Occupational ExposureDoes working around the electromagnetic fields of an MRI scanner carry any health risks? Should exposure be limited? What about pregnant techs and staff?
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Overall Risks
As detailed in another Q&A, there is no convincing evidence that significant long-term biological effects exist for the types of electromagnetic field exposures a staff member would be expected to have in and around an MRI scanner. 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.
Regulatory Limits
There is no shortage of regulatory organizations that have developed and promulgated standards for occupational magnetic field exposures, although most are not binding around the world. The most important organizations include the International Electrotechnical Commission (IEC), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the Institute of Electrical and Electronics Engineers (IEEE). In 2013, the European Council published Directive 2013/35/EU, a sweeping revision of prior regulations on occupational electromagnetic field exposure by which European Union member states are now bound. The EU Directive for the most part mirrors the exposure limits set by the ICNIRP in 2010, but have exceptions for special clinical, research, or engineering situations.
The existing panoply of regulations is confusing and in some cases contradictory. For example, the occupational body static field limit is set at 8T by the IEC, but only 2T by the ICNIRP. For occupational RF exposures, the current limits are 4 W/kg for the IEC, but only 0.4 W/kg for both the ICNIRP and IEEE. The excellent reviews by McRobbie (2012) and Mazar (2016) nicely catalog various occupational limits in various countries and their rationales.
Special Considerations for Pregnant Staff
In 1993 Kanal et al reviewed medical history questionnaires from over 1400 pregnant MRI technologists, nurses and health care workers. They found no statistically significant difference between the rates of spontaneous abortions/miscarriages, or low birth weights, or premature delivery, or infertility, or male offspring between those working as MR employees versus the same population when they were employed elsewhere.
Although there is no definitive evidence that occupational exposure to MR-level magnetic fields may be harmful to a pregnant staff member or her fetus, we have adopted a conservative approach and try to limit exposure throughout pregnancy. We try to keep pregnant personnel out of the scanner rooms especially while scanning is occurring. This practice minimizes their exposure to the higher level of static magnetic fields and eliminates completely their exposure to the time-varying (RF) fields. Being outside the room during scanning also nullifies concerns about acoustic noise on the fetus. If a staff member has strong fears about her occupational exposure during pregnancy, we assign her a clerical position far away from the scanner (a job we refer to as "tech-retary"). Fortunately, there are also other jobs in the MR suite, such as patient preparation and counseling, image archiving and filming, with little or no magnetic field exposure.
Although there is no definitive evidence that occupational exposure to MR-level magnetic fields may be harmful to a pregnant staff member or her fetus, we have adopted a conservative approach and try to limit exposure throughout pregnancy. We try to keep pregnant personnel out of the scanner rooms especially while scanning is occurring. This practice minimizes their exposure to the higher level of static magnetic fields and eliminates completely their exposure to the time-varying (RF) fields. Being outside the room during scanning also nullifies concerns about acoustic noise on the fetus. If a staff member has strong fears about her occupational exposure during pregnancy, we assign her a clerical position far away from the scanner (a job we refer to as "tech-retary"). Fortunately, there are also other jobs in the MR suite, such as patient preparation and counseling, image archiving and filming, with little or no magnetic field exposure.
lReferences
Directive 2013/35/EU of the European Parliament and of the Council of 26 June 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields); 2013 (accessed July 2020)
Frankel J, Wilén J, Mild KH. Assessing exposures to magnetic resonance imaging’s complex mixture of magnetic fields for in vivo, in vitro, and epidemiological studies of health effects for staff and patients. Front Public Health 2018; 6:66. [DOI Link]
Health Canada. Safety Code 6. Limits of human exposure to radio frequency electromagnetic energy in the frequency range from 3 kHz to 300 GHz. 2015. (accessed July 2020)
IEEE International Committee on Electromagnetic Safety Technical Committee. Synopsis of IEEE Std C95.1™-2019 “IEEE standard for safety levels with respect to human exposure to electric, magnetic, and electromagnetic fields, 0 Hz to 300 GHz”. [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 Commission on Non-Ionizing Radiation Protection. ICNIRP Guidelines for limiting exposure to electromagnetic fields (100 kHz to 300 GHZ). Health Phys 2020; 118:483-524. [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 July 2020)
Kanal E, Gillen J, Evans JA, et al. Survey of reproductive health among female MR workers. Radiology 1993; 187:395‑399. [DOI Link]
Mazar H. Human radio frequency exposure limits: an update of reference levels in Europe, USA, Canada, China, Japan and Korea. IEEE International Symposium on Electromagnetic Compatibility 2016: 467-473. [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 nationa/international organizations).
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]
Directive 2013/35/EU of the European Parliament and of the Council of 26 June 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields); 2013 (accessed July 2020)
Frankel J, Wilén J, Mild KH. Assessing exposures to magnetic resonance imaging’s complex mixture of magnetic fields for in vivo, in vitro, and epidemiological studies of health effects for staff and patients. Front Public Health 2018; 6:66. [DOI Link]
Health Canada. Safety Code 6. Limits of human exposure to radio frequency electromagnetic energy in the frequency range from 3 kHz to 300 GHz. 2015. (accessed July 2020)
IEEE International Committee on Electromagnetic Safety Technical Committee. Synopsis of IEEE Std C95.1™-2019 “IEEE standard for safety levels with respect to human exposure to electric, magnetic, and electromagnetic fields, 0 Hz to 300 GHz”. [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 Commission on Non-Ionizing Radiation Protection. ICNIRP Guidelines for limiting exposure to electromagnetic fields (100 kHz to 300 GHZ). Health Phys 2020; 118:483-524. [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 July 2020)
Kanal E, Gillen J, Evans JA, et al. Survey of reproductive health among female MR workers. Radiology 1993; 187:395‑399. [DOI Link]
Mazar H. Human radio frequency exposure limits: an update of reference levels in Europe, USA, Canada, China, Japan and Korea. IEEE International Symposium on Electromagnetic Compatibility 2016: 467-473. [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 nationa/international organizations).
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]
Related Questions
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How do radiofrequency fields affect biological tissues?
Does MRI pose any risk to the developing fetus? Do you scan pregnant patients?
Who regulates/sets standards for MRI equipment and safety?