Diffusion MRI (dMRI) is the only method that allows in vivo segmentation of white matter (WM) fasciculi and assessment of their integrity, based on the water diffusion anisotropy caused by highly ordered bundles of axons. It can therefore be used to study wm decline: loss of integrity and tract thinning. The disambiguation of the volumetric and coherence information in the dmri data is not trivial. We show that it is possible to obtain reasonable estimates of: tract volume (on a sub-voxel level), local fibre orientation and diffusivity parameters linked to the coherence of axonal bundles, using a realistic model for mixed-tissue voxels and a plausible spatial similarity assumption.