MAPT and Parkinson disease: Specifically, in patients with PD we examined DTI‐ALPS' relationship with: (1) white matter macrostructural integrity, assessed using whole‐brain fiber cross‐section and plasma neurofilament light chain (NfL); (2) grey matter atrophy, using cortical thickness; (3) iron accumulation, using QSM; and (4) beta‐amyloid and tau accumulation, using plasma levels of phosphorylated tau at threonine 181 (p‐tau181).