SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI.
OBJECTIVE
To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B(1)(+) excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice.


METHODS
Finite difference time domain (FDTD) simulations were used to evaluate five RF shimming strategies for the pelvis inside a body coil. The results were compared to the theoretical best solution that could be achieved if the electric field inside the patient was known.


RESULTS
Most of the RF shimming strategies were successful. However, between the different strategies a factor of two difference in average SAR reduction, a factor of three difference in local maximum SAR reduction, and a factor of 20 difference in power efficiency was observed. Phase matching was found to be the most promising RF shimming method for the body coil used and patient models.


CONCLUSIONS
RF shimming can reduce the SAR and improve power efficiency in an accurate patient model without knowing the electric field. However, choosing the right method is critical to prevent unexpected behavior in local SAR deposition.
van den Bergen
Bob
B
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands. B.vandenbergen@UMCUtrecht.nl
van den Berg
Cornelis A T
CA
Klomp
Dennis W J
DW
Lagendijk
Jan J W
JJ
D003198:Computer Simulation; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008280:Magnetics; D008953:Models, Anatomic; D010388:Pelvis / Q000033:anatomy & histology*; D019047:Phantoms, Imaging