Specific absorption rate

Last revised by Dr Daniel J Bell on 14 Jun 2022

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
  • RF pulses per unit time: greater SAR with FSE/TSE
  • contact with wall of bore
Precautions

Considerations for increases in body temperature should be made for those with 1:

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

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