Fast slice-selective radio-frequency excitation pulses for mitigating B-1(+) inhomogeneity in the human brain at 7 tesla

A novel radio-frequency (RF) pulse design algorithm is presented that generates fast slice-selective excitation pulses that mitigate B-1(+) inhomogeneity present in the human brain at high field. The method is provided an estimate of the B-1(+) field in an axial slice of the brain and then optimizes the placement of sinc-like spokes in k(z) via an L-1-norm penalty on candidate (k(x), k(y)) locations; an RF pulse and gradients are then designed based on these weighted points. Mitigation pulses are designed and demonstrated at 7T in a head-shaped water phantom and the brain; in each case, the pulses mitigate a significantly non-uniform transmit profile and produce nearly uniform flip angles across the field of excitation (FOX). The main contribution of this work, the sparsity-enforced spoke placement and pulse design algorithm, is derived for conventional single-channel excitation systems and applied in the brain at 7T, but readily extends to lower field systems, nonbrain applications, and multichannel parallel excitation arrays.