Local SAR Reduction in Parallel Excitation Based on Channel-Dependent Tikhonov Parameters

Purpose: To reduce the local specific absorption rate (SAR) obtained with tailored pulses using parallel transmission while obtaining homogenous flip angle distributions. Materials and Methods: Finite-element simulations on a human head model were performed to obtain the individual magnetic and electric field maps for each channel of a parallel transmit array. From those maps, SAR calculations were carried out for spoke pulses designed to homogenize the flip angle in an axial slice of a human brain at 7 T. Based on the assumption that the coil element nearest to the maximum local energy deposition is the dominant contributor to the corresponding hot spot, a set of channel-dependent Tikhonov parameters is optimized. Resulting SAR distributions are compared to the ones obtained when using standard pulse design approaches based on a single Tikhonov parameter. Results: In both the small- and large-tip-angle domain, the simulations show local SAR reductions by over a factor of 2 (4) for a well-centered (off-centered) head model at the expense of roughly 1% increment in flip-angle spread over the slice. Conclusion: Significant SAR reductions can be obtained by optimizing channel-dependent Tikhonov parameters based on the relation between coil elements and SAR hot spot positions.