specific absorption rate

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 . 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/kg.

Considerations for increases in body temperature should be made for patients with :

  • 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