A recent method, denoted in vivo g-ratio-weighted imaging, has related the microscopic g-ratio, only accessible by ex vivo histology, to noninvasive MRI markers for the fiber volume fraction (FVF) and myelin volume fraction (MVF). Different MRI markers have been proposed for g-ratio weighted imaging, leaving open the question which combination of imaging markers is optimal. To address this question, the repeatability and comparability of four g-ratio methods based on different combinations of, respectively, two imaging markers for FVF (tract-fiber density, TFD, and neurite orientation dispersion and density imaging, NODDI) and two imaging markers for MVF (magnetization transfer saturation rate, MT, and, from proton density maps, macromolecular tissue volume, MTV) were tested in a scan-rescan experiment in two groups. Moreover, it was tested how the repeatability and comparability were affected by two key processing steps, namely the masking of unreliable voxels (e.g., due to partial volume effects) at the group level and the calibration value used to link MRI markers to MVF (and FVF). Our data showed that repeatability and comparability depend largely on the marker for the FVF (NODDI outperformed TFD), and that they were improved by masking. Overall, the g-ratio method based on NODDI and MT showed the highest repeatability (90%) and lowest variability between groups (3.5%). Finally, our results indicate that the calibration procedure is crucial, for example, calibration to a lower g-ratio value (g = 0.6) than the commonly used one (g = 0.7) can change not only repeatability and comparability but also the reported dependency on the FVF imaging marker. Hum Brain Mapp 39:24-41, 2018. © 2017 Wiley Periodicals, Inc.

译文

最近的一种称为体内g比加权成像的方法已将仅可通过离体组织学访问的微观g比与纤维体积分数 (FVF) 和髓鞘体积分数 (MVF) 的无创MRI标记相关联。对于g比加权成像,已经提出了不同的MRI标记,从而解决了哪种成像标记组合最佳的问题。为了解决这个问题,分别基于FVF的两个成像标记 (tract-fiber density,TFD和神经突取向色散和密度成像,NODDI) 和MVF的两个成像标记 (磁化传递饱和率,MT,并且,从质子密度图,大分子组织体积,MTV) 在两组的扫描-扫描实验中进行了测试。此外,测试了可重复性和可比性如何受到两个关键处理步骤的影响,即在组水平掩蔽不可靠的体素 (例如,由于部分体积效应) 以及用于将MRI标记链接到MVF (和FVF) 的校准值。我们的数据表明,可重复性和可比性在很大程度上取决于FVF的标记 (NODDI优于TFD),并且通过掩蔽可以改善它们。总体而言,基于NODDI和MT的g比方法显示出最高的可重复性 (90%) 和组之间的最低变异性 (3.5%)。最后,我们的结果表明,校准过程是至关重要的,例如,校准到一个比常用的 (g   =   0.6) 更低的g比值 (g   =   0.7) 不仅可以改变可重复性和可比性,还可以改变报告的对FVF成像标记的依赖性。哼脑马普39:24-41,2018。©2017威利期刊公司

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