Imaging Depression in Parkinson's Disease

This study will compare people with Parkinson's Disease and depression to individuals with Parkinson's Disease without Major Depressive Disorder, Major Depressive Disorder with no Parkinson's, and Healthy Controls. Up to 30 participants will be recruited and enrolled for each of these 4 groups. Participants will be asked to complete one PET and one MRI scan along with study assessments. Once screening and consent is completed, participants will be scheduled for PET and MRI scans. The total duration for participants in this study is 7 hours, including the screening visit and then one additional visit for the scans. There will also be a small subset (opt-in) who will receive a single dose of ketamine to determine the ability of ketamine to target these mechanisms and initiate an associated antidepressant response. If the participant is opting into the ketamine arm, the investigators will ask them to come in for one single-dose of ketamine and complete another PET and MRI scan post-ketamine treatment. The main questions it aims to answer are: 1) Will the Parkinson's depression group exhibit a distinct pattern of synaptic deficits compared to other groups? 2) Will there be differences in functional connectivity across groups? 3) Are there associations between synaptic density and functional connectivity across groups? 4) Will ketamine increase synaptic density in PD-affected and mood-related circuitry, and will that be associated with an antidepressant response?