Fast slice‐selective radio‐frequency excitation pulses for mitigating B inhomogeneity in the human brain at 7 Tesla

Abstract

A novel radio‐frequency (RF) pulse design algorithm is presented that generates fast slice‐selective excitation pulses that mitigate B inhomogeneity present in the human brain at high field. The method is provided an estimate of the B field in an axial slice of the brain and then optimizes the placement of sinc‐like “spokes” in k _z via an L₁‐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 nonuniform 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. Magn Reson Med 59:1355–1364, 2008.