This study will use positron emission tomography (PET) to measure a brain protein called peripheral benzodiazepine receptor (PBR) in patients with multiple sclerosis. PBR is created during the inflammation process, and brain inflammation is a key feature of multiple sclerosis (MS). PBR usually affects one type of brain cell, but it can also cause damage to surrounding areas of the brain in patients with MS. PET studies of PBRs and brain inflammation may help elucidate the role of these brain cells in patients with MS.