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Estimating axon conduction velocity in vivo from microstructural MRI.
Mark Drakesmith1, Robbert Harms2, Suryanarayana Umesh Rudrapatna3
1Cardiff University Brain Research Imaging Centre, Cardiff University, Cardiff, United Kingdom; Neuroscience and Mental Health Research Institute, Cardiff University, Cardiff, United Kingdom.
Neuroimage
|September 23, 2019
Summary
Estimating nerve conduction velocity (CV) using MRI is feasible for large axons, with inner axon diameter and g-ratio explaining 85% of CV variance. Accuracy depends on axon size and fibre population density.
Area of Science:
- Neuroscience
- Biophysics
- Medical Imaging
Background:
- Conduction velocity (CV) is crucial for neural communication.
- Estimating CV non-invasively in humans via MRI is a significant neuroscientific goal.
- Current MRI techniques face challenges in capturing neurophysiological complexity.
Purpose of the Study:
- To determine the variability in CV predictable from MRI-estimable parameters: inner axon diameter (AD) and g-ratio.
- To assess the impact of errors in MRI-based biophysical models on CV estimates.
- To establish conditions for accurate in vivo CV estimation using MRI.
Main Methods:
- Sensitivity analysis on a comprehensive axon electrophysiology model.
- Simulation of diffusion and relaxometry MRI data across various axon morphologies.
- Application of restricted diffusion and relaxation models to derive AD, g-ratio, and axon volume fraction (AVF).
Main Results:
- Inner axon diameter (AD) and g-ratio account for 85% of CV variance.
- CV estimates are most accurate (>95%) with AVF > 0.3, g-ratio 0.6-0.85, and AD > 4μm.
- AVF errors significantly impact CV estimates, especially in sparse populations (AVF<0.3); AD estimation errors are critical for small axons.
Conclusions:
- Accurate CV estimation is achievable in brain regions with large AD.
- Challenges in estimating AD for smaller axons limit CV estimation across the entire central nervous system (CNS).
- Future research should focus on improving AD estimation for small axons.
