Hippocampus‐associated tract degradation follows differential DTI patterns and is influenced by amyloid in MCI patients

Abstract

AbstractBackgroundDiffusion tensor imaging (DTI) has been leveraged to understand microstructural alterations to limbic and cortico‐cortical tracts in Alzheimer’s disease (AD). However, investigation into the key changes and patterns of fractional anisotropy (FA) and mean diffusivity (MD) metrics in the early stages of MCI has not been widely investigated. This study aimed to establish whether the early DTI white matter tract changes in MCI patients are decoupled and whether amyloid positivity (Aβ+/Aβ‐) is associated with early tract degradation.Method171 participants (103 MCI & 73 CN) were recruited from the AD Neuroimaging Initiative (ADNI; adni.loni.usc.edu) database. For each participant, FA and MD maps were obtained at their baseline scan and voxel wise analysis was carried out using FSL (https://fsl.fmrib.ox.ac.uk/fsl/) and Tract‐Based Spatial Statistics (https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/TBSS). Students t‐tests were conducted on the FA and MD changes in hippocampal WM tracts, previously identified as areas of potential early degradation. Comparisons between CN and MCI groups were made along Aβ+ and Aβ‐ groups.ResultWe found evidence for WM degradation, indicated by lower FA scores in MCI participants compared with controls, in the specific regions of interest of the fornix (p = 0.004) and tapetum (p = 0.012). Mean diffusivity was increased in the MCI group compared to CN in areas of the fornix (p < 0.001), consistent with FA alterations, yet differed from areas identified in FA changes owing to changes in the uncinate fasciculus (p = 0.022) and cingulum (p = 0.003). Comparison of DTI metrics in the fornix between amyloid status groups revealed that Aβ+ participants had significantly lower FA compared to Aβ‐ participants (p = 0.047), but MD did not significantly differ.ConclusionThese results confirm the notion that WM tract integrity degradation in the tract apparatus surrounding the hippocampus occur early in the AD trajectory. However, results indicate that FA alterations occur in different patterns to that of MD and that amyloid may influence these patterns. This supports the idea that efforts to develop therapeutics which maintain WM tract integrity early may be key to slowing AD progression.