Letter from the Editor

Which Devices are Safe for Magnetic Resonance Imaging?

Morton Kern, MD, Clinical Editor, Clinical Professor of Medicine, Associate Chief Cardiology, University of California Irvine, Orange, California
Morton Kern, MD, Clinical Editor, Clinical Professor of Medicine, Associate Chief Cardiology, University of California Irvine, Orange, California
We well know that many metallic devices implanted in the heart may create problems in the very intense magnetic field of MR imaging. From several excellent reviews,1,2 we can better appreciate the interaction of magnetic resonance imaging and the metallic devices we implant in patients. It is important to recognize that there are three mechanisms of strong magnetic fields which may produce risk to patients. These are: 1. The attraction effect of a static magnetic field. 2. The electric current generation effect of a gradient magnetic field. 3. The heating effect of focused radiofrequency energy emitted by the MRI scanner. The static magnetic field of most MRI scanners (independent of MRI activation) ranges from 1.5 to 3.0 Tesla. The magnetic strength of these devices (which are always on) is 25-75,000 times the strength of the magnetic field of the earth. The risk from a static magnetic field is the attraction of a ferromagnetic object into the scanner with rapid acceleration, making the object a projectile. The attractive effect may also cause an implanted device to be moved, dislodged, or travel toward the magnet. Similarly, any magnetic component within an implanted device may move toward the magnet and thus change the activity of that device (e.g., pacemakers). The responses of metals vary with higher magnetic field strengths. Some devices which will not be affected by 1.5 Tesla may be greatly affected by a 3.0 Tesla. A gradient magnetic field is used to produce an image from the MR scanner by changing the magnetism (i.e., the intensity of the Tesla) quickly over each second of the measurement period. Recall that electricity can be produced by spinning a magnet inside tightly coiled wires. Thus, the rapidly changing magnetic gradient fields may induce an electrical field within metallic conductive wires and leads, which may then cause arrhythmias. However, this is a relatively uncommon event given the strengths of most MR magnets. Gradient fields may produce microvoltages that can produce ECG aberrations and alter the appearance of the electrocardiogram. Occasionally, inappropriate inhibition of pacemaker function or creation of an arrhythmic artifact on loop recorders may occur in large gradient fields. The third effect of MRI is the generation of radiofrequency energy (RF). During MR imaging, RF energy is pulsed into the body to alter cellular magnetic fields and generate the MR image. The RF energy is pulsed in, and the resultant energy released by the realigning fields of the cells is measured in time and space, hence the MRI image. Some of the RF energy is absorbed and can cause heating of a metallic implant. Certain metallic devices such as pacemaker leads can act as antenna and concentrate radiofrequency energy at one point. Excessive local heating at the tip of these implants has been reported. Fractured pacemaker leads may pose a particular high risk of thermal injury. The concentration of radiofrequency energy is device-dependent and varies with field strength. Is Your Device MR Safe? The American Society for Testing of Materials (ASTM) International has a new set of terms to describe implants subject to MRI. These are: 1) MR safe, 2) MR conditional, and 3) MR unsafe. These names are now applied (but not mandated) to many of implants and devices overseen by the Food and Drug Administration. MR safe means an item poses no known hazards in all MR environments. These include nonconducting, nonmetallic, nonmagnetic items such as plastic forceps, dish or cup. MR conditional means an item has been demonstrated to pose no hazard in a specific MR environment under specified conditions. An MR environment is then defined for the implant. The conditions would include static magnetic field strength, spatial magnetic gradient, or radiofrequency energy and specific radiofrequency absorption rates. Additional conditions may be described for an implant to carry the label MR conditional. An MR unsafe item is that which poses a hazard in all MR environments, including any magnetic items such as a pair of ferromagnetic scissors, clips, or other such tools. Let’s look at some examples. A non-ferromagnetic passive implant carries no concern for MR-related risk of heating, injury or movement. When patients have a weakly ferromagnetic implant, MR examinations generally produce only a weak attractive force to move or dislodge coils or stents which are firmly implanted in tissues, and are unaffected by the attractive magnetic forces of an MR examination. Tissue healing after device implantation provides additional anchoring, and thus it is safest to wait at least six weeks before MR imaging of many of these devices if the true MR implant status is unknown. Table 1 shows common implanted devices and their MR labeling.
References
1. Shellock FG. Reference Manual for Magnetic Resonance Safety, Implants, and Devices: 2006 Edition. Los Angeles: Biomedical Research Publishing Group; 2006.

2. Kanal E, Borgstede JP, Barkovich AJ, et al. American College of Radiology White Paper on MR, Safety: 2004 update and revisions. AJR Am J Roentgenol 2004 May; 182(5):1111-1114.