Specific absorption rate | Radiology Reference Article | Radiopaedia.org (2024)

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 frequencyor 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:

• cardiovascular disease

• hypertension

• diabetes mellitus

• increased age

• obesity

• fever

• impaired ability to perspire

• pregnancy (risk for fetal heating)

• drug regimens that may affect thermoregulatory capabilities (e.g. diuretics, tranquilisers, 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

• ensuring scanner ventilation system is turned on