認知神經老化對大腦灰質體積與白質結構的影響

Abstract

在這篇論文中，我們利用解剖性 （structural）與擴散性（diffusion）核磁共振造影技術（Magnetic Resonance Imaging）探討生物老化歷程對大腦結構的影響。結果發現，年長者和年輕人的不同腦區灰質的體積，在左右半球的不對稱性上有不少差異。大腦的前部體積減少較後部明顯，這項觀察結果跟大部份的文獻一致，但我們觀察到大腦的運動區及感覺區也有體積的萎縮，這兩個區域的體積在過去的文獻中，常被認為是不受老化的影響。這說明了隨著年紀漸增，大腦體積雖然會減少，但個別腦區的體積衰退並不是均勻一致的，每個腦區受到老化的影響並不相同。此外，運用擴散性張量磁振造影（Diffusion Tensor Imaging）技術分析了老化對大腦白質纖維束完整性的影響，我們選擇的是“部分非等向性（Fractional anisotropy, FA）”指標，這個指標數值代表白質中神經纖維束的傳遞效率，數值愈高，傳遞速率愈佳。結果發現，老化對大腦白質纖維束的影響，在腦區前部較腦區後部為多，右腦受到的影響也較左腦大。此外，進一步將胼胝體分為十區，用tractography分析了各區域中神經纖維束連結的來源，發現胼胝體的神經纖維束來源雖然有前、後區的差異，但是老年人顯示出比年輕人更大的個別差異。本篇論文提供生物老化影響個別區域大腦灰質體積與白質結構的證據，並觀察到顯著的個別差異，我們認為這些腦造影結果部分支持老化的右腦衰退理論。

In this thesis, we utilized structural and diffusion magnetic resonance imaging (MRI) techniques to investigate age-related changes in gray-matter brain volumes and white matter microstructural integrity. We reported eleven cortical regions to exhibit volumetric hemispheric asymmetry between young and old groups. We also found that age-related brain volume shrinkage appears to be greater in the anterior part of the aging brain and more moderate in the posterior part of the aging brain, consistent with previous findings. However, we found significant volumetric changes within precentral and postcentral gyrus which have been identified to be related motor and sensory functions. These two areas are often reported to be preserved with age. Moreover, examining white matter microstructure by using diffusion tensor imaging (MRI) to examine white matter microstructural integrity. In voxel-wise fractional anisotropy (FA) analysis, we found significantly larger FA values in the anterior than the posterior part of the brain and larger FA values in right hemisphere than those in the left hemisphere. Finally, the results of probabilistic tractography indicates that, despite integral fibres of anterior part of corpus callosum mainly stem from anterior part of the cortex, we reported greater individual differences in older adults compared to young adults. In sum, these results provide neuroimaging evidence of age-related volumetric changes in gray-matter and microstructural variations in white-matter. The findings are partially consistent with the hypothesis of right hemisphere decline in aging.