Both diffusion tensor and NODDI models are fit voxel-wise, and IDPs of the various model outputs are extracted from a set of white matter tracts. Tensor fits utilize the b=1000 s/mm2 data, producing maps including fractional anisotropy, tensor mode and mean diffusivity. The NODDI16 model is fit using the AMICO (Accelerated Microstructure Imaging via Convex Optimization) tool52, with outputs including intra-cellular volume fraction (which is often interpreted to reflect neurite density) and orientation dispersion (a measure of within-voxel disorganization). For tractography, a parametric approach is first used to estimate fibre orientations. The generalised ball & stick model is fit to the multi-shell data, estimating up to 3 crossing fibre orientations per voxel.17, 69 Tractography is then performed in a probabilistic manner to estimate white matter pathways using the voxel-wise orientations.
