Specific absorption rate (SAR) is the rate that electromagnetic energy in the radiofrequency is absorbed by tissues during MR image acquisition represented as watts per kilogram (W/kg). Both the International Electrotechnical Commission and the USA's Food and Drug Administration limit the amount of energy absorbed during the body over the course of a single examination to 1°C/kg 1,2. For a 1°C rise in body temperature, the body can be exposed to 4 W/kg.
For example high SAR sequences of a 3 T MRI deposits approximately between 1.9-2.5 W/kg3.
Considerations for increases in body temperature should be made for patients with 1:
- cardiovascular disease
- increased age
- impaired ability to perspire
- pregnancy (risk for fetal heating)
- drug regimes that may affect thermoregulatory capabilities (e.g. diuretics, tranquillizers, vasodilators)
- extensive tattoos
- plaster or fibreglass casts
- implanted organ devices
Precautions to reduce the SAR to patients can include:
- taking breaks between high SAR sequences
- alternating between low SAR and high SAR sequences
- reducing the flip angle
- reducing slice numbers
- reducing pulse number and duration
- reducing pulse frequency
- ensuring the patient is lightly dressed
- ensure scanner ventilation system is turned on
- 1. Jerry Allison, Nathan Yanasak. What MRI Sequences Produce the Highest Specific Absorption Rate (SAR), and Is There Something We Should Be Doing to Reduce the SAR During Standard Examinations?. (2015) American Journal of Roentgenology. 205 (2): W140. doi:10.2214/AJR.14.14173 - Pubmed
- 2. U.S. Food and Drug Administration. www.fda.gov. Accessed on 10/09/2019.
- 3. Youngseob Seo, Zhiyue J. Wang. MRI scanner‐independent specific absorption rate measurements using diffusion coefficients. (2017) Journal of Applied Clinical Medical Physics. 18 (4): 224. doi:10.1002/acm2.12095 - Pubmed