Department of Neuroradiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
Siemens Healthineers GmbH, Erlangen, Germany.
Magn Reson Med. 2021 Jun;85(6):3140-3153. doi: 10.1002/mrm.28643. Epub 2021 Jan 5.
To mitigate spatial flip angle (FA) variations under strict specific absorption rate (SAR) constraints for ultra-high field MRI using a combination of universal parallel transmit (pTx) pulses and fast subject-specific optimization.
Data sets consisting of B , maps, and virtual observation point (VOP) data were acquired from 72 subjects (study groups of 48/12 healthy Europeans/Asians and 12 Europeans with pathological or incidental findings) using an 8Tx/32Rx head coil on a 7T whole-body MR system. Combined optimization values (COV) were defined as combination of spiral-nonselective (SPINS) trajectory parameters and an energy regularization weight. A set of COV was optimized universally by simulating the individual RF pulse optimizations of 12 training data sets (healthy Europeans). Subsequently, corresponding universal pulses (UPs) were calculated. Using COV and UPs, individually optimized pulses (IOPs) were calculated during the sequence preparation phase (maximum 15 s). Two different UPs and IOPs were evaluated by calculating their normalized root-mean-square error (NRMSE) of the FA and SAR in simulations of all data sets. Seven additional subjects were examined using an MPRAGE sequence that uses the designed pTx excitation pulses and a conventional adiabatic inversion.
All pTx pulses resulted in decreased mean NRMSE compared to a circularly polarized (CP) pulse (CP = ~28%, UPs = ~17%, and IOPs = ~12%). UPs and IOPs improved homogeneity for all subjects. Differences in NRMSE between study groups were much lower than differences between different pulse types.
UPs can be used to generate fast online-customized (FOCUS) pulses gaining lower NRMSE and/or lower SAR values.
在严格的特定吸收率 (SAR) 约束下，通过通用并行发射 (pTx) 脉冲和快速的基于个体的优化，减轻超高场 MRI 中的空间翻转角 (FA) 变化。
使用 7T 全身磁共振系统的 8Tx/32Rx 头部线圈，从 72 名受试者（48/12 名健康的欧洲人/亚洲人和 12 名有病理或偶发发现的欧洲人组成的研究组）中采集 B1 图、虚拟观察点 (VOP) 数据和数据。组合优化值 (COV) 定义为螺旋非选择性 (SPINS) 轨迹参数和能量正则化权重的组合。通过模拟 12 个训练数据集（健康的欧洲人）的个体 RF 脉冲优化，对一组 COV 进行了通用优化。随后计算了相应的通用脉冲 (UP)。使用 COV 和 UPs，在序列准备阶段（最多 15 秒）计算个体优化脉冲 (IOP)。在所有数据集的模拟中，通过计算它们的 FA 和 SAR 的归一化均方根误差 (NRMSE)，评估了两种不同的 UP 和 IOP。使用设计的 pTx 激励脉冲和传统的绝热反转，对另外 7 名受试者进行了 MPRAGE 序列检查。
与圆极化 (CP) 脉冲相比（CP = ~28%，UPs = ~17%，IOPs = ~12%），所有 pTx 脉冲都导致平均 NRMSE 降低。UPs 和 IOPs 提高了所有受试者的均匀性。研究组之间的 NRMSE 差异远小于不同脉冲类型之间的差异。
UPs 可用于生成具有更低 NRMSE 和/或更低 SAR 值的快速在线定制 (FOCUS) 脉冲。
