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White Matter Differences in Networks in Elders with Mild Cognitive Impairment and Alzheimer's Disease

BACKGROUND
Alzheimer's disease is associated with impairment of large -scale brain networks, disruption in structural connections and functional disconnection between distant brain regions. While decreased functional connectivity has been thoroughly investigated and reported by existing functional neuroimaging literature, the present study investigated network-based differences due to the structural changes in white matter pathways in Alzheimer's disease patients. We hypothesize that diffusion metrics of disrupted tracts that go through cognitive networks related with intrinsic awareness, motor movement and executive control can be utilized as biomarkers to distinguish prodromal stage from Alzheimer's disease stage.


METHODS
Diffusion MRI data of a total 154 subjects including patients with clinical Alzheimer's disease (n=47) and patients with mild cognitive impairment (n=107) was used. To study structural changes associated with white matter fiber pathways voxel-averaged diffusion metrics and fiber density metrics were calculated.


RESULTS
Study revealed that AD patients exhibit disruptions in intra-hemispheric tracts and projection fiber tracts as suggested by diffusion indices. Our whole brain analysis revealed that network differences within default mode network, sensory motor network and fronto-parietal networks are associated with disruption in Inferior fronto-occipital fasciculus, corticospinal tract and Superior longitudinal fasciculus.


CONCLUSION
Diffusion metrics appear to be more sensitive than fiber density metrics in differentiating the structural changes in the white matter. Decreased fractional anisotropy along with increased mean diffusivity and radial diffusivity in forceps minor, corticospinal tract and inferior fronto-occipital fasciculus as an imaging biomarker would be ideal to distinguish Alzheimer's disease patients from mild cognitive impairment patients.