Bioeffects of Static Magnetic FieldsDo the strong static magnetic fields used in MRI pose any health risks?
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Excluding several transient sensory disturbances described below, as well as the secondary risk of projectile injuries, the static magnetic fields used in MRI probably create no significant adverse health effects on patients or staff. This conclusion is supported by multiple recent review papers, several of which are provided in the References. It should be noted, however, that evidence does exist for some low-level biological effects at the microscopic level in certain cell suspension and animal experiments, but these typically employ fields that are stronger or at longer exposures than humans receive in the MR environment. Like many questions in science, the final answers are not in, but the fact that over 400 million MRI exams have been safely performed worldwide to date provides some reassurance that static fields in MRI pose minimal risks.
To have a biological effect, static magnetic fields must operate through one of several known physical mechanisms:
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1. Lorentz Force. The Lorentz force is experienced by charged particles moving through a magnetic field, discussed in a prior Q&A. In theory the Lorentz force could have subtle effects on cellular transmembrane potentials and transport through ionic channels, but as yet no conclusive evidence this occurs at the field strengths used in clinical MRI. However, it is likely that the Lorentz force does act effect ionic currents in the middle ear endolymph and be responsible for the transient sensations of vertigo/dizziness/nausea commonly noted at 7.0T and above. A full Q&A is devoted to this topic.
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Lorentz force (F) on an electric current (I) passing through a magnetic field (B).
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When blood flows through a magnetic field the Lorentz force differentially deflects positive and negative ions toward opposite sides of the vessel. This creates a potential difference known as the magneto-hydrodynamic (MHD) effect. As described in another Q&A, the MHD effect produces artifactual changes in recorded EKG voltages but no harmful effects on the body.
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2. Motion-Induced Currents. The Faraday-Lenz Law states that an electric field (or current) is induced in a conductor whenever it moves through a static magnetic field. This mechanism its thought responsible for generation of visual light flashes (magnetophosphenes) during eye motion (discussed in this related Q&A) and metallic taste by tongue motion (discussed in this related Q&A).
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Faraday-Lenz Law
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Translational (Ftrans) and rotational (Frot) forces on a magnetizable object placed in an external magnetic field (Bo)
3. Susceptibility-induced forces. Susceptibility (χ) is an intrinsic property of matter describing how it becomes polarized when when placed in an external magnetic field. The polarization of diamagnetic materials (such as water, simple salts and calcium) opposes the applied field, resulting an a repulsive force. The polarization of paramagnetic and ferromagnetic materials (such as ferrites and hemosiderin) augments the applied field, resulting in an attractive force. For either repulsion or attraction, the result may be translation or rotational movement of the molecule, cell, or anatomic structure.
Most of the human body is composed of diamagnetic materials with susceptibilities close to zero, and the repulsive force from an external field is weak compared to gravity and thermal motions. Paramagnetic and ferromagnetic materials, however, have much larger susceptibilities, and forces on them may be moderate. This is especially the case for materials whose susceptibilities are highly directionally dependent (anisotropic).
"Stacking" of sickle cells perpendicular to an external magnetic field due to susceptibility-related anisotropy
Sickle cell erythrocytes, which stack themselves perpendicular to an external magnetic field, are perhaps the most easily visualized demonstration of anisotropic susceptibility. Other examples include retinal rods and proteins/polypeptides (especially those containing an α-helix or harboring a peripheral paramagnetic ion). Anisotropic susceptibility can produce forces 100x stronger than isotropic susceptibility, so this potential mechanism cannot be cursorily dismissed. However, it remains speculative whether anisotropic torques on DNA, proteins, and the like due to strong magnetic fields produce clinically significant effects. In the 1980's several authorities expressed concern about performing MRI scans in patients with sickle cell disease, worried that magnetically-induced cell stacking might potentiate thrombosis. These concerns proved unfounded, however, and now many thousands of sickle cell patients have been safely scanned without incident.
Regulatory Limits for Patients and Staff
The two most important international organizations promulgating regulatory guidelines for patient and occupational exposure to static magnetic fields are the International Electrotechnical Commission (IEC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP). Unfortunately, their recommendations differ. For example, the IEC defines 3T as the upper limit for scanning patients in normal operating mode, while the ICNIRP has a more lenient limit of 4T. An even greater discordance occurs for staff, with 8T set as the maximum exposure for head and trunk by the IEC, but only 2T allowed in non-controlled mode by the ICNIRP.
The typical MRI worker is not commonly in the magnet room during scanning; therefore she is only occasionally exposed to gradient or radiofrequency fields. The main occupational exposure is from being around the static (Bo) fringe field intermittently for most of the day. With self-shielded scanners, the peak static field exposure for staff is unlikely to exceed 50% of Bo, and the average exposure over a shift would be only a few mT.
For MR staff working around scanners with field strengths of 3T or higher, however, short term sensory effects (including nausea and dizziness) have been clearly demonstrated. For example, the incidence of vertigo among staff working at 1.5T, 3.0T, and 7.0T has been reported to be about 4%, 8%, and 24% respectively. These effects can be mitigated by moving slowly around the scanner or at the scanner bore to minimize dB/dt in the fringe field.
