Keywords: Microstructure, Modelling, Axons, Diffusion, Validation, TBI, Segmentation, Electron microscopy
Motivation: Interpreting diffusion MRI (dMRI) in terms of brain tissue micro-geometry.
Goal(s): To identify cellular features that govern diffusion measurements among myriads of parameters specifying tissue microstructure.
Approach: We found how axonal micro-geometry is manifested in a dMRI measurement by analytically solving the diffusion equation in a tube with a randomly varying cross-section.
Results: We identify a specific power-law approach of the time-dependent diffusion coefficient along the axon to its long-time limit. The average inverse cross-section and the variance of long-range cross-sectional fluctuations govern the diffusive dynamics. We quantify changes in these non-trivial geometrical features, associated with axonal beading, in a rat TBI model.
Impact: Beading is a characteristic feature of numerous neurodegenerative diseases triggered by different pathological conditions and injuries. Here, we detect geometrical changes in the axonal micro-geometry, accessible via the along-tract diffusivity from the diffusion tensor using clinically feasible diffusion weightings.
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