# Specific absorption rate (SAR)

## What is specific absorption rate (SAR), and what are the mandated limits?

The specific absorption rate (SAR) is defined as the rate at which energy is absorbed by tissue when exposed to a radiofrequency pulse, such as during MRI or therapeutic US. It is described in units of watts per kilogram (W/kg). For example, 1 W/kg would increase the temperature of a slab of tissue by 1° C/hr. SAR is proportional to the square of the magnetic field strength, so it increases by a factor of 4 when switching from a 1.5-T scanner to a 3-T scanner. For MRI, SAR limits are defined for the whole body, local anatomy, or extremities. Different limits are specified by the FDA and the International Electrotechnical Commission (IEC)