Study of Biodistribution, Metabolism, Excretion and Brain Uptake11C-M503

This study uses a radioactive imaging agent called 11C-M503 and PET brain scans to examine how a specific brain protein is distributed and processed in people with Parkinson's disease (PD), multiple system atrophy (MSA), or progressive supranuclear palsy (PSP) — three neurodegenerative conditions that share some overlapping features. The goal is to better understand how this protein behaves in these diseases and potentially develop better diagnostic tools. **You may be eligible if...** - You are between 40 and 85 years old - You have a confirmed or suspected diagnosis of Parkinson's disease, MSA, or PSP - You are willing to provide written consent (or have a designated study partner who can assist) - You are willing to consider (though not required) brain donation after death **You may NOT be eligible if...** - You have a pacemaker or other devices incompatible with PET/MRI scanning - You have other serious medical conditions that make study procedures unsafe - You do not meet the diagnosis-specific criteria for PD, MSA, or PSP as determined by the study committee Talk to your doctor to see if this trial is right for you.