The method matters: free water imaging in Parkinson’s disease is not a binary verdict

A recent comparison of neuromelanin and free water (FW) imaging in Parkinson's disease concluded that FW imaging underperforms. We argue this reflects methodological constraints, not biological truth. Three issues compromise FW sensitivity: use of a registration template that degrades substantia nigra localization, an age imbalance between groups that favors neuromelanin detection, and repetition times insufficient to capture the FW compartment. With unmet acquisition requirements, the technique was never correctly applied.