[IC‐P‐161]: MEAN DIFFUSIVITY IN CORTICAL GRAY MATTER IN ALZHEIMER's DISEASE: THE IMPORTANCE OF PARTIAL VOLUME CORRECTION

Abstract

Mean diffusivity (MD) measured by diffusion tensor imaging can reflect microstructural alterations of the brain's gray matter (GM). Therefore, GM MD may be a sensitive marker of neurodegeneration related to Alzheimer's Disease (AD). However, due to partial volume effects (PVE), differences in MD may be overestimated because of a higher degree of brain atrophy in AD patients and in cases with mild cognitive impairment (MCI). Here, we evaluated GM MD changes in AD and MCI compared with healthy controls, and the effect of partial volume correction (PVC) on diagnostic utility of MD.We determined region of interest (ROI) and voxel-wise group differences and diagnostic accuracy of MD and volume measures between matched samples of 39 AD, 39 MCI and 39 healthy subjects before and after PVC. Additionally, we assessed whether effects of GM MD values on diagnosis were mediated by volume.ROI and voxel-wise group differences were reduced after PVC. When using these ROIs for predicting group separation in logistic models, both PVE corrected and uncorrected GM MD values yielded a poorer diagnostic accuracy in single predictor models than regional volume. For the discrimination of AD patients and healthy controls, the effect of GM MD on diagnosis was significantly mediated by volume of hippocampus and posterior cingulate ROIs.Our results suggest that GM MD measurements are strongly confounded by PVE in the presence of brain atrophy, underlining the necessity of PVC when using these measurements as specific metrics of microstructural tissue degeneration. Independently of PVC, regional MD was not superior to regional volume in separating prodromal and clinical stages of AD from healthy controls.

Cortical microstructure in young onset Alzheimer's disease using neurite orientation dispersion and density imaging

“…Associations between cortical thickness and diffusion imaging metrics have been reported in brain regions associated with atrophy in AD previously (Jacobs et al, ). One potential interpretation of such results is that the decreases in cortical thickness may result in decreases in diffusion metrics because of partial volume effects rather than a purely biological association (Henf et al, ; Jeon et al, ). This has been a longstanding criticism of diffusion MRI studies focusing on the cerebral cortex in neurodegenerative disease (Weston et al, ), given the thin nature of cortex and the limited spatial resolution of diffusion MRI acquisition sequences (2.5 mm × 2.5 mm × 2.5 mm voxel size in this study) leading to increased risk of contamination with non‐grey‐matter voxels.…”

“…This and related modelling approaches in the context of grey matter provides information regarding the structural organization of neuronal dendritic trees. Furthermore, the capacity to model the fraction of free water reduces the risk of partial volume effects from CSF contamination influencing NODDI metrics, something which is not explicitly considered within the DTI model (Zhang et al, ) and often significantly influence DTI based studies of grey matter in AD (Henf, Grothe, Brueggan, Teipel, & Dyrba, ; Jeon et al, ). Differences in cortical NODDI metrics have been reported in ageing (Nazeri et al, ), Parkinson's disease (Kamagata et al, ), schizophrenia (Nazeri et al, ), multiple sclerosis (Granberg et al, ), and transgenic murine models of AD tauopathy (Colgan et al, ).…”

Integrated magnetic resonance imaging and [11C]‐PBR28 positron emission tomographic imaging in amyotrophic lateral sclerosis

“…These diffusion changes probably represent the loss of gray and white matter integrity in the motor regions in ALS and PLS. Mean diffusivity measures were used to evaluate gray matter integrity in other neurodegenerative diseases . The diffusion results (ie, increased diffusivity and decreased fractional anisotropy) are indicators of the neuronal damage seen in ALS and PLS.…”