Magnetic Resonance Safety

Magnetic resonance imaging (MRI) has proved to be a powerful diagnostic imaging tool in children and adults. MRI uses low-energy nonionizing radio waves, and as such it is particularly well suited for pediatric and longitudinal imaging studies. MRI exploits a wide variety of intrinsic tissue-specific properties to generate a wide spectrum of tissue contrast, thus providing detailed information about anatomy, physiology, and function. For clinical MRI examinations, the acquisition techniques used and the specific acquisition parameters selected are manipulated to generate the type of image contrast most relevant to the underlying medical question. Unlike MRI, ultrasound is limited by the restriction of air and bony anatomy, and radiography and computed tomography uses ionizing radiation. Compared with other diagnostic imaging modalities, MRI has proved to be superior at imaging soft tissues. In addition, MRI enables images to be obtained in any plane and can provide true volumetric data acquisition.

MRI, when performed following the right conditions and guidelines, presents no apparent safety risks, and millions of MRI examinations are performed each year without incident. However, when not appropriately managed, safety concerns exist from the following: each of the three electromagnetic fields ( Table 2.1 ), external contrast agent (typically gadolinium-based), associated acoustic noise, and other issues (e.g., sedation). Notably, most MR-related injuries and the few fatalities that have occurred primarily were a result of the failure to adhere to MR safety guidelines for the MRI environment, or they occurred because inaccurate or outdated MR safety-related information was used for biomedical implants and devices.

TABLE 2.1

Principal Mechanisms of Interaction of the Three Main MRI Energy Fields With Tissues and Some Related Effects

From Bushong SC. Biologic effects of magnetic resonance imaging. In: MR safety, magnetic resonance imaging: physical and biological principles . 3rd ed. St Louis: Mosby; 2003.

Magnetic Resonance–Related Electromagnetic Field	Mechanism of Interaction	Potential Effects
Static magnetic field (T)	Polarization/magnetization	Elevated electrocardiographic T wave and/or transient sensory effects (e.g., vertigo, nausea, phosphenes, and metallic taste)
Transient gradient magnetic field (dB/dt)	Induced currents Acoustic noise	Peripheral nerve stimulation Physiologic stress, anxiety, temporary hearing loss
Radio frequency field (specific absorption rate)	Thermal heating	Local burns

To prevent similar adverse MR safety-related incidents from occurring, it is imperative that the potential safety risks intrinsic to the MR environment be understood and respected by users of this powerful imaging technology.

Safety Considerations: Electromagnetic Fields

Main Static Magnetic Field

Biologic Effects of Static Magnetic Fields

The majority of MRI systems in clinical use today operate at main or static magnetic field strengths of 0.2 to 3.0 tesla (T), with 1.5 T and 3.0 T units being used predominantly. The static field of a 1.5 T scanner is approximately 30,000 times stronger than Earth's magnetic field (roughly 0.00005 T). The most recent United States Food and Drug Administration (FDA) guidelines state that, for adults, children, and infants older than 1 month, diagnostic MRI systems that operate at or below 8 T are considered to be a nonsignificant risk. For neonates, the limit is 4 T. In practice, 3 T is the highest field strength in common clinical use. Initially, some concern was expressed that the strong magnetic fields might have irreversible detrimental biologic/health effects in humans, including alterations in cell growth and morphology, cell reproduction and teratogenicity, DNA structure and gene expression, prenatal 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. However, a comprehensive review of the literature indicates that short-term exposures (of a duration comparable to an MR examination) to high-static magnetic fields produce no appreciable long-term detrimental biologic effects.

Although no lasting adverse effects of short-term exposures to high magnetic field strengths have been reported, several relatively transient reversible biologic effects are known to occur, including electrocardiographic changes and benign sensory effects. These effects have been reported primarily at field strengths greater than 2 T, and the sensory effects appear to occur most often when a person's head is moved rapidly within the static magnetic field. The elevation in electrocardiographic T waves is believed to be due to magnetohydrodynamic phenomena. The voltage associated with the magnetohydrodynamic effect is not considered hazardous at the magnetic field strengths approved for clinical use. However, at higher field strengths, the possibility exists that the induced potential might exceed 40 mV (voltage produced across large arteries are on the order of 5 mV/T), which is the threshold for depolarization of cardiac muscle.

Several short-term, relatively benign sensory effects have been reported at higher field strengths, including vertigo, nausea, headaches, flashing lights (also known as magnetophosphenes), a metallic taste, and/or sensation in dental fillings.

Interaction of the Main Magnetic Field With Ferromagnetic Objects

Torque and Attractive Forces.

The field associated with an MR scanner can be separated into two spatial regions defined by the types of interaction with ferromagnetic objects that predominate in each region. The first region surrounds the magnet isocenter and is contained within the bore of the MRI scanner. The magnetic field in this region essentially is temporally constant and homogeneous in strength. A magnetic object introduced into this region of the static field is subjected to a torque that acts to align it with the magnetic field, just as magnetic material aligns itself with the poles of a permanent bar magnet or a compass needle aligns itself with the Earth's magnetic field. Hence any nonspherical metallic object that enters the spatially homogenous static region of the field will be subjected to a rotational force or torque such that its long axis will be aligned with that of the main magnetic field. If the metallic implants are not sufficiently anchored by the surrounding tissue (e.g., bone in the case of an orthopedic implant), the resultant rotational motion will cause trauma to the surrounding tissue. The magnitude of these effects depends on the geometry and mass of the object, as well as the characteristics of the MR system's magnetic field.

