Bundle-Specific Axon Diameter Index as a New Contrast to Differentiate White Matter Tracts

Area of Science:

• Neuroimaging
• Neuroscience
• Biophysics

Background:

• Central nervous system pathways have varying axon diameter spectra.
• Non-invasive in vivo diffusion-weighted magnetic resonance imaging (dMRI) can estimate axon diameter index.
• Current voxel-wise dMRI methods struggle with partial volume effects, leading to inaccurate estimates within white matter bundles.

Purpose of the Study:

• To develop a novel microstructure-informed tractography approach, COMMIT-AxSize, for resolving axon diameter index estimates at the streamline level.
• To make axon diameter index estimates invariant along white matter streamline trajectories.
• To provide a more robust estimation of the axon diameter index by jointly considering tissue microstructure and white matter connectivity.

Main Methods:

• Developed COMMIT-AxSize, a microstructure-informed tractography method.
• Applied the method to estimate axon diameter index at the streamline level.
• Validated estimates against histological data in the corpus callosum and posterior limb of the internal capsule.

Main Results:

• COMMIT-AxSize successfully resolved streamline-level axon diameter index estimates.
• Estimates were invariant along individual white matter streamline trajectories.
• Demonstrated favorable performance by comparing results with histological measurements.

Conclusions:

• The proposed COMMIT-AxSize method offers a more robust estimation of the axon diameter index for white matter pathways.
• Streamline-level estimates overcome limitations of voxel-wise approaches, reducing partial volume effects.
• This approach provides complementary information to existing mean value calculations for white matter bundles.