The introduction of magnetic resonance (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. Most clinical MR systems in use today operate at fields ranging from 0.2- to 3-Tesla. According to the guidelines from the U.S. Food and Drug Administration, clinical MR systems using static magnetic fields up to 8.0-Tesla are considered a “non-significant risk” for adult patients. The exposure of research subjects to fields above this level requires approval of the research protocol by an Institutional Review Board and the informed consent of the subjects.
Currently, the most powerful MR system used for human subjects operates at 9.4-Tesla, but there are plans to develop even higher field strength scanners (e.g., greater than 11-Tesla). Several investigations now describe physiologic findings obtained in human subjects, including volunteers, patients, and workers relative to exposures to the 9.4-Tesla MR system. For the short-term exposures experienced by volunteers and patients, no readily demonstrated health risks were identified. However, a study conducted to evaluate sensory symptoms and vestibular function in workers exposed to the 9.4-Tesla MR system revealed that all participants noted sensory symptoms related to the exposure. The investigators concluded that, while the workers experienced sensory symptoms, it is unclear whether long-term vestibular damage or other changes occurred. The higher rates of vestibular changes may argue for improved worker surveillance and exposure control.
With respect to short-term exposures, the available information that pertains to the effects of static magnetic fields on biological tissues is extensive. Investigations include studies on alterations in cell growth and morphology, cell reproduction and teratogenicity, DNA structure and gene expression, pre- and post-natal reproduction and development, blood brain barrier permeability, nerve activity, cognitive function and behavior, cardiovascular dynamics, hematological indices, temperature regulation, circadian rhythms, immune responsiveness, neurological processing of visual and auditory information, and other biological processes. The majority of these studies concluded that exposures to static magnetic fields produce no substantial or harmful bioeffects. Although there have been reports of potentially injurious effects of static magnetic fields on isolated cells or organisms, no effect has been verified or firmly established as a scientific fact. The documented serious injuries and few fatalities that have occurred with MR system magnets were in association with the inadvertent introduction or presence of ferromagnetic objects (e.g., oxygen tanks, wheelchairs, aneurysm clips, etc.) into the MR environment.
Regarding the effects of long-term exposures to static magnetic fields, there are several physical mechanisms of interaction between tissues and static magnetic fields that could theoretically lead to pathological changes in human subjects. However, quantitative analysis of these mechanisms indicates that they are below the threshold of significance with respect to long-term, adverse bioeffects.
Presently, the peer-reviewed literature does not contain carefully controlled studies that support 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. Thus, several groups are now directing attention to performing these important studies and preliminary results have appeared in the literature.
In addition, although there is no evidence for a cumulative effect of magnetic field exposure on health, further studies of the exposed populations (MR healthcare professionals, patients that undergo repeat studies, interventional MR users, etc.) will help establish guidelines for occupational and patient exposures to powerful static magnetic fields. Recently, the results of several investigations have been published that have addressed occupational exposures to MR systems including those operating at 1.5-, 3-, and 7-Tesla.
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