  Objective: Accurate estimation of SAR is critical to safeguarding vulnerable
patients who require an MRI procedure. The increased static field strength and
RF duty cycle capabilities in modern MRI scanners mean that systems can easily
exceed safe SAR levels for patients. Advisory protocols routinely used to
establish quality assurance protocols are not required to advise on the testing
of MRI SAR levels and is not routinely measured in annual medical physics
quality assurance checks. This study aims to develop a head phantom and
protocol that can independently verify global SAR for MRI clinical scanners.
  Methods: A four-channel birdcage head coil was used for RF transmission and
signal reception. Proton resonance shift thermometry was used to estimate SAR.
The SAR estimates were verified by comparing results against two other
independent measures, then applied to a further four scanners at field
strengths of 1.5 T and 3 T.
  Results: Scanner output SAR values ranged from 0.42 to 1.52 W/kg. Percentage
SAR differences between independently estimated values and those calculated by
the scanners differed by 0-2.3%.
  Conclusion: We have developed a quality assurance protocol to independently
verify the SAR output of MRI scanners.
