Mapping the aggregate g-ratio of white matter tracts using multi-modal MRI

Area of Science:

• Neuroimaging
• Neuroscience
• Biophysics

Background:

• The g-ratio, myelin inner-to-outer diameter ratio, influences action potential conduction speed.
• Multi-modal MRI enables in vivo mapping of g-ratio across the human brain.
• Conventional tractometry for g-ratio is limited by partial volume effects in voxels with multiple fiber populations.

Purpose of the Study:

• To introduce and validate a novel COMMIT-based tract-specific g-ratio mapping approach.
• To compare the COMMIT method against conventional tractometry in healthy adults.
• To assess the impact of the COMMIT approach on reducing biases from crossing white matter fibers.

Main Methods:

• Utilized the Convex Optimization Modeling for Microstructure Informed Tractography (COMMIT) framework.
• Derived tract-specific axonal and myelin volumes to compute aggregate g-ratio.
• Compared COMMIT-based g-ratio mapping with conventional tractometry in 10 healthy adults.

Main Results:

• The COMMIT approach preserves spatial distribution and enhances inter-tract contrast compared to tractometry.
• Scan-rescan data demonstrate high repeatability for medium to large caliber tracts.
• Short and large caliber tracts exhibit lower g-ratios with COMMIT, contrasting with tractometry findings.

Conclusions:

• The COMMIT-based tract-specific g-ratio mapping advances white matter analysis by mitigating partial volume effects.
• This technique provides a more accurate representation of myelin characteristics in complex fiber networks.
• The findings support the use of COMMIT for detailed investigation of white matter microstructure and its relation to function.