Earplugs, when properly used, can decrease noise by 10–30 dB, which usually affords adequate protection for MR environments with relatively loud MR systems. Regardless of the technique used, facilities operating with MR systems that generate substantial acoustic noise should require all patients undergoing an examination to wear a protective hearing device. Exposure of staff members, health care workers, and other individuals (eg, relatives, visitors) to loud MR systems is also of concern (49,56). Therefore, these individuals should likewise be required to use an appropriate means of hearing protection if they remain in the room during the operation of these units (49).

	BIOLOGIC EFFECTS OF RF FIELDS

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
The majority of the RF power transmitted for MR imaging or spectroscopy (eg, carbon decoupling, fast spin-echo pulse sequences, magnetization transfer contrast pulse sequences) is transformed into heat within the patient’s tissues as a result of resistive losses (11,67). Not surprisingly, the primary biologic effects associated with exposure to RF radiation are related to the thermogenic qualities of this electromagnetic field (11,67–77).

Prior to 1985, there were no published reports concerning thermal or other physiologic responses of human subjects exposed to RF radiation during MR procedures. Since then, many investigations have been conducted to characterize the thermal effects of MR procedure–related heating (68–74,78). This topic has been reviewed by Schaefer (67,76) and Shellock (11).

MR Procedures and Specific Absorption Rate of RF Radiation
Thermoregulatory and other physiologic changes that a human subject exhibits in response to exposure to RF radiation are dependent on the amount of energy that is absorbed. The dosimetric term used to describe the absorption of RF radiation is the specific absorption rate (SAR) (11,67,76,79). The SAR is the mass normalized rate at which RF power is coupled to biologic tissue and is typically expressed in watts per kilogram. The relative amount of RF radiation that an individual encounters during an MR procedure is usually characterized with respect to the whole-body averaged and peak SAR levels (ie, the SAR averaged in 1 g of tissue).

Measurements or estimates of SAR are not trivial, particularly in human subjects. There are several methods of determining this parameter for the purpose of RF energy dosimetry in association with MR procedures (67,76,79,80). The SAR that is produced during an MR procedure is a complex function of numerous variables, including the frequency (ie, determined by the strength of the static magnetic field of the MR system), the repetition time, the type of RF coil used, the volume of tissue contained within the coil, the configuration of the anatomic region exposed, and the orientation of the body to the field vectors, as well as other factors (11,67,76,79,80).

Thermophysiologic Responses to MR Procedure–related Heating
Thermophysiologic responses to MR procedure–related heating depend on multiple physiologic, physical, and environmental factors (11,67,76,77). These include the duration of exposure, the rate at which energy is deposited, the status of the patient’s thermoregulatory system, the presence of an underlying health condition, and the ambient conditions within the MR system.

With regard to the thermoregulatory system, when subjected to a thermal challenge the human body loses heat by means of convection, conduction, radiation, and evaporation. Each of these mechanisms is responsible to a varying degree for heat dissipation as the body attempts to maintain thermal homeostasis (11,67,77,79). If the thermoregulatory effectors are not capable of totally dissipating the heat load, then there is an accumulation, or storage, of heat along with an elevation in local and/or overall tissue temperatures (11,76,77).

Various underlying health conditions may affect an individual’s ability to tolerate a thermal challenge, including cardiovascular disease, hypertension, diabetes, fever, old age, and obesity (81–85). In addition, medications such as diuretics, ß-blockers, calcium blockers, amphetamines, muscle relaxants, and sedatives can also greatly alter thermoregulatory responses to a heat load. In fact, certain medications have a synergistic effect with respect to tissue heating if the heating is specifically caused by exposure to RF radiation (86).

The environmental conditions that exist in and around the MR system will also affect the tissue temperature changes associated with RF-induced heating. During an MR procedure, the amount of tissue heating that occurs and the concomitant exposure to RF energy that is tolerable are dependent on environmental factors that include ambient temperature, relative humidity, and airflow.

