Specific absorption rate

Dr Henry Knipe and Dr David Chang et al.

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
  • hypertension
  • diabetes
  • increased age
  • obesity
  • fever
  • 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
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Article information

rID: 70933
Section: Physics
Synonyms or Alternate Spellings:
  • Specific absorption rate (SAR)

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