White matter (WM) fiber bundles change dynamically with age. These changes could be driven by alterations in axonal diameter, axonal density, and myelin content. In this study, we applied a novel fixel‐based analysis (FBA) framework to examine these changes throughout the adult lifespan. Using diffusion‐weighted images from a cohort of 293 healthy volunteers (89 males/204 females) from ages 21 to 86 years old, we performed FBA to analyze age‐related changes in microscopic fiber density (FD) and macroscopic fiber morphology (fiber cross section [FC]). Our results showed significant and widespread age‐related alterations in FD and FC across the whole brain. Interestingly, some fiber bundles such as the anterior thalamic radiation, corpus callosum, and superior longitudinal fasciculus only showed significant negative relationship with age in FD values, but not in FC. On the other hand, some segments of the cerebello‐thalamo‐cortical pathway only showed significant negative relationship with age in FC, but not in FD. Analysis at the tract‐level also showed that major fiber tract groups predominantly distributed in the frontal lobe (cingulum, forceps minor) exhibited greater vulnerability to the aging process than the others. Differences in FC and the combined measure of FD and cross section values observed between sexes were mostly driven by differences in brain sizes although male participants tended to exhibit steeper negative linear relationship with age in FD as compared to female participants. Overall, these findings provide further insights into the structural changes the brain's WM undergoes due to the aging process.

中文翻译：

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成年期白质纤维密度和形态的变化：基于固定素的横截面分析。

白质 (WM) 纤维束随年龄动态变化。这些变化可能是由轴突直径、轴突密度和髓鞘含量的改变驱动的。在这项研究中，我们应用了一种新的基于固定素的分析 (FBA) 框架来检查整个成人生命周期中的这些变化。使用来自 21 至 86 岁的 293 名健康志愿者（89 名男性/204 名女性）队列的扩散加权图像，我们进行了 FBA 以分析显微纤维密度 (FD) 和宏观纤维形态（纤维交叉[FC] 部分）。我们的结果显示整个大脑中 FD 和 FC 的显着且广泛的与年龄相关的改变。有趣的是，一些纤维束如前丘脑放射、胼胝体、和上纵束仅在 FD 值中与年龄呈显着负相关，而在 FC 中则没有。另一方面，小脑-丘脑-皮质通路的某些部分仅在 FC 中与年龄呈显着负相关，而在 FD 中则没有。纤维束水平的分析还表明，主要分布在额叶（扣带、小钳子）的主要纤维束组比其他纤维束更容易受到衰老过程的影响。FC 的差异以及在两性之间观察到的 FD 和横截面值的组合测量值主要是由大脑大小的差异驱动的，尽管与女性参与者相比，男性参与者倾向于表现出与 FD 年龄更陡峭的负线性关系。总体而言，这些发现提供了对大脑结构变化的进一步见解。