Specific Absorption Rate

What is Specific Absorption Rate?

Specific Absorption Rate, often shortened to SAR, is an estimate of the the RF energy deposited into the patient while scanning that may result in body temperature rise. SAR is not an exact 'amount' of RF, nor a specific level of heating, but an estimate based on patient weight, orientation, pulse sequence, acquisition parameters, and scan time. Each MR vendor utilizes different modeling for estimating SAR, so there may not be a 1-to-1 comparison between scanners, but broadly several things regarding SAR are constant:

SAR will increase with field strength

SAR will be higher with pulse sequences that use a lot of strong RF pulses

SAR will be higher if strong RF pulses are closely spaced

SAR will be higher as the number of slices increases

SAR will be higher with many consecutive pulse sequences

SAR and Scanning

On even relatively older scanners, SAR is tracked throughout the scan, typically in 15 minute intervals. If the scanner senses SAR limits being exceeded, scanning will be prohibited and a 'cool down' time will have to elapse before scanning can resume. When scanning veterinary patients, body weights are typically far lower than the SAR model was meant to be used with, leading to inaccurate estimates and SAR errors preventing scanning unnecessarily. Additionally, animals under anesthesia do not have the same thermoregulatory capacity as when they're awake, often leading to patients being too cold despite what the scanner SAR level reads. Generally, the best approach is to maintain adequate patient temperature with blankets and warmers, and mitigate SAR errors so they don't prolong anesthesia time.

Reducing SAR While Scanning

The first part of knowing how to reduce SAR errors is knowing where and when they are likely to occur:

Small animals; <10kgs

RF heavy pulse sequences: Fast spin echo, HASTE/ SS-FSE, FIESTA/TruFISP, IR-FSE

T1 weighting: Short TR FSE's

Large axial stacks of T1's

Post contrast imaging: lots of T1's back to back

SAR reduction strategies are pretty straightforward: reduce the amount, frequency, and strength of RF pulses!

Increase TR (but stay within contrast ranges!)

Reduce the number of slices

Reduce the refocus flip angle for FSE's

Use a GRE instead of an FSE

Choose Low SAR RF mode

Space out T1 FSE with T2's or GRE's