Imaging the relationship of axon diameter and myelination in macaque and human brain

Motivation: Axon diameter and myelination are essential for conduction of action potentials and therefore related to brain function. However, the relationships between them in white matter (WM) across different species are not well understood. Goal(s): To investigate the relationship between axon diameter and myelination in human and macaque brain WM. Approach: We estimate axon diameter and myelin water fraction (MWF), and derive fiber g-ratio, using macaque and human brain data acquired on a preclinical scanner. Results: Microstructure parameters exhibit consistent patterns across WM tracts and species. Regions with smaller axons tend to have higher packing density and MWF; fiber g-ratio is relatively stable. Impact: The weak correlations between dMRI measures and MWF suggest they can provide complementary information about fiber morphology. The regional variations of these microstructure measures will be baseline for investigating changes in abnormal tissue conditions such as demyelination and axonal loss.