In the second region (the fringe field), the strong magnetic field strength drops off rapidly as the distance from the magnet increases, producing a large spatial gradient that typically is greatest in regions immediately adjacent to the magnet. Modern day scanners have actively shielded main magnetic fields, which significantly increases the spatial gradient. Metallic objects introduced into this magnetic field gradient experience a translational (attractive) and rotational force. The translation will be in the direction of the higher field strength. The strength of the attractive/translational force will depend on the size of the object and its location within the gradient field and will increase rapidly as the object approaches the magnet. As such, the metallic object will be accelerated along the direction of the spatial gradients in the static field and quickly can become a dangerous projectile. Depending on the size and composition of the metallic object, these attractive/translational effects may begin as soon as the object is introduced into the scan room. Once the object enters the uniform field within the bore of the magnet, acceleration will cease and the object will come to a stop.

The potential hazards associated with the spatial gradients are greatest for high magnetic field strengths and large fringe fields. In general, the forces increase approximately as the square of the field strength but will vary depending on the composition of the object. For example, at 3 T compared with 1.5 T, the force on a paramagnetic material (e.g., a stainless steel scalpel) is 5 times greater, whereas the force on a ferromagnetic object (e.g., a steel wrench) is 2.5 times greater. Finally, even for a given field strength, the magnitude and footprint of the spatial gradients can vary between magnet configurations. For example, it has been reported that short-bore MR systems have significantly higher spatial gradients than long-bore scanners, particularly those operating at 3 T. This finding highlights the multiple nuances and factors that must be considered when evaluating and establishing MR safety guidelines and operating procedures for any object before it may be introduced into a specific MRI environment. It is a common misconception that the magnet is only “on” when the images are being acquired. However, the magnet is always on, and signage to indicate the “Magnet is ON” should be displayed conspicuously on the door to the MR scan room.

Missile/Projectile Effect.

The projectile/missile effect, wherein a ferromagnetic object is accelerated toward the isocenter of the magnet, is the most widely recognized and publicized safety hazard associated with MRI. Objects constructed partially or entirely of ferromagnetic materials (e.g., iron, nickel, cobalt, and the rare earth metals chromium, gadolinium, and dysprosium) are strongly attracted to the magnet bore. Steel objects also are highly ferromagnetic, as are some medical grades of stainless steel. Notably, working a metal by machining, molding, and/or bending it, for example, can alter its magnetic properties and, as a result, some forms of nonmagnetic stainless steel can be made magnetic. For this reason, the MR safety of a given metallic object must be evaluated in its final form. If any doubt remains regarding the presence of ferromagnetic material, the object should be checked with a permanent magnet and/or a ferromagnetic wand as a final precautionary measure before it enters the scan room.

Any individual in the path between the object and the center of the magnet (e.g., the patient lying in the magnet bore) can be seriously injured or killed. Numerous instances of MR-related accidents involving objects such as scalpels, scissors, oxygen tanks, intravenous line poles, wheelchairs, transport carts, floor buffers, mop buckets, vacuum cleaners, other medical devices, and even firearms have been documented. Any injuries related to ferromagnetic projectiles must be documented; the necessary forms can be accessed on the FDA website. More comprehensive information regarding these online resources is provided in a later section of this chapter.

To prevent potentially devastating MR-related accidents from occurring, it is essential that access to areas that exceed the 5 Gauss line (see next section) must be rigorously restricted and vigilantly supervised at all times. All patients, accompanying family members, and hospital personnel with a demonstrated need to enter the MR environment must be appropriately educated to the potential hazards and rigorously screened for contraindications.

5 Gauss Line.

Failure to adhere to safe practices within the MR environment has led to most of the MR-related accidents and fatalities. These incidents have involved the inappropriate introduction of metallic objects and/or implanted medical devices into the scan room. Five gauss (G) (1G = 0.0001 T) and below are considered “safe” levels of static magnetic field exposure for the general public. The 5 G line identifies the perimeter around an MR scanner within which the static magnetic fields are higher than 5 G; it is the most commonly recognized MR safety policy. Safety considerations require that this distance from the magnet be determined and that areas with field strength greater than 5 G be identified with appropriate and conspicuous signage. Importantly, the 5 G line extends in three dimensions; thus when the MR area is sited, the extent of the fringe field to the floors above and below the magnet must be considered. Because of the potential hazards, access to this area must be rigorously controlled and supervised.

