Department of Chemical and Biological Physics, The Weizmann Institute of Science, Rehovot, Israel.
Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University Grossman School of Medicine, New York, New York, USA.
NMR Biomed. 2021 Jun;34(6):e4507. doi: 10.1002/nbm.4507. Epub 2021 Mar 23.
H-MRSI is commonly performed with gradient phase encoding, due to its simplicity and minimal radio frequency (RF) heating (specific absorption rate). Its two well-known main problems-(i) "voxel bleed" due to the intrinsic point-spread function, and (ii) chemical shift displacement error (CSDE) when slice-selective RF pulses are used, which worsens with increasing volume of interest (VOI) size-have long become accepted as unavoidable. Both problems can be mitigated with Hadamard multislice RF encoding. This is demonstrated and quantified with numerical simulations, in a multislice phantom and in five healthy young adult volunteers at 3 T, targeting a 2-cm thick temporal lobe VOI through the bilateral hippocampus. This frequently targeted region (e.g. in epilepsy and Alzheimer's disease) is subject to strong, 1-2 ppm.cm regional B0, susceptibility gradients that can dramatically reduce the signal-to-noise ratio (SNR) and water suppression effectiveness. The chemical shift imaging (CSI) sequence used a 3-ms Shinnar-Le Roux (SLR) 90° RF pulse, acquiring eight steps in the slice direction. The Hadamard sequence acquired two overlapping slices using the same SLR 90° pulses, under twofold stronger gradients that proportionally halved the CSDE. Both sequences used 2D 20 × 20 rosette spectroscopic imaging (RSI) for in-plane spatial localization and both used RF and gradient performance characteristics that are easily met by all modern MRI instruments. The results show that Hadamard spectroscopic imaging (HSI) suffered dramatically less signal bleed within the VOI compared with CSI (<1% vs. approximately 26% in simulations; and 5%-8% vs. >50%) in a phantom specifically designed to test these effects. The voxels' SNR per unit volume per unit time was also 40% higher for HSI. In a group of five healthy volunteers, we show that HSI with in-plane 2D-RSI facilitates fast, 3D multivoxel encoding at submilliliter spatial resolution, over the bilateral human hippocampus, in under 10 min, with negligible CSDE, spectral and spatial contamination and more than 6% improved SNR per unit time per unit volume.
H-MRSI 通常采用梯度相位编码，因为它简单且射频（RF）加热（比吸收率）最小。其两个众所周知的主要问题 - （i）由于固有的点扩散函数导致的“体素渗血”，以及（ii）当使用切片选择性 RF 脉冲时的化学位移位移误差（CSDE），随着感兴趣体积（VOI）尺寸的增加而恶化 - 长期以来被认为是不可避免的。这两个问题都可以通过 Hadamard 多切片 RF 编码来缓解。在多切片体模和五名健康年轻成年人志愿者中，在 3T 下进行了演示和量化，通过双侧海马体对 2cm 厚的颞叶 VOI 进行靶向。这个经常靶向的区域（例如在癫痫和阿尔茨海默病中）受到强烈的 1-2ppm.cm 局部 B0、磁化率梯度的影响，这些梯度会极大地降低信号噪声比（SNR）和水抑制效果。化学位移成像（CSI）序列使用 3ms Shinnar-Le Roux（SLR）90°RF 脉冲，在切片方向上采集 8 个步骤。Hadamard 序列使用相同的 SLR 90°脉冲在两个重叠切片中采集数据，使用两倍强的梯度，CSDE 成比例减半。两个序列都使用 2D 20×20 梅花形光谱成像（RSI）进行平面内空间定位，并且都使用很容易满足所有现代 MRI 仪器的 RF 和梯度性能特性。结果表明，Hadamard 光谱成像（HSI）在 VOI 内的信号渗血明显少于 CSI（<1%比模拟中约 26%；5%-8%比>50%）；在专门设计用于测试这些效果的体模中。HSI 的体素 SNR 也比 CSI 高 40%。在五名健康志愿者中，我们展示了 HSI 与平面内 2D-RSI 相结合，在不到 10 分钟的时间内，以亚毫升级空间分辨率，在双侧人类海马体上进行快速、3D 多体素编码，具有可忽略的 CSDE、光谱和空间污染以及超过 6%的 SNR 改善。
