MS cannot be diagnosed by MRI
alone. The revised (2010) McDonald Criteria sets out the formula
, if you will, for diagnosing this disease. Further, MS is a diagnosis of exclusion, meaning that its many mimics (eg Lyme disease, Lupus, Myasthenia Gravis, Sjogren's Syndrome, B12 deficiency, etc etc etc) for which there are more conclusive tests, must be ruled out.
Wiki has a good overview of the McDonald Criteria.