Patients can not have a relapse during 30 days before initiation of treatment. If a relapse occurs during this period and eligibility criteria are otherwise fulfilled, treatment (day one) will be delayed while corticosteroids are administered. If corticosteroids are administered, the MRI during that period will not be considered. An additional MRI will be added at 4 weeks following the completion of corticosteroids, to maintain a total of four MRI's that are analyzed in the baseline period. In the event of relapse, the baseline period will be prolonged, as necessary, to meet these criteria.