Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, Minneapolis, MN, USA.
The Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University, Baltimore, MD, USA.
NMR Biomed. 2021 May;34(5):e4218. doi: 10.1002/nbm.4218. Epub 2019 Dec 18.
The semi-adiabatic localization by adiabatic selective refocusing (sLASER) sequence provides single-shot full intensity signal with clean localization and minimal chemical shift displacement error and was recommended by the international MRS Consensus Group as the preferred localization sequence at high- and ultra-high fields. Across-vendor standardization of the sLASER sequence at 3 tesla has been challenging due to the B requirements of the adiabatic inversion pulses and maximum B limitations on some platforms. The aims of this study were to design a short-echo sLASER sequence that can be executed within a B limit of 15 μT by taking advantage of gradient-modulated RF pulses, to implement it on three major platforms and to evaluate the between-vendor reproducibility of its perfomance with phantoms and in vivo. In addition, voxel-based first and second order B shimming and voxel-based B adjustments of RF pulses were implemented on all platforms. Amongst the gradient-modulated pulses considered (GOIA, FOCI and BASSI), GOIA-WURST was identified as the optimal refocusing pulse that provides good voxel selection within a maximum B of 15 μT based on localization efficiency, contamination error and ripple artifacts of the inversion profile. An sLASER sequence (30 ms echo time) that incorporates VAPOR water suppression and 3D outer volume suppression was implemented with identical parameters (RF pulse type and duration, spoiler gradients and inter-pulse delays) on GE, Philips and Siemens and generated identical spectra on the GE 'Braino' phantom between vendors. High-quality spectra were consistently obtained in multiple regions (cerebellar white matter, hippocampus, pons, posterior cingulate cortex and putamen) in the human brain across vendors (5 subjects scanned per vendor per region; mean signal-to-noise ratio > 33; mean water linewidth between 6.5 Hz to 11.4 Hz). The harmonized sLASER protocol is expected to produce high reproducibility of MRS across sites thereby allowing large multi-site studies with clinical cohorts.
半绝热局部化的绝热选择性重聚焦（sLASER）序列提供单次全强度信号，具有清晰的局部化和最小的化学位移位移误差，并且被国际 MRS 共识小组推荐为高场和超高场的首选局部化序列。由于绝热反转脉冲的 B 要求和某些平台上的最大 B 限制，在 3 特斯拉下对 sLASER 序列进行跨供应商标准化一直具有挑战性。本研究的目的是设计一种短回波 sLASER 序列，该序列可以利用梯度调制 RF 脉冲在 15 μT 的 B 限制内执行，在三个主要平台上实现，并使用体模和体内评估其性能的跨供应商可重复性。此外，在所有平台上都实现了基于体素的一阶和二阶 B 匀场和基于体素的 RF 脉冲 B 调整。在所考虑的梯度调制脉冲中（GOIA、FOCI 和 BASSI），GOIA-WURST 被确定为最佳重聚焦脉冲，它在最大 B 为 15 μT 的情况下提供了良好的体素选择，基于定位效率、反转轮廓的污染误差和波纹伪影。一种包含 VAPOR 水抑制和 3D 外体积抑制的 sLASER 序列（30ms 回波时间）采用相同的参数（RF 脉冲类型和持续时间、扰频梯度和脉冲间延迟）在 GE、飞利浦和西门子上实现，并在 GE 'Braino' 体模上生成相同的谱在供应商之间。在跨供应商的情况下（每个供应商每个区域 5 名受试者；平均信噪比>33；平均水线宽在 6.5Hz 至 11.4Hz 之间），在多个区域（小脑白质、海马、脑桥、后扣带回皮层和壳核）中始终获得高质量的谱。预计协调的 sLASER 协议将在站点之间产生 MRS 的高度可重复性，从而允许对具有临床队列的大型多站点研究。
