Department of Radiology and Biomedical Imaging, Magnetic Resonance Research Center, Yale University School of Medicine, New Haven, CT, USA.
Department of Radiology and Biomedical Imaging, Magnetic Resonance Research Center, Yale University School of Medicine, New Haven, CT, USA.
J Magn Reson. 2022 Aug;341:107247. doi: 10.1016/j.jmr.2022.107247. Epub 2022 Jun 3.
Gradient modulated RF pulses, especially gradient offset independent adiabaticity (GOIA) pulses, are increasingly gaining attention for high field clinical magnetic resonance spectroscopy and spectroscopic imaging (MRS/MRSI) due to the lower peak B amplitude and associated power demands achievable relative to its non-modulated adiabatic full passage counterparts. In this work we describe the development of two GOIA RF pulses: 1) A power efficient, 3.0 ms wideband uniform rate with smooth truncation (WURST) modulated RF pulse with 15 kHz bandwidth compatible with a clinically feasible peak B amplitude of 0.87 kHz (or 20 µT), and 2) A highly selective asymmetric 6.66 ms RF pulse with 20 kHz bandwidth designed to achieve a single-sided, fractional transition width of only 1.7%. Effects of potential asynchrony between RF and gradient-modulated (GM) waveforms for 3 ms GOIA-WURST RF pulses was evaluated by simulation and experimentally. Results demonstrate that a 20+ µs asynchrony between RF and GM functions substantially degrades inversion performance when using large RF offsets to achieve translation. A projection-based method is presented that allows a quick calibration of RF and GM asynchrony on pre-clinical/clinical MR systems. The asymmetric GOIA pulse was implemented within a multi-pulse OVS sequence to achieve power efficient, highly-selective, and B and T-independent signal suppression for extracranial lipid suppression. The developed GOIA pulses were utilized with linear gradient modulation (X, Y, Z gradient fields), and with second-order-field modulations (Z2, X2Y2 gradient fields) to provide elliptically-shaped regions-of-interest for MRS and MRSI acquisitions. Both described GOIA-RF pulses have substantial clinical value; specifically, the 3.0 ms GOIA-WURST pulse is beneficial to realize short TE sLASER localized proton MRS/MRSI sequences, and the asymmetric GOIA RF pulse has applications in highly selective outer volume signal suppression to allow interrogation of tissue proximal to extracranial lipids with full-intensity.
梯度调制射频脉冲，特别是梯度偏移独立绝热性（GOIA）脉冲，由于其可实现的较低峰值 B 幅度和相关功率需求，相对于其未调制绝热全通过对应物，在高场临床磁共振波谱和波谱成像（MRS/MRSI）中越来越受到关注。在这项工作中，我们描述了两种 GOIA RF 脉冲的开发：1）一种具有高效率、3.0ms 宽带均匀速率和平滑截断（WURST）调制的 RF 脉冲，带宽为 15kHz，与临床可行的峰值 B 幅度 0.87kHz（或 20μT）兼容，以及 2）一种高度选择性的不对称 6.66ms RF 脉冲，带宽为 20kHz，旨在实现单边、仅 1.7%的分数转换宽度。通过模拟和实验评估了潜在的 RF 和梯度调制（GM）波形之间的异步对 3ms GOIA-WURST RF 脉冲的影响。结果表明，当使用大 RF 偏移量实现平移时，RF 和 GM 功能之间 20+µs 的异步会大大降低反转性能。提出了一种基于投影的方法，可快速校准临床前/临床 MR 系统中的 RF 和 GM 异步。不对称 GOIA 脉冲在多脉冲 OVS 序列中实现，用于实现颅外脂质抑制的高效、高选择性和 B 和 T 独立的信号抑制。所开发的 GOIA 脉冲与线性梯度调制（X、Y、Z 梯度场）一起使用，并与二阶场调制（Z2、X2Y2 梯度场）一起使用，为 MRS 和 MRSI 采集提供了椭圆形感兴趣区域。所描述的两种 GOIA-RF 脉冲都具有重要的临床价值；具体而言，3.0ms GOIA-WURST 脉冲有益于实现短 TE sLASER 局部质子 MRS/MRSI 序列，而不对称 GOIA RF 脉冲在高度选择性的外体积信号抑制中具有应用，允许用全强度对颅外脂质附近的组织进行询问。
