Zelinski Adam C, Wald Lawrence L, Setsompop Kawin, Alagappan Vijayanand, Gagoski Borjan A, Goyal Vivek K, Adalsteinsson Elfar
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Magn Reson Med. 2008 Jun;59(6):1355-64. doi: 10.1002/mrm.21585.
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 kz via an L1-norm penalty on candidate (kx, ky) 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.
提出了一种新型射频（RF）脉冲设计算法，该算法可生成快速切片选择性激发脉冲，以减轻高场下人脑中存在的B + 1不均匀性。该方法先对脑轴向切片中的B + 1场进行估计，然后通过对候选（kx，ky）位置施加L1范数惩罚来优化kz中类似 sinc 的“辐条”的放置；然后基于这些加权点设计RF脉冲和梯度。在头部形状的水模体和大脑中于7T下设计并演示了减轻脉冲；在每种情况下，这些脉冲都减轻了明显不均匀的发射分布，并在激发场（FOX）中产生了几乎均匀的翻转角。这项工作的主要贡献，即稀疏强制辐条放置和脉冲设计算法，是针对传统单通道激发系统推导出来的，并应用于7T下的脑部，但很容易扩展到低场系统、非脑部应用和多通道并行激发阵列。
