The study seeks to test PBR expression measured by [(11)C]PBR28 PET imaging as a marker of neuroinflammation in patients with MS. Primary outcome measures include 1) correlation between increased PBR expression and gadolinium-enhancing lesions on MRI, and 2) correlation between increased PBR expression and previously and/or persistently gadolinium-enhancing lesions on MRI, 3) PBR expression in subjects with multiple sclerosis compared to that in healthy volunteers.