Diffusion restriction along fibres: How coherent is the corpus callosum?

PURPOSE. Diffusion weighted imaging aims to unravel the microstructural properties of white matter in the brain by detecting alterations to diffusive motion along different orientations. Increasingly sophisticated biophysical models are used to estimate properties like fibre orientation dispersion in addition to mean orientation. Most models assume an explicit or implicit assumption that water is able to diffuse freely along the primary fibre orientation. However, microstructural analysis with histology and electron microscopy in both rodent and human brains demonstrated a considerable amount of dispersion even in the corpus callosum [1,2,3], which is often used as a test-bed for diffusion models [4] based on its assumed extreme fibre coherence. The present study aims to investigate the coherence of fibre orientations at multiple scales in the human corpus callosum using two modalities: diffusion-time (∆) MRI measurements and direct estimation of fibre orientation from optical microscopy (polarized light imaging, PLI). A secondary aim of this work is to demonstrate the potential for using PLI-MRI comparisons in the same tissue sample to inform biophysical modeling aimed at in vivo diffusion MRI.