Introduction
Gait impairments have been reported with Alzheimer's disease (AD)\cite{Della2004,Higuma_2013,Sheridan_2003}. Though AD is considered a disease of the gray matter (GM), emerging research indicates that white matter (WM) damage is more severe than originally estimated (\cite{Caso2015,Sachdev2013}. The ediopathology of the movement impairment in AD is unclear. A number of factors may be involved that may or may not include the white matter. Deficits could be the result of inability to plan movement due to loss of cognitive function, or may be due to pathology in motor areas of the brain, such as the basal ganglia, or due to neuropathologies in motor pathways.
The purpose of this study to use tractography to determine whether neural pathways directly involved in movement control are compromised in AD patients. Fractional anisotropy (FA) is a measure of overall directionality and is typically higher in WM along axons. FA is also highly sensitive to microstructural changes and is used as an indicator of compromise in WM tracts. Lower FA values in motor pathways, such as the corticospinal tract and thalamic radiations, may provide insights into gait impairments in AD patients. Mean diffusivity (MD) is an inverse measure of membrane density, and while MD values are similar in both GM and WM, they are sensitive to cellular necrosis so that higher MD values may indicate axonal degeneration. For comparison, FA and MD values in non-motor WM tracts will also be considered.
Methods
Participants
Data for this study is from a subset of individuals who participated in the European DTI Study on Dementia \cite{Brueggen_2017}, a multi-center study which included patients diagnosed with either Alzheimer's Disease (AD) or mild cognitive impairment (MCI), as well as healthy controls (HC). The data included here consists of neuroimages from 26 such individuals--10 individuals (8 female, 2 male), ages 63-85 (avg. = 75.1 yrs., SD = 6.69), diagnosed with AD, and 16 (9 female, 7 male) healthy controls, ages 53-72 (avg. = 64.38 yrs., SD = 6.11).
MRI Acquisitions
The MRI data were acquired using a 3T Siemens TIM-Trio (Siemens Medical Inc., Erlangen, Germany). Scans included T1-weighted structural images and T2 diffusion-weighted images (DWI).