Free-water levels in the substantia nigra provide a non-invasive biomarker for PD
 

"... 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:
7 - Imaging biomarker for Parkinson’s could aid in testing drugs to slow disease’s progression - University of Florida News

Open-access research paper:
Progression marker of Parkinson’s disease: a 4-year multi-site imaging study | Brain | Oxford Academic

It may be non-invasive, but it certainly wouldn't be cheap!

Still, as they say in the press release, it could be of good use during clinical trials of disease-modifying therapies (i.e. to establish the "before" and "after" baselines).