IEEE Trans Med Imaging. 2018 Aug;37(8):1809-1821. doi: 10.1109/TMI.2018.2828112. Epub 2018 Apr 18.
High peak RF amplitude and excessive specific absorption rate (SAR) are two critical concerns for hardware implementation and patient safety in scientific and clinical research for high field MRI using parallel transmissions (pTX). In this paper, we introduce a squeezing strategy to reduce peak RF amplitude and integrated RF power via direct reshaping of the k-space trajectory. In the existing peak RF / integrated RF power optimization methods gradient amplitude or slew rate is reduced, but the k-space trajectory remains unchanged. Unlike these traditional methods, we worked directly in the excitation k-space to reshape k-space traversal by a squeezing vector in order to achieve peak RF and total RF power optimization, using a particle swarm optimization algorithm. The squeezing strategy was applied to the conventional variable density spiral (CVDS) and the variable rate selective excitation (VERSE) trajectories, dubbed SVDS (squeezed variable density spiral) and SVERSE (squeezing trajectory with VERSE), respectively, for different excitation profiles of small or large tip angles. Pulse acceleration and off-resonance effects were evaluated for an 8-ch pTX via Bloch simulation. CVDS, VERSE, SVDS, and SVERSE pulses were implemented on a 3T scanner with a 2-ch pTX. Phantom and in vivo experiments were performed for reduced FOV (rFOV) imaging. The results show that SVDS pulses simultaneously reduce integrated RF power and peak RF by about 30% on average compared to CVDS pulses for a square pattern ( $80\times80$ mm2) with flip angles of 30°, 90°, and 180°. Compared with the VERSE method under the same peak RF constraints, the SVDS method reduces integrated RF power by an average of 20% for small tip excitations for profiles of slice, rectangular, square, and circle, and has slightly reduced excitation accuracy slightly (about 0.6%, from 6.8% to 7.4%). The SVERSE method shortens the duration of the VERSE pulse by 12.8% at large ti p angle (180°). Feasibility for rFOV imaging was demonstrated with phantom and in vivo experiments with squeezed pulses.
高峰值射频幅度和过高的比吸收率（SAR）是使用并行传输（pTX）进行高磁场 MRI 科学和临床研究中硬件实现和患者安全的两个关键问题。在本文中，我们介绍了一种挤压策略，通过直接对 k 空间轨迹进行整形来降低峰值 RF 幅度和集成 RF 功率。在现有的峰值 RF/集成 RF 功率优化方法中，降低了梯度幅度或摆率，但 k 空间轨迹保持不变。与这些传统方法不同，我们直接在激发 k 空间中工作，通过挤压矢量重塑 k 空间遍历，以使用粒子群优化算法实现峰值 RF 和总 RF 功率优化。挤压策略分别应用于传统的可变密度螺旋（CVDS）和可变率选择性激发（VERSE）轨迹，分别称为 SVDS（挤压可变密度螺旋）和 SVERSE（具有 VERSE 的挤压轨迹），用于小或大翻转角的不同激发轮廓。通过 Bloch 模拟评估了用于 8 通道 pTX 的脉冲加速和离频效应。在具有 2 通道 pTX 的 3T 扫描仪上实现了 CVDS、VERSE、SVDS 和 SVERSE 脉冲。进行了幻影和体内实验以进行小视野（rFOV）成像。结果表明，与 CVDS 脉冲相比，SVDS 脉冲在翻转角为 30°、90°和 180°的正方形图案（80×80mm2）中，平均可将集成 RF 功率和峰值 RF 降低约 30%。与相同峰值 RF 约束下的 VERSE 方法相比，SVDS 方法对于小翻转角的切片、矩形、正方形和圆形轮廓，平均可将集成 RF 功率降低 20%，并且略微降低了激发精度（约 0.6%，从 6.8%降至 7.4%）。在大翻转角（180°）下，SVERSE 方法将 VERSE 脉冲的持续时间缩短了 12.8%。通过使用挤压脉冲的幻影和体内实验证明了 rFOV 成像的可行性。
