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

Area of Science:

• Neuroscience
• Biophysics
• Medical Imaging

Background:

• Axon diameter and myelin thickness are crucial for nerve impulse conduction velocity.
• Non-invasive MRI methods are valuable for studying brain microstructure.
• Previous studies suggest a close relationship between axon diameter and myelin thickness.

Purpose of the Study:

• To investigate the relationships between MRI-based estimates of axon diameter and myelin thickness across white-matter tracts.
• To explore correlations between axon diameter index, intra-axonal signal fraction, myelin water fraction (MWF), and g-ratio.
• To establish a reference for microstructural variations in control samples for disease-related changes.

Main Methods:

• Utilized ultra-high-gradient strength diffusion MRI (dMRI) and multi-echo spin-echo MRI on ex vivo macaque and human brain samples.
• Estimated axon diameter index, intra-axonal signal fraction, MWF, and aggregate g-ratio.
• Analyzed correlations between these microstructural parameters across white-matter tracts.

Main Results:

• Found weak but consistent correlations between axon diameter index and other microstructural parameters across WM tracts.
• In well-myelinated regions, larger axon diameter indices correlated with lower packing density, lower MWF, and higher g-ratio.
• Intra-axonal signal fractions and MWF showed inconsistent correlations across different samples.

Conclusions:

• MRI-based measures of axon geometry and myelination provide complementary information on fiber morphology.
• The observed relationships align with electron microscopy findings.
• Combining these advanced MRI measures can aid in differentiating diseases like demyelination and axonal damage.