MR Procedure–related Heating and Human Subjects
To our knowledge, the first study of human thermal response to RF radiation–induced heating during an MR procedure was conducted by Schaefer et al (87). Temperature changes and other physiologic parameters were assessed in volunteer subjects exposed to relatively high, whole-body, averaged SARs (approximately 4.0 W/kg). The data indicated that there were no excessive temperature elevations or other deleterious physiologic consequences related to these exposures to RF radiation (87).

Several studies were subsequently conducted with volunteer subjects and patients undergoing clinical MR procedures with the intent of obtaining information that would be applicable to patient populations typically encountered in the MR setting (68–75). These investigations demonstrated that changes in body temperature were relatively minor (ie, <0.6°C). While there was a tendency for statistically significant increases in skin temperatures to occur, these were of no serious physiologic consequence.

Of interest, various studies reported a poor correlation between body or skin temperature changes versus whole-body averaged SARs during clinical MR procedures (69,73). These findings are not surprising considering the range of thermophysiologic responses possible to a given SAR that are dependent on the individual’s thermoregulatory system and the presence of one or more underlying condition(s) that can alter or impair the ability to dissipate heat.

An extensive investigation was conducted in volunteer subjects exposed to a 1.5-T 64-MHz MR procedure with a whole-body averaged SAR of 6.0 W/kg (75), which, to our knowledge, is the highest level of RF energy to which human subjects have ever been exposed with an MR system. This excessive amount of RF radiation was achieved by using nonclinical MR imaging parameters (75). Tympanic membrane temperature, six different skin temperatures, heart rate, blood pressure, oxygen saturation, and skin blood flow were monitored (75). The findings indicated that an MR procedure performed at a whole-body averaged SAR of 6.0 W/kg can be physiologically tolerated by an individual with normal thermoregulatory function (75).

MR Procedure–related Heating and Very High Field Strength MR Systems
There are over 200 MR systems operating with a static magnetic field strength of 3 T, several operating at 4 T, a few operating at 7 T, one operating at 8 T (74), and at least one MR unit that operates at a field strength higher than 8 T is in the final stage of installation (likely completed by the time this article is published). For a given application, these very high field strength systems are capable of generating RF power depositions that greatly exceed those associated with a 1.5-T MR system. Of note, with the doubling of field strength (eg, 1.5 vs 3.0 T), the RF power deposition increases four times for a given MR imaging pulse sequence. Therefore, investigations are needed for characterization of thermal responses in human subjects to determine potential thermogenic hazards associated with the use of these powerful MR devices. To date, however, with the exception of work conducted at 8 T by Kangarlu et al (74), there has been virtually no investigation of MR procedure–related heating with regard to very high field strength MR systems.

	MR SAFETY AND PATIENT CARE

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
Screening Patients for MR Procedures and Individuals for the MR Environment
The establishment of thorough and effective screening procedures for patients and other individuals is one of the most critical components of a program to guard the safety of all those preparing to undergo MR procedures or to enter the MR environment (5,13,15–17,89). An important aspect of protecting individuals from MR system–related accidents and injuries involves an understanding of the risks associated with the various implants, devices, accessories, and other objects that may cause problems in this setting (5,13,15–17). This requires obtaining information and documentation about these objects in order to provide the safest MR setting possible. In addition, because MR-related incidents have been due to deficiencies in screening methods and/or a lack of proper control of access to the MR environment (especially with regard to preventing personal items and other potentially problematic objects from entering the MR room) (3,4), it is crucial to set up procedures and guidelines to prevent such incidents from occurring. Various guidelines and recommendations have been developed to facilitate the screening process (15,17,88,89).

Screening patients for MR.—Certain aspects of screening patients for MR procedures may take place during the scheduling process. This must be conducted by a health care worker who is specially trained in MR safety (17,88,89). That is, this individual should be (a) trained to understand the potential hazards and issues associated with the MR environment and MR procedures and (b) familiar with the information contained on screening forms for patients and individuals. During this time, it may be ascertained if the patient has any implant that may be contraindicated for the MR procedure (eg, ferromagnetic aneurysm clip, pacemaker) or if there is any condition that requires careful consideration (eg, patient is pregnant or has a disability). Preliminary screening helps to prevent scheduling of patients who may be inappropriate candidates for MR examinations.

