Tract-specific g-ratio using COMMIT: comparison with conventional g-ratio tractometry

Motivation: Tractometry is used to estimate the microstructural properties of white matter tracts from volumetric images. However, it has significant limitations due to multi-fiber voxels that bias tract measurements. Goal(s): We aim to estimate the tract-specific g-ratio, a ratio of the inner and outer radius of the myelin sheath. Approach: Building on the COMMIT framework, we disentangle the microstructural features of individual white matter tracts to estimate tract-specific g-ratio. Results: Tract-specific g-ratio had higher contrast between tracts and had a stronger correlation with tract caliber (i.e. the axonal cross-sectional area between two nodes derived from COMMIT) and length in comparison to tractometry. Impact: By using this novel COMMIT-based pipeline to analyze diffusion and myelin-sensitive MRI data, we anticipate that tract-specific g-ratio measures will be more sensitive to subtle differences in g-ratio across tracts and individuals due to the elimination of partial volume effects.