Up to 32 subjects with MS will undergo screening TSPO genotype analysis by PCR to accrue up to 16 MS subjects with confirmed high- or mixed- affinity binding genotype (HL or HH genotype) who will undergo a brain [(11)C]PBR28 PET and MRI imaging. Subjects with MS will undergo follow-up brain [(11)C]PBR28 PET and MRI at approximately 4-month interval. Subjects will undergo clinical examination at each PET imaging time point to assess clinical measures of disability and impairment. Up to an equal number of age-matched healthy volunteers will undergo screening TSPO genotype analysis by PCR. Up to 16 healthy volunteers with confirmed high- or mixed- affinity binding genotype (HL or HH genotype)undergo test retest brain [(11)C]PBR28 PET studies to determine the reproducibility of [(11)C]PBR28 PET in a healthy population compared to MS.