The patients will undergo monthly contrast enhanced MRI for the first nine months and thereafter at months 12 and 24. They will also be examined by clinical and laboratory tests at six months intervals in addition to month 9 (3 months after start of IFNB treatment). Fatigue and QoL registration will be performed at baseline and at months 6, 12 and 24. Tests for circulating neutralising antibodies against interferon-beta will be performed during the study.