Specific absorption rate
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Specific absorption rate (SAR) is the rate that electromagnetic energy in the radiofrequency pulses is absorbed by tissues during MR image acquisition measured in watts per kilogram (W/kg).
Both the International Electrotechnical Commission and the Food and Drug Administration (in the USA) limit the amount of energy absorbed during the body over the course of a single examination to 1°C/kg 1,2. In general to prevent a greater than 1°C rise in body temperature, the body cannot be exposed to greater than 4 watts per kilogram.
For example, high SAR sequences of a 3 T MRI deposits ~1.9-2.5 W/kg 3.
Risk factors for increased SAR
Specific absorption rate proportionately increases with certain parameters:
square of the Larmor frequency or B0, i.e. worse as main field increases
square of the B1 pulse, worse with larger flip angles
size and shape of the patient: larger SAR with obesity
contact with wall of bore
Considerations for increases in body temperature should be made for those with 1:
impaired ability to perspire
pregnancy (risk for fetal heating)
drug regimens that may affect thermoregulatory capabilities (e.g. diuretics, tranquilisers, vasodilators)
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
ensuring 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