Interplay between MRI-based axon diameter and myelination estimates in macaque and human brain

TL;DR

This study investigates the relationship between MRI-based estimates of axon diameter and myelination in macaque and human brains, revealing weak but consistent correlations across white-matter tracts and suggesting these measures provide complementary microstructural information.

Contribution

It demonstrates the feasibility of using ultra-high gradient diffusion MRI to explore relationships between axon and myelin microstructure across the brain.

Findings

Weak but consistent correlations between axon diameter and other microstructural measures.

Larger axon diameters associated with lower packing density and myelin water fraction.

MRI measures of axon geometry and myelination provide complementary information.

Abstract

Axon diameter and myelin thickness 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. Electron microscopy studies suggest that axon diameter and myelin thickness are closely related to each other. However, the relationship between MRI-based estimates of these microstructural measures, known to be relative indices, have not been investigated across the brain mainly due to methodological limitations. In recent years, studies using ultra-high gradient strength diffusion MRI (dMRI) have demonstrated improved estimation of axon diameter index 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 across white-matter tracts, as estimated with MRI-based methods. 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 found that the correlations between axon diameter index and other microstructural imaging parameters were weak but consistent across WM tracts in samples estimated with sufficient signal to noise ratio. In well-myelinated regions, tissue voxels with larger axon diameter indices were associated with lower packing density, lower MWF and a tendency of higher g-ratio. We also found that intra-axonal signal fractions and MWF were not consistently correlated when assessed in different samples. 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 electron microscopy studies in smaller field of views.