  Axon diameter and myelin thickness are closely related microstructural tissue
properties that affect the conduction velocity of action potentials in the
nervous system. Imaging them non-invasively with MRI-based methods is thus
valuable for studying brain microstructure and function. However, the
relationship between MRI-based axon diameter and myelination measures has not
been investigated across the brain, mainly due to methodological limitations in
estimating axon diameters. In recent years, studies using ultra-high gradient
strength diffusion MRI (dMRI) have demonstrated improved estimation of axon
diameter across white-matter (WM) tracts in the human brain, making such
investigations feasible. In this study, we aim to investigate relationships
between tissue microstructure properties with MRI-based methods and compare the
imaging findings to histological evidence from the literature. We collected
dMRI with ultra-high gradient strength and multi-echo spin-echo MRI on ex vivo
macaque and human brain samples on a preclinical scanner. From these data, we
estimated axon diameter, intra-axonal signal fraction, myelin water fraction
(MWF) and aggregate g-ratio and investigated their correlations. We found that
the microstructural imaging parameters exhibited consistent patterns across WM
tracts and species. Overall, the findings suggest that MRI-based axon geometry
and myelination measures can provide complementary information about fiber
morphology, and the relationships between these measures agree with prior
histological evidence.
