Visualizing and characterizing white matter fiber structure and architecture in the human pyramidal tract using diffusion tensor MRI

We used diffusion tensor imaging to assess diffusion anisotropy in the pyra midal tract in ten young, and ten elderly subjects (five males and five fem ales in each group). The purpose of this study was to define normative valu es for anisotropy at different anatomic levels of the brainstem as well as to assess differences due to age, gender, and laterality. In all subjects, anisotropy was highest in the cerebral peduncle, lowest in the caudal pens, and intermediate in the medulla, In the pens and medulla the regional vari ability was high, with significant differences in anisotropy even between c ontiguous slices. Multifactorial ANOVA (performed using the average value o f anisotropy within each region of interest) revealed that elderly subjects had significantly lower values than young subjects in the cerebral peduncl e, with no differences in the pens and medulla, No significant differences in anisotropy due to gender and side were found. The differences in anisotr opy at different levels of the brainstem reflect differences in the local a rchitecture of white matter fibers. Anisotropy is high in the cerebral pedu ncle because fibers have a highly ordered arrangement, while in the pens an d medulla, anisotropy is lower because the local fiber architecture is less coherent due to the presence of other fibers and nuclei. The biologic mean ing of the intergroup differences in anisotropy is discussed in light of th e structure and architecture of the tissue under investigation. We also con sider potential sources of,artifacts, such as noise and motion, partial vol ume contamination, anatomic mismatching, and the use of inappropriate stati stical tests. We conclude that the age-related decrease in anisotropy in th e cerebral peduncle is not artifactual but rather reflects subtle structura l changes of the aging white matter, Our study however shows that caution m ust be exercised in interpreting diffusion anisotropy data, (C) 1999 Elsevi er Science Inc.