At the facility, it is advisable for every patient to undergo comprehensive screening in preparation for the MR examination. Comprehensive patient screening involves the use of a printed form to document the screening procedure, a review of the information on the screening form, and an oral interview to verify the information and allow discussion of any question or concern that the patient may have (15,88,89). A health care worker trained in MR safety must conduct this aspect of patient screening. Various forms have been developed for screening patients in preparation for MR procedures (5,15,17–19,88,89). An example of a recently developed form for this use is shown in Figure 1 (18,19).

View larger version (61K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Example of an MR procedure screening form for patients. (Reprinted, with permission, from the Institute for Magnetic Resonance Safety, Education, and Research.)

In the event that the patient is comatose or unable to communicate, the form should be completed by the most qualified individual (eg, physician, family member) with knowledge of the patient’s medical history and present condition. If the screening information is inadequate, it is advisable to look for surgical scars on the patient and/or to obtain conventional radiographs of the skull and/or chest to search for implants that may be particularly hazardous in the MR environment (eg, aneurysm clip, cardiac pacemaker).

After completion of the screening form used for patients, a health care worker trained in MR safety must review the contents of the form. Next, an oral interview should be conducted by the MR safety–trained health care worker to verify the information on the form and to allow discussion of any question or concern that the patient may have before undergoing the MR procedure. This allows for clarification or confirmation of the answers to the questions posed to the patient so that there is no miscommunication regarding important MR safety issues. In addition, because the patient may not be fully aware of the medical terminology used for a particular implant or device, it is imperative that this particular information on the form be discussed during the oral interview.

It should be noted that having undergone a previous MR procedure without incident does not guarantee a safe subsequent MR examination. Various factors (eg, static magnetic field strength of the MR system, orientation of the patient, orientation of a metallic implant or object) can substantially change the scenario (17,88,89). Therefore, a comprehensive screening procedure must be conducted each time a patient prepares to undergo an MR procedure. This is not an inconsequential matter, because a surgical intervention or accident involving a metallic foreign body may have occurred that could affect the safety of the patient entering the MR environment.

Screening individuals for the MR environment.—Similar to the procedure conducted for screening patients, all other individuals (eg, MR technologists, patient’s family members, visitors, allied health professionals, maintenance workers, custodial workers, firefighters, security officers) should undergo screening by using appropriate guidelines before being allowed into the MR environment (17–19). This involves the use of a printed form to document the screening procedure, a review of the information on the form, and an oral interview to verify the information and allow discussion of any question or concern that the individual may have before entry to the MR environment is permitted.

In general, MR screening forms were developed with patients in mind and, therefore, contain many questions that are inappropriate or confusing to other individuals who may need to enter the MR environment. Therefore, a screening form was recently created for individuals who need to enter the MR environment and/or MR system room (Fig 2) (18,19). To prevent problems that may occur in individuals who respond to the MR facility during emergencies, a procedure should be in place to screen these individuals well in advance of their entry to the MR environment.

View larger version (81K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Example of a screening form for individuals who must enter an MR environment. (Reprinted, with permission, from the Institute for Magnetic Resonance Safety, Education, and Research.)

In the past, any individual or patient suspected of having an orbital foreign body typically underwent screening with conventional radiography of the orbits to determine whether a metallic object was present. Thus, screening radiographs of the orbits were obtained routinely not only in individuals who had a history of injury from a foreign body but also in those who simply had a history of exposure to metallic objects, such as welders, grinders, metal workers, sculptors, and others. Obviously, conventional radiographs of the orbits may have been obtained unnecessarily in many individuals because of this policy.

