SAR LimitsWhat are the absolute SAR limits you are not permitted to exceed?
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In the 1990s the International Electrotechnical Commission (IEC) and US Food and Drug Administration (FDA) jointly worked to develop the first international standards for MR equipment safety, codified as IEC 60601-2-33 (latest revision 6/2015). This document classified SAR into various subtypes (shown in the table below) and established three modes of scanner operation based on perceived risk to subjects: 1) normal mode, 2) first-level controlled mode, and 3) second level controlled mode. Criteria for each operating mode are based principally around specific absorption rate (SAR), but also include factors such as static field strength (Bo), rates of time-varying gradients (dB/dt), area of the body imaged, and temperature rise. These non-SAR criteria for various operating modes and supervisory requirements are discussed more completely in the next Q&A.
Classification of SAR types according to the International Electrotechnical Commission (IEC)
The IEC SAR limits are given in the table below. Partial body limits are variable (see caption below graph for explanation). For 2nd Level Controlled Operating Mode specific upper limits are not given. This is considered the responsibility of the local Institutional Review Board (IRB) who authorize and oversee research and safety issues.
IEC SAR Limits (W/kg).All values are based on the environmental temperature being ≤ 25ºC. For higher temperatures, the First Level whole body limit (4 W/kg) only is derated by 0.25 W/kg for each 1 ºC rise until the Normal operating level limit (2 W/kg) is reached. Partial body limits are scaled according to Mass Ratio (R) = RF-exposed patient mass ÷ total patient mass, giving ranges: a = [10 − 8×R] W/kg and b = [10 − 6×R] W/kg. Transmit/receive knee and wrist coils are covered under the "partial body" limits.
References
Adair ER, Black DR. Thermoregulatory responses to RF energy absorption. Bioelectromagnetics 2003; 24(S6):S17-S38. [DOI Link]
Chou CK, Bassen H, Osepchuk J, et al. Radio frequency electromagnetic exposure: tutorial review on experimental dosimetry. Bioelectromagnetics 1996; 17:195-208. [DOI link]
Foster KR, Glaser R. Thermal mechanisms of interaction of radio frequency energy with biological systems with relevance to exposure guidelines. Health Phys 2007; 92:609-620. [DOI link]
Health Protection Agency. Protection of patients and volunteers undergoing MRI procedures. Documents of the Health Protection Agency Radiation, Chemical and Environmental Hazards, August 2008. [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)
Medicines and Healthcare Products Regulatory Agency (MHRA). Safety guidelines for magnetic resonance imaging equipment in clinical use. 2015. (See Appendix 2 for comparison of exposure limits suggested by HPA, IEC, and ICNIRP) [link]
Adair ER, Black DR. Thermoregulatory responses to RF energy absorption. Bioelectromagnetics 2003; 24(S6):S17-S38. [DOI Link]
Chou CK, Bassen H, Osepchuk J, et al. Radio frequency electromagnetic exposure: tutorial review on experimental dosimetry. Bioelectromagnetics 1996; 17:195-208. [DOI link]
Foster KR, Glaser R. Thermal mechanisms of interaction of radio frequency energy with biological systems with relevance to exposure guidelines. Health Phys 2007; 92:609-620. [DOI link]
Health Protection Agency. Protection of patients and volunteers undergoing MRI procedures. Documents of the Health Protection Agency Radiation, Chemical and Environmental Hazards, August 2008. [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)
Medicines and Healthcare Products Regulatory Agency (MHRA). Safety guidelines for magnetic resonance imaging equipment in clinical use. 2015. (See Appendix 2 for comparison of exposure limits suggested by HPA, IEC, and ICNIRP) [link]
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