However, not all MS activity can be picked up by an MRI scan.
This is how the National MS Society explains it :-
“On the other hand, a normal MRI does not absolutely rule out MS. About 5% of
people, who are confirmed to have MS on the basis of other criteria, do not show any lesions in the brain on MRI. These people may have lesions in the spinal cord or may have lesions that cannot be detected by MRI.”