Seidenwurm et al (93) presented research and a new set of guidelines for radiographic screening of individuals suspected of having metallic foreign bodies. Their investigation addressed the cost-effectiveness of the use of a clinical versus a radiographic technique to screen individuals for orbital foreign bodies before an MR procedure (93). The costs of screening were determined on the basis of published data, disability rating guides, and results of a practice survey. A sensitivity analysis was performed for each variable. For their analysis, the benefit of screening was prevention of immediate, permanent, nonameliorable, or unilateral blindness. Seidenwurm et al (93) implemented the following policy: "If a patient reports injury from an ocular foreign body that was subsequently removed by a doctor or that resulted in negative findings on any examination, we perform MR imaging... Those persons with a history of injury and no subsequent negative eye examination are screened radiographically." The findings of their study indicated that the use of clinical screening before radiography increased the cost-effectiveness of foreign body screening by an order of magnitude (ie, assuming base-case ocular foreign body removal rates). Of note is that Seidenwurm et al have performed approximately 100 000 MR procedures using this protocol without incident.

Thus, an occupational history of exposure to metallic fragments, by itself, is not sufficient to mandate radiographic orbital screening (92,93). Therefore, current practice guidelines for foreign body screening should be altered in consideration of this information and because radiographic screening before MR procedures on the basis of occupational exposure alone is not clinically necessary, nor is it cost-effective (92,93).

Updated guidelines for orbital foreign body screening.—The procedure to follow with regard to a patient suspected of having an orbital foreign body involves a clinical screening protocol that entails asking the patient if he or she has had an ocular injury (93). If an ocular injury from a metallic object was sustained, the patient is asked if a medical examination was conducted at the time of the injury and if he or she was informed by the doctor that the object was completely removed (93). If (a) there was no injury, (b) the individual was informed that the ophthalmologic examination results were normal, or (c) the foreign body was removed at the time of the injury, the patient then proceeds to MR imaging. On the basis of the results of the clinical screening protocol, the patient should be screened with conventional radiography if an ocular injury related to a metallic object was sustained and the patient was not informed that the postinjury eye examination result was normal (93). In this case, the MR examination is postponed and the patient is scheduled for screening radiography.

Excessive Heating and Burns Associated with MR Procedures
The use of RF coils, physiologic monitors, electronically activated devices, and external accessories or objects made from conductive materials has caused excessive heating that resulted in burn injuries to patients undergoing MR procedures (3–6,94–101). Heating of implants and similar devices may also occur in association with MR procedures, but this tends to be problematic primarily for objects made from conductive materials that have an elongated shape, such as electrodes, leads, guidewires, and certain types of catheters (eg, catheters with thermistors or other conducting components) (102–108).

More than 30 incidents of excessive heating have been reported in patients undergoing MR procedures in the United States that were unrelated to equipment problems or the presence of conductive external or internal implants or materials (3,4,109). These incidents include first-, second-, and third-degree burns experienced by patients. In many of these cases, the reports pertaining to these incidents indicated that the limbs or other body parts of the patients were in direct contact with body RF coils or other RF transmit coils of the MR systems or that there were skin-to-skin contact points suspected to be responsible for these injuries (3,4,109).

In consideration of these injuries, guidelines have been developed to prevent excessive heating and burns related to MR procedures (Appendix A) (19). The adoption of these guidelines will help to ensure that patient safety is maintained, especially as more conductive materials and electronically activated devices are used in association with MR procedures.

Tattoos and Permanent Cosmetics
Traditional (ie, decorative) and cosmetic tattoo procedures have been performed for thousands of years. Cosmetic tattooing or "permanent cosmetics" are used to reshape, recolor, recreate, or modify eye shadow, eyeliner, eyebrows, lips, beauty marks, and cheek blush. In addition, permanent cosmetics are used to hide scars and for other aesthetic applications (110,111).

There is considerable controversy regarding the MR safety aspects of tattoos and permanent cosmetics (112–121). Problems related to MR procedures and tattoos and permanent cosmetics are associated with the use of iron oxide or other metal-based pigments. Because a small number of patients with permanent cosmetics who underwent MR procedures (fewer than 10 documented cases) experienced transient skin irritation, cutaneous swelling, or heating sensations (3,4), many radiologists have refused to perform MR procedures in individuals with permanent cosmetics (Shellock FG, unpublished observations, 2002). Obviously, this undue concern for possible adverse events prevents patients with permanent cosmetics from having access to a potentially important diagnostic imaging modality (115).

In a study conducted by Tope and Shellock (115), the frequency and severity of adverse events associated with MR imaging were determined in a population of subjects with permanent cosmetics. A questionnaire was distributed to clients of cosmetic tattoo technicians. One hundred thirty-five (13.1%) study subjects underwent MR imaging after having permanent cosmetics applied. Of these, only two (1.5%) experienced problems associated with MR imaging: One subject reported a sensation of "slight tingling" and the other subject reported a sensation of "burning," both transient in nature (115). On the basis of these findings, as well as of other available information (3,4), it is apparent that MR procedures may be performed in patients with permanent cosmetics without any serious soft-tissue reactions or adverse events. Therefore, the presence of permanent cosmetics should not prevent patients from undergoing MR procedures.

Of interest, decorative tattoos tend to cause worse problems (including first- and second-degree burns) in patients undergoing MR procedures than do cosmetic tattoos. For example, Kreidstein et al (119) reported that a patient experienced a sudden burning pain at the site of a decorative tattoo during MR imaging of the lumbar spine at 1.5 T. Surprisingly, in order to permit completion of the MR examination, an excision of the tattooed skin was performed (119). The authors of this report stated, "Theoretically, the application of a pressure dressing of the tattoo may prevent any tissue distortion due to ferromagnetic pull" (119). However, this simple and relatively benign procedure was not attempted in this patient. The authors also indicated that "in some cases, removal of the tattoo may be the most practical means of allowing MRI" (119). Kanal and Shellock (120) commented on this report in a letter to the editor, suggesting that the response to this situation was "rather aggressive." Clearly, the trauma, expense, and morbidity associated with excision of a tattoo far exceed those that may be associated with MR-related tattoo interactions.

Because of the relatively remote possibility of an incident occurring in a patient with permanent cosmetics or a tattoo and due to the relatively minor short-term complication or adverse event that may develop (ie, transient cutaneous redness and swelling) (3,4,115), the patient should be permitted to undergo an MR procedure. Any problem regarding performance of an MR procedure in a patient with permanent cosmetics or a tattoo should not prevent the examination, because the diagnostic information that is provided by this modality may be crucial for the care of the patient. For patients in whom MR-related heating may occur, it is advisable to apply an ice pack or cold compress to the site of the tattoo or permanent cosmetics as a precautionary measure, since this a relatively innocuous procedure that adds little risk, time delay, or expense to the MR examination and could reduce the possibility of thermal injury (although, to date, there are no empiric data to support this).

Information on this topic has also been provided to patients by the FDA Center for Food Safety and Applied Nutrition, Office of Cosmetics and Colors fact sheet (116), as follows: "The risks of avoiding an MRI when your doctor has recommended one are likely to be much greater than the risks of complications from an interaction between the MRI and tattoo or permanent makeup. Instead of avoiding an MRI, individuals who have tattoos or permanent makeup should inform the radiologist or technician of this fact in order to take appropriate precautions, avoid complications, and assure the best results."

Pregnant Patients and MR Procedures
MR procedures have been used to evaluate obstetric, placental, and fetal abnormalities in pregnant patients for more than 18 years (122–125). Initially, there were substantial technical problems with the use of MR imaging, due primarily to the presence of image degradation caused by fetal motion. However, several technologic improvements, including the development of high-performance gradient systems and rapid pulse sequences, provided advances that were especially useful for imaging pregnant patients. Thus, high-quality MR studies for obstetric and fetal applications may now be accomplished routinely in the clinical setting (125).

