"... on diffusion MRI scans there is an increase over one year in "free-water", or fluid unconstrained by brain tissue, in a part of the brain called the substantia nigra ..."
"... in a subgroup of Parkinson's disease patients who have been followed for up to four years, ... analysis of the diffusion MRI data revealed that free-water in the posterior substantia nigra continued to increase."
Uni. of Florida press release: