252. Prediction of subject-specific SAR distribution in MSK MR exam at 7 T

Abstract

Purpose we predict SAR during MRI exam using a 7 T 1H 298 MHz eight-channel degenerate birdcage coil1 combining SAR simulations with subject-specific measured (RF) B 1 + maps. Materials and Methods We simulated the coil1 in CST MW Suite, loaded by a model of human knee (Fig. 1, top). B 1 + was calculated in an axial slice crossing the patella. The maximum local SAR for an Axial “Zero” Time-of-Echo (ZTE) sequence “SILENT”2 was calculated. We acquired ∣ B 1 , map + ∣ maps of an adult (female) knee with a Bloch-Siegert sequence on 7 axial slices, centered on the same slice of the simulation, on a GE MR950 7T human system. For each slice a coefficient C, proportional to avg ( ∣ B 1 , map + ∣ ) / B 1 , nominal + , was used to scale the SAR simulated3. Results Fig. 1 shows: bottom left, simulated B 1 + magnitude; bottom center, local SAR for an input of 1 W per channel; bottom right, simulated B 1 , nominal + magnitude for a FA = 90° (length = 3.2 ms) sinc-pulse in the slice previously chosen. Fig. 2 shows the subject-specific measured ∣ B 1 , map + ∣ for a FA = 90° sinc-pulse. The predicted SAR obtained with scaled B 1 + maps are 0.50 W/kg (global) and 3.68 W/kg (maximum). Conclusions we obtained a good agreement between simulated and measured in vivo B 1 + maps, and we were able to calculate the distribution of SAR exposure, a safety MRI parameter not available in current exams, where only global SAR is provided, combining simulations and subject-specific measurements. Limits on global and local SAR (20 W/kg) were met for this sequence [1] , [2] , [3] .