Diagnostic imaging is often required during pregnancy (122). Thus, it is not uncommon to consider the use of an MR procedure in a pregnant patient. Safety issues exist that are related to possible adverse biologic effects associated with exposure to the static magnetic, gradient magnetic, and RF electromagnetic fields used for MR procedures (5,13,122). As such, many laboratory and clinical research investigations have been conducted to determine the effects of the use of unenhanced MR procedures during pregnancy (29,34–36,126,127). The overall findings from these studies indicate that there is no substantial evidence of injury or harm to the fetus; however, additional research on this topic is warranted.

Guidelines for MR in pregnant patients.—In 1991, the Safety Committee of the Society for Magnetic Resonance Imaging issued a document entitled "Policies, Guidelines, and Recommendations for MR Imaging Safety and Patient Management" (13), which stated that "MR imaging may be used in pregnant women if other nonionizing forms of diagnostic imaging are inadequate or if the examination provides important information that would otherwise require exposure to ionizing radiation (eg, fluoroscopy, computed tomography). Pregnant patients should be informed that, to date, there has been no indication that the use of clinical MR imaging during pregnancy has produced deleterious effects." These guidelines have been subsequently adopted by the American College of Radiology and are considered to be the standard of care with respect to the use of MR procedures in pregnant patients.

Accordingly, in cases where the referring physician and attending radiologist can defend that the findings of the MR procedure have the potential to affect the care of the mother or fetus (eg, to address important clinical problems or help identify potential complications, anomalies, or complex fetal disorders), the MR procedure may be performed with oral and written informed consent, regardless of the trimester (13,122).

	MR PROCEDURES AND IMPLANTS AND DEVICES

TOP ABSTRACT INTRODUCTION BIOLOGIC EFFECTS OF STATIC... BIOLOGIC EFFECTS OF GRADIENT... ACOUSTIC NOISE BIOLOGIC EFFECTS OF RF... MR SAFETY AND PATIENT... MR PROCEDURES AND IMPLANTS... CONCLUSIONS ESSENTIALS APPENDIX A APPENDIX B REFERENCES

 
The MR environment may be unsafe for individuals with certain biomedical implants or devices, owing primarily to movement or dislodgment of objects made from ferromagnetic materials (3–7,103,128–143). As previously stated, while excessive heating and the induction of electric currents may also present risks to patients with implants or devices, these problems are typically associated with implants that have elongated configurations and/or are electronically activated (eg, neurostimulation systems, cardiac pacemakers) (94,101–103).

To date, more than 1200 objects have been tested for MR safety, with over 200 evaluated at 3 T or higher (5–7,18,128–142). This information is available to MR health care professionals and others as published reports, compiled lists, and, in its entirety, online at www.MRIsafety.com. The topic of MR safety for implants and devices was recently reviewed by Shellock (5,103). As such, the intent for the material presented in the current review is to provide information for implants and devices for which there may be controversy or confusion, with an update on objects tested at 3 T or higher.

Evaluation of Implants and Devices for Safety in the MR Environment
The evaluation of an implant or device with regard to the MR environment is not a trivial matter. The proper assessment of an object typically entails characterization of magnetic field interactions (translational attraction and torque), MR-related heating, induced electric currents, and artifacts. A thorough evaluation of the effects of the MR environment on the functional and operational aspects of certain implants and devices may also be necessary. It is important to note that an object demonstrated to be safe according to one set of MR conditions may be unsafe under more "extreme" conditions (eg, stronger static magnetic field, greater level of RF power deposition, faster gradient field, different RF transmission coil). Accordingly, the specific test conditions for a given implant or device must be known before one makes a decision regarding whether a particular object is safe for an individual in the MR environment.

Magnetic field–related issues.—Magnetic field–related translational attraction and torque are known to present hazards to individuals with certain implants or devices (5–7). Currently, MR systems used in clinical and research settings operate with a static magnetic field that ranges from 0.2 to 8.0 T. Most previous ex vivo tests performed to assess objects for MR safety used units with a static magnetic field of 1.5 T or lower (5,103). Accordingly, this could present problems, insofar as it is possible that an object that displayed "weakly" ferromagnetic qualities in association with a 1.5-T MR system may exhibit substantial