Compounds with high affinity for aggregated alpha-synuclein, after radiolabeling with C-11 or F-18, 1- 125, 1-123 and Br-76, could be used as PET tracers to measure the level and distribution of aggregated alpha-synuclein in patients with Parkinson’s disease and related diseases such as DLB and MSA. Determining the level and distribution of aggregated alpha-synuclein could be useful for early diagnosis and for monitoring response to therapy in these diseases. They could also potentially be useful for treatment of Parkinson Disease and related diseases.