Imaging Institute, Cleveland Clinic, 9500 Euclid Ave, Cleveland, OH 44195, USA; Division of Biomedical Engineering, Hankuk University of Foreign Studies, Yongin, Republic of Korea.
Imaging Institute, Cleveland Clinic, 9500 Euclid Ave, Cleveland, OH 44195, USA.
Neuroimage. 2018 Mar;168:242-249. doi: 10.1016/j.neuroimage.2016.09.009. Epub 2016 Sep 12.
As the use of ultra-high field (UHF; ≥7T) magnetic resonance (MR) imaging expands, there is an increasing need to establish high-resolution MR imaging protocols for patients with neurological disease. Magnetization transfer (MT) imaging has been used to provide information about changes in the magnitude of the restricted protons that are caused by tissue damages. Several studies have found that MTR has a good sensitivity to measure changes in myelin concentration within the brain. Because of the much higher specific absorption rate (SAR) of tissue and longer acquisition time required for UHF, however, in-vivo studies using conventional pulsed MT sequences at UHF have not been well utilized. In this study, we introduce a new MT data acquisition approach using a 7T MR system, variable density magnetization transfer (vdMT) imaging, which can be reasonably included in a routine patient scan protocol with a much shorter scan time and reduced discomfort to the patient. To reduce SAR and scan time while maintaining at least similar MTR image quality to that obtained with the conventional method, a higher density of MT RF pulses was applied in the center of k-space, and sparsely applied MT RF pulses were used in the outer part of k-space. The fraction of k-space receiving 100% MT RF density and TR were optimized based on in-vivo ROI analysis, and results were confirmed with high-resolution MTR map using a vdMT approach from healthy controls and patients with multiple sclerosis (MS). The experimental results confirmed that vdMT imaging can cover a whole brain volume in an acceptable scan time for routine patient scans while providing MTR image quality at least similar to that obtained with conventional MT imaging (correlation coefficient=0.95 in an agar-gel phantom [MT offset frequency=1kH], 0.90 in a postmortem MS brain, and 0.85 in the 4 healthy volunteers). MS lesions were associated with signal reductions in the postmortem MS brains and in the patients with MS. In this study, we have described a new approach for acquiring high-resolution MTR map of the whole brain volume using 7T MR imaging. This vdMT method provides similar image quality to that obtained with the conventional method, and shortens the scan time by reducing SAR. These results suggest that vdMT approach is a method that could be used for UHF scans of patients with neurological disease.
随着超高磁场（UHF；≥7T）磁共振（MR）成像的应用不断扩大，为神经疾病患者建立高分辨率 MR 成像协议的需求日益增加。磁化转移（MT）成像是一种用于提供组织损伤导致的受限质子幅度变化信息的方法。多项研究发现，MTR 对测量脑内髓鞘浓度的变化具有很好的敏感性。然而，由于 UHF 下组织的比吸收率（SAR）高得多，并且采集时间更长，因此，在 UHF 下使用传统脉冲 MT 序列的体内研究尚未得到很好的应用。在这项研究中，我们介绍了一种新的 MT 数据采集方法，使用 7T MR 系统，可变密度磁化转移（vdMT）成像，可以合理地包含在常规患者扫描协议中，扫描时间更短，患者不适感降低。为了在保持与传统方法相似的 MTR 图像质量的同时降低 SAR 和扫描时间，在 k 空间的中心应用了更高密度的 MT RF 脉冲，在 k 空间的外部稀疏地应用了 MT RF 脉冲。基于体内 ROI 分析，优化了接收 100% MT RF 密度和 TR 的 k 空间分数，并使用来自健康对照者和多发性硬化症（MS）患者的 vdMT 方法的高分辨率 MTR 图对结果进行了验证。实验结果证实，vdMT 成像可以在可接受的常规患者扫描时间内覆盖整个脑体积，同时提供至少与传统 MT 成像相当的 MTR 图像质量（在琼脂凝胶体模中 [MT 偏移频率=1kH] 的相关系数=0.95，在死后 MS 脑内的相关系数=0.90，在 4 名健康志愿者中的相关系数=0.85）。MS 病变与死后 MS 脑和 MS 患者的信号降低有关。在这项研究中，我们描述了一种使用 7T MR 成像获取整个脑体积高分辨率 MTR 图的新方法。这种 vdMT 方法提供与传统方法相似的图像质量，并通过降低 SAR 缩短扫描时间。这些结果表明，vdMT 方法是一种可用于神经疾病患者 UHF 扫描的方法。