Time-Varying Gradient Magnetic Fields

During imaging acquisition, gradient magnetic fields are transiently imposed along the main magnetic field to spatially localize and encode the spins. These gradient magnetic fields (current clinical gradient strengths vary between 30–90 mT/m) are much weaker than the static magnetic field and are generated by gradient coils located inside the magnet bore. Each orthogonal axis (i.e., x, y , and z ) has a pair of gradient coils. For a given direction, a linear gradient magnetic field is generated within the main magnetic field by applying electrical current in opposite directions to the coil pair over a short time interval. When this gradient magnetic field is applied, the magnetic field intensity changes rapidly, giving rise to a time-varying magnetic field. The rate of change in a magnetic field (dB) occurs over time (dt) and usually is measured and reported in units of dB/dt expressed in milliteslas per meter per millisecond (mT/m/msec). During the rise time of the magnetic field, an electrical current can be induced in any electrical conductor (e.g., implanted medical device, wire, human body). Notably, the magnitude of the time-varying magnetic field along each of the three spatial directions (i.e., x, y, and z ) is zero at the center of the magnet and increases linearly with increasing distance from the isocenter.

Because the human body is a conductor, the rapidly switching magnetic fields can induce electrical fields and current in a patient (as described by Faraday's law of induction) that may lead to the stimulation of muscle and nerve tissues. The mean threshold levels (measured in tesla per second) for various stimulations are 3600 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. However, the exact values differ significantly among individuals. Experience has shown that sufficient dB/dt levels can produce brief muscle twitches and peripheral nerve stimulation (PNS) perceptible as a “tingling” or “tapping” sensation. This sensation can become uncomfortable and/or painful as the gradient magnetic field increases to 50% to 100% above perception thresholds. For these reasons, threshold sensations, when reported, should not be ignored, because they may readily escalate to uncomfortable levels. Although cardiac stimulation is a concern, studies conducted in dogs have shown that the cardiac stimulation threshold for the most sensitive 1% of the population is 20 times, and the mean defibrillation threshold is 500 times, the energy required for PNS. The exceedingly strong and/or rapidly switching gradient magnetic fields necessary to achieve cardiac stimulation threshold levels are more than an order of magnitude greater than those used in commercially available MR systems.

In practice, the potential for PNS is greatest for fast imaging (e.g., echo planar imaging [EPI]) acquisitions, particularly when oblique imaging planes are obtained where the combined contributions of gradients from more than one axis result in a higher effective slew and/or when the readout (i.e., frequency encode) lies along the cranio-caudal direction.

Normal imaging sequences (e.g., conventional spin echo and gradient echo) induce currents of a few tens of milliamperes per square meter, a level far below that present in the normal brain and heart tissue. However, as noted previously, PNS is quite possible for the rapid imaging techniques such as EPI and for high-performance gradients. These effects can be controlled by limiting the maximum rate of change in the magnetic field gradients. The current FDA standard is based on the threshold for sensation, rather than a specific numerical value of dB/dt. Specifically, “current FDA guidance limits the time rate of change of magnetic field (dB/dt) to levels which do not result in painful peripheral nerve stimulation.” This policy reflects in part the complexity of modeling and calculating the current distribution in the body associated with the pulsed gradient fields. Correspondingly, dB/dt levels below that resulting in painful stimulation are considered a nonsignificant risk by the FDA. It is important to note that dB/dt is a function of the gradient strength and rise time and not of the static field strength. Thus dB/dt is of equal concern for both lower and higher field scanners.

Clinically, two operating modes for the radio frequency (RF) and gradient field levels used for imaging are permissible, normal and first level controlled ( Table 2.2 ). In normal mode, the system output levels are below those that will cause physiologic stress and therefore are appropriate for all subjects. Alternatively, in first level controlled mode, one of the MR system outputs (e.g., dB/dt) may reach a value that may cause physiologic stress and thus may not be appropriate for the most medically compromised patients. Operation in this mode requires authorization by appropriate clinical staff and vigilant medical supervision during the examination. Institutional Review Board approval is required for operation above these levels (second level controlled). In practice, the commercial MR systems monitor dB/dt throughout the examination and generate a warning for the specific protocols for which PNS is likely. For normal operating mode, the dB/dt threshold level is set at 80% of the mean threshold value for PNS stimulation, and it is set at 100% for the first level. In addition, the MR operator should remain in constant verbal contact with the patient and instruct him or her to report any tingling, muscle twitching, or painful sensations that occur during scanning. This contact is not possible for infants and for sedated, noncommunicative, or otherwise compromised patients. Operationally, dB/dt can be reduced by increasing the field of view and/or slice thickness, reducing the matrix size, and decreasing receiver bandwidth.

TABLE 2.2

International Electrotechnical Commission/Food and Drug Administration Operating Modes for Magnetic Resonance Imaging Diagnostic Equipment

From Center for Devices and Radiological Health, Food and Drug Administration. FDA guidelines for magnetic resonance equipment safety : Available at http://www.aapm.org/meetings/02AM/pdf/8356-48054.pdf .

Operating Mode	Conditions/Qualifications
Normal mode	Will not cause stress; suitable for all patients
First level controlled mode	May cause stress; requires medical supervision and positive action by operator to enter
Second level controlled mode	Institutional Review Board approval required

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