This study will use positron emission tomography (PET) and magnetic resonance imaging (MRI) to measure peripheral benzodiazepine receptors (PBRs) in the brain. PBRs were initially found in peripheral organs such as kidneys, endocrine glands and lungs, but later studies identified PBRs in the CNS. PBRs can be a marker to detect nervous system inflammation. Development of a test to image PBRs may improve the management of brain disorders such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, and others in which inflammation is involved in progression of the disease.