The specific absorption rate (SAR) is the rate at which radiofrequency (RF) energy is absorbed by tissues during MR image acquisition, which is measured in watts per kilogram (W/kg) ⁴. It is used as indirect measure of RF heating, which is an MRI safety concern ⁴.

Technique

The International Electrotechnical Commission (IEC) and the USA's Food and Drug Administration (FDA) limit the amount of energy absorbed by the body for a single MRI examination to 1°C/kg ^1,2. In general, to prevent a rise in body temperature >1°C, the body cannot be exposed to more than 4 watts per kilogram. For example, high SAR sequences of a 3 T MRI deposits ~1.9-2.5 W/kg ³.

SAR models are proprietary, and absolute SAR values are not interchangeable between vendors ⁴.

Physical principles

SAR proportionately increases with certain parameters ¹:

• square of the Larmor frequency or B₀, i.e. worse as the 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 the bore wall

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 ¹:

• 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