NEFL and Cognitive impairment: Having confirmed the association between DTI‐ALPS index and poor cognitive outcomes in PD, we aimed to assess how impaired glymphatic clearance (reflected by lower DTI‐ALPS) relates to processes we have previously shown to correlate with cognitive impairment in our cohort4, 6, 33, 34; specifically loss of white matter macrostructural integrity (using fiber cross‐section and plasma NfL), iron accumulation (using QSM), amyloid co‐pathology (using plasma p‐tau181), and grey matter atrophy (using cortical thickness).