Role of MR imaging in the diagnosis of MS

The distribution of lesions in MS, as seen with MR, closely follows the histopathology, with periventricular lesions, frequently ovoid in shape, and typical involvement of the optic nerve, spinal cord, temporal lobes, brain stem and cerebellum. T2-weighted MR imaging is the most sensitive paraclinical test to demonstrate dissemination of the disease in space, but its specificity is only moderate. The presence of four or more lesions at the time of the first symptom strongly supports the diagnosis and the actual number of lesions has bearing on the future course of the disease. Gadolinium enhancement depicts the earliest inflammatory phase of MS lesions and indicates dissociation of the disease in time, the other prerequisite for the diagnosis of MS. The presence of both enhancing and non-enhancing lesions increases the specificity of MR, as does increasing the minimum number of T2 lesions to 9. (Sub)cortical lesions are frequently seen in MS (which is not purely a white matter disease) and are quite specific, as are infratentorial lesions. The choice of MR criteria used depends strongly on the clinical situation. Given the low prevalence of MS compared with small vessel disease, incidental lesions virtually never represent MS. A normal T2-weighted scan in a patient with a low clinical suspicion of MS is strongly suggestive of non-MS disease. Standard T2-weighted imaging also suffices to ascertain MS in typical cases. More specific imaging findings, such as gadolinium enhancement, infratentorial lesions and lesions in the optic nerve and spinal cord further increase the level of confidence; such confirmation might be sought after when treatment with potential side effects is contemplated.