The typical MRI worker is not commonly in the magnet room during scanning; therefore she is only occasionally exposed to gradient or radiofrequency fields. The main occupational exposure is from being around the static (Bo) fringe field intermittently for most of the day. With self-shielded scanners, the peak static field exposure for staff is unlikely to exceed 50% of Bo, and the average exposure over a shift would be only a few mT.
For MR staff working around scanners with field strengths of 3T or higher, however, short term sensory effects (including nausea and dizziness) have been clearly demonstrated. For example, the incidence of vertigo among staff working at 1.5T, 3.0T, and 7.0T has been reported to be about 4%, 8%, and 24% respectively. These effects can be mitigated by moving slowly around the scanner or at the scanner bore to minimize dB/dt in the fringe field.
References
European Commission Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). Potential health effects of exposure to electromagnetic fields (EMF). European Commission, 2015. [DOI link]
International Commission on Non-Ionizing Radiation Protection. ICNIRP Guidelines on limits of exposure to static magnetic fields. Health Phys 2009; 96:504-514. [DOI Link]
International Electrotechnical Commission. IEC 60601-2-33:2010: Medical Electrical Equipment - Part 2-33: Particular Requirements for the Basic Safety and Essential Performance of Magnetic Resonance Equipment for Medical Diagnosis. 3rd ed. with amendments. International Electrotechnical Commission; 2015. (accessed September 2020)
IEEE International Committee on Electromagnetic Safety. Expert Reviews. Available at [this link]. (Comprehensive listing of over 90 high quality expert reviews by various consensus panels and government agencies world-wide published between 2010 and 2020.)
Grant A, Metzger GJ, van de Moortele P-F, et al. 10.5 T MRI static field effects on human cognitive, vestibular, and physiological function. Magn Reson Imaging 2020; 73:163-176. [DOI LINK]
McRobbie DW. Occupational exposure in MRI. Br J Radiol 2012; 85:293-312. [DOI Link] (excellent review, now somewhat out of date, but multiple tables show the relatively diverse and complex criteria for magnetic field exposures of various national/international organizations).
Murayama M. Molecular mechanism of red cell “sickling”. Science 1966; 153:145-149. [Stable URL]
Schaap K, Christopher-De Vries Y, Crozier S, et al. Exposure to static and time-varying magnetic fields from workin in the static magnetic stray fields of MRI scanners: a comprehensive survey in the Netherlands. Ann Occup Hyg 2014; 58:1094-1110. [DOI Link]
Schaap K, Christopher-De Vries Y, Mason CK, et al. Occupational exposure of healthcare and research staff to static magnetic stray fields from 1.5-7 tesla MRI scanners is associated with transient symptoms. Occup Environ Med 2014; 71:423-429. [DOI Link]
Schenck JF. Safety of strong, static magnetic fields. J Magn Reson Imaging 2000; 12:2-19. [DOI Link]
SSM’s Scientific Council on Electromagnetic Fields. Report number 2020:04: Recent research on EMF and health risk. Available at [this link].
European Commission Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). Potential health effects of exposure to electromagnetic fields (EMF). European Commission, 2015. [DOI link]
International Commission on Non-Ionizing Radiation Protection. ICNIRP Guidelines on limits of exposure to static magnetic fields. Health Phys 2009; 96:504-514. [DOI Link]
International Electrotechnical Commission. IEC 60601-2-33:2010: Medical Electrical Equipment - Part 2-33: Particular Requirements for the Basic Safety and Essential Performance of Magnetic Resonance Equipment for Medical Diagnosis. 3rd ed. with amendments. International Electrotechnical Commission; 2015. (accessed September 2020)
IEEE International Committee on Electromagnetic Safety. Expert Reviews. Available at [this link]. (Comprehensive listing of over 90 high quality expert reviews by various consensus panels and government agencies world-wide published between 2010 and 2020.)
Grant A, Metzger GJ, van de Moortele P-F, et al. 10.5 T MRI static field effects on human cognitive, vestibular, and physiological function. Magn Reson Imaging 2020; 73:163-176. [DOI LINK]
McRobbie DW. Occupational exposure in MRI. Br J Radiol 2012; 85:293-312. [DOI Link] (excellent review, now somewhat out of date, but multiple tables show the relatively diverse and complex criteria for magnetic field exposures of various national/international organizations).
Murayama M. Molecular mechanism of red cell “sickling”. Science 1966; 153:145-149. [Stable URL]
Schaap K, Christopher-De Vries Y, Crozier S, et al. Exposure to static and time-varying magnetic fields from workin in the static magnetic stray fields of MRI scanners: a comprehensive survey in the Netherlands. Ann Occup Hyg 2014; 58:1094-1110. [DOI Link]
Schaap K, Christopher-De Vries Y, Mason CK, et al. Occupational exposure of healthcare and research staff to static magnetic stray fields from 1.5-7 tesla MRI scanners is associated with transient symptoms. Occup Environ Med 2014; 71:423-429. [DOI Link]
Schenck JF. Safety of strong, static magnetic fields. J Magn Reson Imaging 2000; 12:2-19. [DOI Link]
SSM’s Scientific Council on Electromagnetic Fields. Report number 2020:04: Recent research on EMF and health risk. Available at [this link].
Related Questions
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