Department of Medical Biophysics, Western University, London, Ontario, Canada.
Center for Functional and Metabolic Mapping, Western University, London, Ontario, Canada.
Magn Reson Med. 2023 Feb;89(2):756-766. doi: 10.1002/mrm.29473. Epub 2022 Oct 5.
Oscillating gradient spin-echo (OGSE) sequences have demonstrated an ability to probe time-dependent microstructural features, although they often suffer from low SNR due to increased TEs. In this work we introduce frequency-tuned bipolar (FTB) gradients as a variation of oscillating gradients with reduced TE and demonstrate their utility by mapping the frequency dispersion of kurtosis in human subjects.
An FTB oscillating gradient waveform is presented that provides encoding of 1.5 net oscillation periods, thereby reducing the TE of the acquisition. Simulations were performed to determine an optimal protocol based on the SNR of kurtosis frequency dispersion-defined as the difference in kurtosis between pulsed and oscillating gradient acquisitions. Healthy human subjects were scanned at 7T using pulsed gradient and an optimized 23 Hz FTB protocol, which featured a maximum b-value of 2500 s/mm . In addition, to directly compare existing methods, measurements using traditional cosine OGSE were also acquired.
FTB oscillating gradients demonstrated equivalent frequency-dependent diffusion measurements compared with cosine-modulated OGSE while enabling a significant reduction in TE. Optimization and in vivo results suggest that FTB gradients provide increased SNR of kurtosis dispersion maps compared with traditional cosine OGSE. The optimized FTB gradient protocol demonstrated consistent reductions in apparent kurtosis values and increased diffusivity in generated frequency dispersion maps.
This work presents an alternative to traditional cosine OGSE sequences, enabling more time-efficient acquisitions of frequency-dependent diffusion quantities as demonstrated through in vivo kurtosis frequency dispersion maps.
虽然摆动梯度自旋回波（OGSE）序列已经证明了探测时变微观结构特征的能力，但由于 TE 增加，它们通常会受到低 SNR 的影响。在这项工作中，我们引入了频率调谐双极（FTB）梯度作为摆动梯度的一种变体，通过在人体受试者中绘制峰度的频率色散来证明其效用。
提出了一种 FTB 摆动梯度波形，该波形提供了 1.5 个净振荡周期的编码，从而减少了采集的 TE。进行了模拟以根据峰度频率色散的 SNR 确定最佳协议-定义为脉冲和摆动梯度采集之间的峰度差异。在 7T 下使用脉冲梯度和优化的 23Hz FTB 协议对健康人体受试者进行了扫描，该协议的最大 b 值为 2500s/mm 。此外，为了直接比较现有方法，还采集了使用传统余弦 OGSE 的测量结果。
FTB 摆动梯度与余弦调制 OGSE 相比，在具有等效的频率依赖性扩散测量的同时，能够显著减少 TE。优化和体内结果表明，与传统的余弦 OGSE 相比，FTB 梯度提供了更高的峰度弥散图的 SNR。优化的 FTB 梯度协议在生成的频率色散图中显示出一致的表观峰度值降低和扩散性增加。
这项工作提出了一种替代传统余弦 OGSE 序列的方法，通过在体内峰度频率色散图中证明了更高效的时间依赖性扩散量采集，从而实现了这一目标。
