Peripheral Nerve Diffusion Tensor Imaging: Assessment of Axon and Myelin Sheath Integrity

Area of Science:

• Neuroimaging
• Biomarkers
• Peripheral Nerve Imaging

Background:

• Median nerve integrity in the carpal tunnel is crucial for hand function.
• Electrophysiology is the standard for assessing nerve damage but lacks specificity for differentiating axon vs. myelin injury.
• Diffusion tensor imaging (DTI) offers potential for non-invasive in-vivo assessment of nerve microstructure.

Purpose of the Study:

• To evaluate DTI parameters as biomarkers for median nerve axon and myelin sheath integrity.
• To correlate DTI findings with electrophysiological measures in healthy subjects.
• To assess the sensitivity of DTI in detecting subtle structural changes in the carpal tunnel.

Main Methods:

• 3T MRI with DTI was performed on 30 healthy wrists.
• Manual segmentation of the median nerve allowed quantitative analysis of DTI parameters (FA, AD, RD, MD).
• Correlations were computed between DTI metrics and electrophysiological parameters (SNAP, CMAP, dml, sNCV).

Main Results:

• DTI parameters exhibited a proximal-to-distal profile with extrema in the carpal tunnel center.
• Axial diffusivity (AD) correlated with compound muscle action potential (CMAP), indicating axon integrity.
• Radial diffusivity (RD) and fractional anisotropy (FA) correlated with sensory nerve conduction velocity (sNCV) and distal motor latency (dml), indicating myelin sheath integrity.

Conclusions:

• DTI parameters (AD, RD, FA) effectively differentiate between axon and myelin sheath integrity.
• DTI reveals subtle structural integrity changes in the carpal tunnel even in healthy individuals.
• DTI shows promise for clinical application in diagnosing peripheral neuropathies with mixed axonal and demyelinating components.