The Edmond and Lily Safra Center for Brain Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel.
Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, USA; Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA.
Neuroimage. 2018 Nov 15;182:304-313. doi: 10.1016/j.neuroimage.2017.06.076. Epub 2017 Jun 30.
Recent years have seen a growing interest in relating MRI measurements to the structural-biophysical properties of white matter fibers. The fiber g-ratio, defined as the ratio between the inner and outer radii of the axon myelin sheath, is an important structural property of white matter, affecting signal conduction. Recently proposed modeling methods that use a combination of quantitative-MRI signals, enable a measurement of the fiber g-ratio in vivo. Here we use an MRI-based g-ratio estimation to observe the variance of the g-ratio within the corpus callosum, and evaluate sex and age related differences. To estimate the g-ratio we used a model (Stikov et al., 2011; Duval et al., 2017) based on two different WM microstructure parameters: the relative amounts of myelin (myelin volume fraction, MVF) and fibers (fiber volume fraction, FVF) in a voxel. We derived the FVF from the fractional anisotropy (FA), and estimated the MVF by using the lipid and macromolecular tissue volume (MTV), calculated from the proton density (Mezer et al., 2013). In comparison to other methods of estimating the MVF, MTV represents a stable parameter with a straightforward route of acquisition. To establish our model, we first compared histological MVF measurements (West et al., 2016) with the MRI derived MTV. We then implemented our model on a large database of 92 subjects (44 males), aged 7 to 81, in order to evaluate age and sex related changes within the corpus callosum. Our results show that the MTV provides a good estimation of MVF for calculating g-ratio, and produced values from the corpus callosum that correspond to those found in animals ex vivo and are close to the theoretical optimum, as well as to published in vivo data. Our results demonstrate that the MTV derived g-ratio provides a simple and reliable in vivo g-ratio-weighted (GR*) measurement in humans. In agreement with theoretical predictions, and unlike other tissue parameters measured with MRI, the g-ratio estimations were found to be relatively stable with age, and we found no support for a significant sexual dimorphism with age.
近年来，人们越来越关注将 MRI 测量值与白质纤维的结构生物物理特性联系起来。纤维 g-ratio 定义为轴突髓鞘的内半径与外半径之比，是白质的一个重要结构特性，影响信号传导。最近提出的使用定量 MRI 信号组合的建模方法，能够在体内测量纤维 g-ratio。在这里，我们使用基于 MRI 的 g-ratio 估计来观察胼胝体内部 g-ratio 的变化，并评估性别和年龄相关的差异。为了估计 g-ratio，我们使用了一种基于两个不同的 WM 微观结构参数的模型（Stikov 等人，2011 年；Duval 等人，2017 年）：一个体素内髓鞘的相对量（髓鞘体积分数，MVF）和纤维（纤维体积分数，FVF）。我们从各向异性分数（FA）中推导出 FVF，并通过质子密度（Mezer 等人，2013 年）计算出的脂质和大分子组织体积（MTV）来估计 MVF。与其他估计 MVF 的方法相比，MTV 是一种稳定的参数，获取方法简单直接。为了建立我们的模型，我们首先将组织学 MVF 测量值（West 等人，2016 年）与 MRI 衍生的 MTV 进行了比较。然后，我们在一个由 92 名受试者（44 名男性）组成的大型数据库上实施了我们的模型，这些受试者年龄在 7 至 81 岁之间，以评估胼胝体内部的年龄和性别相关变化。我们的结果表明，MTV 为计算 g-ratio 提供了 MVF 的良好估计值，并为胼胝体提供了与动物离体值相对应的值，并且接近理论最佳值，也与已发表的体内数据相接近。我们的结果表明，MTV 衍生的 g-ratio 为人类提供了一种简单可靠的体内 g-ratio 加权（GR*）测量方法。与理论预测一致，与其他用 MRI 测量的组织参数不同，g-ratio 估计值随年龄的变化相对稳定，我们没有发现年龄相关的显著性别二态性的支持证据。
