Fig. 3. The interpretation of fixel-based metrics changes.

a Representative images show the enlargement of fiber orientation distributions’ amplitude, which reflects an increase in intra-axonal volume¹², in the corticospinal tract (CST) in a patient with tremor-dominant Parkinson’s disease (TD-PD) compared with healthy control. b The schematic represents a fiber-bundle cross section (blue circles represent axons, and the grid represents imaging voxels). The increase in intra-axonal volume might indicate a fiber-bundle enlargement due to increases in axon number, as indexed by greater fiber cross section (FC) and a combined measure of fiber density and cross section (FDC).