The investigators hypothesize that IT rituximab therapy in patients with progressive forms of MS could disrupt ectopic lymphoid follicles in the meninges and thus slow progression of the disease, which is particularly important because there exist no FDA-approved therapies for progressive MS. The investigators hypothesize that using magnetic resonance imaging (MRI) to identify those with enhancing meningeal lesions will provide a biomarker to select patients who might be most likely to respond to IT rituximab and to use these lesions to monitor therapeutic response.