Abstract
This study aimed to characterize age-related white matter changes by evaluating patterns of overlap between the linear association of age with fractional anisotropy (FA) with mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD). Specifically, we assessed patterns of overlap between diffusion measures of normal appearing white matter by covarying for white matter hyperintensity (WMH) load, as WMHs are thought to increase with age and impact diffusion measures. Seventy-nine healthy adults aged between 18 and 75 years took part in the study. Diffusion tensor imaging (DTI) data were based on 61 directions acquired with a b-value of 2,000. We found five main patterns of overlap: FA alone (15.95%); FA and RD (31.90%); FA and AD (12.99%); FA, RD, and AD (27.93%); and FA, RD, and MD (8.79%). We showed that cognitively healthy aging adults had low WMH load, which subsequently had minimal effect on diffusion measures. We discuss how patterns of overlap may reflect underlying biological changes observed with aging such as loss of myelination, axonal damage, as well as mild microstructural and chronic white matter impairments. This study contributes to understanding the underlying causes of degeneration in specific regions of the brain and highlights the importance of considering the impact of WMHs in aging studies of white matter.
Highlights
Diffusion tensor imaging (DTI) is a neuroimaging technique, which allows for non-invasive, in vivo, investigation of white matter [1,2,3]
We investigated other diffusion measures extracted from data preprocessed in ExploreDTI such as mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity (AD)
All statistically significant linear correlations of diffusion measures with age reported in the results include white matter hyperintensity (WMH) load, NART scores and gender as covariates to account for any changes in diffusion measures of white matter that may be related to them
Summary
Diffusion tensor imaging (DTI) is a neuroimaging technique, which allows for non-invasive, in vivo, investigation of white matter [1,2,3]. DTI measures are based on random motion of water molecules, where within the brain diffusion of water is less restricted, or more isotropic, in areas of grey matter and CSF, and more restricted, or more anisotropic, in areas of white matter. The diffusion tensor is a 3x3 covariance matrix used to model diffusion within a voxel, in which there are 3 positive eigenvalues (λ1, λ2, λ3) and 3 orthogonal eigenvectors (ε1, ε2, ε3). The eigenvalues of the tensor give the diffusivity in the direction of each eigenvector. Together they describe diffusion probability using an ellipsoid, where the axes of the ellipsoid are aligned with the eigenvectors, and the major eigenvector (λ1) represents the principal diffusion direction
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