Simulation-based Evaluation of SAR and Flip Angle Homogeneity for Five Transmit Head Arrays at 14 T

Introduction: Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.

Methods: Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and k-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity.

Results: For RF shimming, the 16L array performs best. For k-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays.

Discussion And Conclusion: For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in k-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by k-points at the expense of larger power deposition. For k-points, the dipole arrays seem to outperform loop coil arrays.