A simple estimate of axon size with diffusion MRI

Area of Science:

• Neuroimaging
• Biophysics
• Diffusion-weighted MRI

Background:

• Noninvasive estimation of mean axon diameter is crucial for understanding white matter plasticity, development, and pathology.
• Current diffusion-weighted MRI (DW-MRI) methods require complex models and numerous diffusion encodings.
• Existing techniques struggle to disentangle intra-axonal and extra-axonal signal contributions.

Purpose of the Study:

• To develop a straightforward diffusion-weighted MRI (DW-MRI) metric for estimating mean axon diameter.
• To validate this metric using Monte Carlo simulations.
• To assess the metric's sensitivity to various microstructural parameters.

Main Methods:

• Utilized Monte Carlo simulations to model diffusion MRI signals.
• Investigated the change in radial apparent diffusion coefficient (ΔD⊥) at different effective diffusion times.
• Analyzed the relationship between ΔD⊥ and mean axon diameter.

Main Results:

• Discovered a monotonic relationship between ΔD⊥ and mean axon diameter within a relevant physiological range.
• Demonstrated that ΔD⊥ is insensitive to changes in extra-axonal volume fraction, axon diameter distribution, g-ratio, and myelin water.
• Confirmed a monotonic relationship for both intra-axonal and extra-axonal signal compartments.

Conclusions:

• The proposed ΔD⊥ metric provides a simple and robust method for noninvasively estimating mean axon diameter using DW-MRI.
• This metric holds promise for advancing research into white matter microstructure and its alterations in disease.
• The findings suggest potential for improved diagnostic and monitoring tools in neurological conditions.