AID‐P, MRPI, and plasma NfL were investigated and compared as diagnostic biomarkers. AID‐P, an automated diffusion MRI analysis protocol that samples multiple brain regions, performed better than MRPI and NfL in distinguishing PD vs. MSA‐P/PSP and MSA‐P vs. PSP. All three biomarker measures were associated with disease severity. The gene discussed is NEFL; the disease is Parkinson disease.