On the safety margin of using simplified human head models for local SAR simulations of B1-shimming at 7 Tesla

Abstract

PurposesUsing simplified human models significantly alleviates the difficulty of rendering human models for subject-specific local specific absorption rate (SAR) simulations. Although its accuracy has been demonstrated with the birdcage mode combination of RF transmitters, its accuracy in general B1-shimming, where numerous phase and magnitude combinations can take place, is yet unknown. MethodsThe electromagnetic fields of a 7-Tesla eight-channel brain imaging array were simulated by using four detailed human models from the Virtual Family and their two-, three-, and four-tissue simplifications. The 10-g averaged local SAR was computed for each case with 1,000 sets of uniformly distributed random B1-shimming parameters. Linear regression was applied to relate the local SAR obtained by using detailed and simplified human models. The 99% confidence prediction interval was determined as the safety margin in order to cover the largest local SAR variability introduced by using simplified human models. ResultsThe local SAR computed by using three- and four-tissue simplifications are strongly correlated with those computed by using detailed models. Safety margins of 0.38 and 0.45W/kg/W were found appropriate for each case being considered. ConclusionsThe proposed procedure can be applied to evaluate the safety margin of the local SAR simulated by using simplified human models. However, discretion needs to be exercised since the safety margins in some cases may represent more than 50% overestimation.