University of British Columbia MRI Research Centre, 2221 Wesbrook Mall, M10 Purdy Pavilion, Vancouver, BC V6T 2B5, Canada.
Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada; Radiology, University of British Columbia, 2775 Laurel Street, 11th Floor, Vancouver, BC V5Z 1M9, Canada; ICORD (International Collaboration on Repair Discoveries), 818 W. 10th Ave., Vancouver, BC V5Z 1M9, Canada.
Neuroimage. 2021 Aug 1;236:118046. doi: 10.1016/j.neuroimage.2021.118046. Epub 2021 Apr 20.
The promise of inhomogeneous magnetization transfer (ihMT) as a new myelin imaging method was studied in ex vivo human brain tissue and in relation to myelin water fraction (MWF). The temperature dependence of both methods was characterized, as well as their correspondence with a histological measure of myelin content. Unfiltered and filtered ihMT protocols were studied by adjusting the saturation scheme to preserve or attenuate signal from tissue with short dipolar relaxation time T.
ihMT ratio (ihMTR) and MWF maps were acquired at 7 T from formalin-fixed human brain samples at 22.5 °C, 30 °C and 37 °C. The impact of temperature on unfiltered ihMTR, filtered ihMTR and MWF was investigated and compared to myelin basic protein staining.
Unfiltered ihMTR exhibited no temperature dependence, whereas filtered ihMTR increased with increasing temperature. MWF decreased at higher temperature, with an increasing prevalence of areas where the myelin water signal was unreliably determined, likely related to a reduction in T peak separability at higher temperatures ex vivo. MWF and ihMTR showed similar per-sample correlation with myelin staining at room temperature. At 37 C, filtered ihMTR was more strongly correlated with myelin staining and had increased dynamic range compared to unfiltered ihMTR.
Given the temperature dependence of filtered ihMT, increased dynamic range, and strong myelin specificity that persists at higher temperatures, we recommend carefully controlled temperatures close to 37 °C for filtered ihMT acquisitions. Unfiltered ihMT may also be useful, due to its independence from temperature, higher amplitude values, and sensitivity to short T components. Ex vivo myelin water imaging should be performed at room temperature, to avoid fitting issues found at higher temperatures.
本研究旨在探讨非均匀磁化转移（ihMT）作为一种新的髓鞘成像方法的潜力，并与髓鞘水分数（MWF）相关联。本研究对这两种方法的温度依赖性进行了特征描述，并研究了它们与髓鞘含量的组织学测量的对应关系。本研究通过调整饱和方案来研究未过滤和过滤的 ihMT 方案，以保留或衰减具有短偶极弛豫时间 T 的组织的信号。
在 7T 下，本研究从福尔马林固定的人脑样本中，在 22.5°C、30°C 和 37°C 下采集 ihMT 比（ihMTR）和 MWF 图。研究了温度对未过滤 ihMTR、过滤 ihMTR 和 MWF 的影响，并与髓鞘碱性蛋白染色进行了比较。
未过滤 ihMTR 没有表现出温度依赖性，而过滤 ihMTR 随着温度的升高而增加。MWF 在较高温度下降低，并且越来越多的区域中髓鞘水信号无法可靠确定，这可能与较高温度下体外 T 峰可分离性降低有关。MWF 和 ihMTR 在室温下与髓鞘染色具有相似的样本间相关性。在 37°C 时，过滤 ihMTR 与髓鞘染色的相关性更强，并且与未过滤 ihMTR 相比具有更大的动态范围。
鉴于过滤 ihMT 的温度依赖性、更高的动态范围以及在较高温度下保持的强烈髓鞘特异性，我们建议在接近 37°C 的温度下对过滤 ihMT 进行仔细控制。由于未过滤 ihMT 对温度不敏感，幅度值更高，并且对短 T 成分敏感，因此也可能有用。体外髓鞘水成像应在室温下进行，以避免在较高温度下出现拟合问题。
