Neurodegenerative disorders have a substantial immune component, implicating both peripheral and central immune systems. Despite neuroinflammation being linked to greater dementia risk, the interaction of central and peripheral inflammatory processes to influence disease courses remain ill-understood. A key site-of-interest for this interaction is the choroid plexus. Beyond secreting cerebrospinal fluid (CSF), it has been proposed as a gateway for peripheral immune cell entry into the central nervous system (CNS). As such, increases in choroid volume measurable with structural magnetic resonance imaging may provide a non-invasive, low-cost measure to identify pathological neuroinflammation – and may capture both peripheral and central immune components. Previous research in multiple sclerosis and psychosis demonstrated increased choroid volumes being related to neuroinflammation, greater disease burden and lymphocyte incursion. However, it has not been systematically mapped how peripheral and CNS inflammation relate independently and synergistically to choroid volume, and how this captures disease progression. Taking a cross-diagnostic approach, we are performing blinded high-fidelity segmentations of choroid volume in T1-weighted MRI scans from participants with Parkinson’s Disease (n=40), Dementia with Lewy Bodies (n=23), and Progressive Supranuclear Palsy (n=17). Using multimodal statistical approaches, we test the relationship of these data to I) biospecimen measures of inflammation (lymphocyte phenotypes from flow cytometry and cytokine profiles from ELISA in blood and CSF); II) microglial activation measured with TSPO positron emission tomography; III) CSF chemokine measurements relevant to peripheral and central immune cell chemotaxis; and IV) longitudinal cognitive measures. The diagnostic and prognostic utility data will be presented during the FENS congress.