PET Brain Imaging in Multiple Sclerosis, Alzheimer's Disease, and Other Neurological and Neuropsychiatric Diseases

The specific aims of the study are: Primary: To determine the presence and regional distribution of microglial activation, as assessed by Fluorine-18 (18F) labeled "Peripheral Benzodiazepine Receptor 06" (PBR06) -PET, in subjects with active Relapsing Remitting Multiple Sclerosis (RRMS), Secondary Progressive Multiple Sclerosis (SPMS), and Alzheimer's Disease (AD) as compared to healthy controls Secondary: 1. To assess the relationship between microglial activation and clinical variables including disease severity and comorbidities (such as pain, fatigue and/or depression), as well as clinical MRI findings (such as lesions and atrophy) 2. A pilot substudy aims to establish the non-inferiority of \[F-18\]PBR06 as compared with Carbon-11 \[C-11\] labeled "Peripheral Benzodiazepine Receptor 28" (PBR28) PET in patients with RRMS. Hypothesis: The working hypothesis is that there is microglial activation in multiple sclerosis and Alzheimer's disease as compared to healthy controls and that the pattern/ regional distribution of microglial activation is different in Multiple Sclerosis (MS) versus AD and correlates with disease severity and comorbidities. In addition, the investigators hypothesize that \[F-18\]PBR06-PET scans will be at least as good as \[C-11\]PBR28-PET scans, the current gold standard.