Magnetic Resonance Imaging
Gary Zeman, ScD, CHP
Magnetic resonance imaging (MRI) involves a sophisticated use of electromagnetic fields to produce images of internal anatomy with exceptional spatial and contrast resolution. Nonionizing radiation safety issues associated with MRI arise from the three electromagnetic fields used to produce the images: a strong, static magnetic (B) field to align the nuclear magnetic moments of atomic nuclei in the body; a time-varying magnetic (dB/dt) field to scan the point of interest within the body; and a radiofrequency (RF) field to measure the nuclear magnetic resonance signal from the point of interest.
The strength of B fields is measured in units of tesla (T), where 1 T is equal to 10,000 gauss. (The earth's geomagnetic field strength is about 0.5 gauss or 5×10–5 T.) B fields commonly used in MRI (up to 2 T) have not been found to produce adverse effects directly in humans. Instead, indirect hazards associated with B fields in MRI are due to the forces exhibited on ferromagnetic materials and on moving electrical charges.
For patients undergoing MRI, one concern is metallic objects (such as surgical clips) within the body, the presence of which can contraindicate the use of MRI. Another concern has been abnormalities in electrocardiograms (EKGs), probably a consequence of potentials induced by movement of blood (an electrical conductor) in the presence of the B field. The U.S. Food and Drug Administration, Center for Devices and Radiological Health (CDRH) has established a guidance level for static B fields of 4 T, above which MRI studies could present risk to patients.
For those working near MRI units, B fields also pose concerns. Any metallic object in the scan room can become a hazard if it is forcefully attracted to the scanner. Even outside the scan room, fringing B fields can affect the operation of cardiac pacemakers. Current recommendations of the American Conference of Governmental Industrial Hygienists are that worker exposure to static B fields should not exceed 60 millitesla (mT) to the whole body and 600 mT to limbs, on a daily time-weighted basis, and short-term exposures should not exceed 2 T to the whole body and 5 T to limbs. Wearers of medical electronic devices should be limited to 0.5 mT fields.
Time-varying B (dB/dt) fields induce voltages and associated electrical currents within the body, which in turn can cause nerve or muscle stimulation. Potential concerns include painful peripheral nerve stimulation and a startle reaction, either of which can cause a patient to move and interfere with image acquisition. The CDRH level of concern for dB/dt is 20 T s–1. Manufacturers wishing to exceed the guidance level are required to demonstrate that the rate of change of the B field does not cause peripheral nerve stimulation.
For RF fields, the associated hazards are not unique to MRI scanners and are addressed by limitations on the specific absorption rate (SAR) of RF energy in the body. Current CDRH guidance levels for SARs delivered by MRI scanners depend on body part, ranging from 3 W kg–1 averaged over the head for 10 minutes to 12 W kg–1 in any 1 g of tissue in the extremities for 15 minutes. CDRH guidance levels for patient SARs during MRI scans are comparable to the maximum permissible exposure levels for controlled areas recommended by ANSI/IEEE C95.1 (1992), which are 0.4 W kg–1 averaged over the whole body, 8 W kg–1 for any 1 g of tissue, and 20 W kg–1 averaged over any 10 g of tissue in the extremities.
The three nonionizing electromagnetic fields associated with MRI imaging (static B, time-varying B, and RF), given careful attention and under appropriate limitation, have resulted in this major medical technology gaining widespread acceptance and use throughout the world without a concomitant risk of adverse health effects.
For further reading—The 1997 Health Physics Society Summer School had two papers on MRI concepts and safety: "Physical Concepts of Magnetic Resonance Imaging" by Gary D. Fullerton and "MRI Safety and Planning Issues" by Cindy L. Bowser. Both were published in Nonionizing Radiation: An Overview of the Physics and Biology, edited by K. Hardy, M. Meltz, and R. Glickman, Medical Physics Publishing, Madison, Wisconsin, 1997.
An excellent source of information on research on the safety of MRI units is a General Electric website.
Two CDRH reports which contain information on safety requirements for MRI units are "Guidance for the Submission of Premarket Notifications for Magnetic Resonance" and "A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems."
