Multiple sclerosis (MS) is classically characterized by a sudden or gradual onset of neurologic dysfunction of various systems with a remitting and relapsing course which is highly variable that gradually results in mild to severe deficits or even death. The age of onset is usually in the second decade, and females outnumber males two to one. MS is seen most commonly in the Northern United States and Northern Europe. MS is an inflammatory disease of the CNS with 95% of lesions located in the white matter. An autoimmunologic process is presumed, but the precise pathogenicity is unknown. Histologically, active MS lesions show inflammation, demyelination and edema and have indistinct margins. Chronic inactive plaques consist of demyelination and fibrillary gliosis and have sharper margins.

MR detects characteristic lesions in 95% of patients. MR is more sensitive than visual, auditory, and somatosensory evoked potentials, although brain stem auditory evoked potentials may be more sensitive for detecting brain stem lesions. However, the use of motion compensation techniques has greatly improved detection of brain stem and posterior fossa lesions. Laboratory analysis of CSF for MS profile is positive in 54-74% of cases of MS. Double-dose CT scanning has been positive in 29-62% of definite MS cases. MR is the preferred imaging modality to study suspected MS, both for its improved lesion detection and its ability to exclude other pathologies such as midline tumors or Arnold-Chiari malformations which may simulate the signs and symptoms of MS.

Often the appearance of MS on MR is characteristic and the diagnosis is facilitated by pattern recognition. There may be one or many lesions varying in size from several millimeters to large contiguous lesions. Plaques vary in shape; they may be round, oblong, linear, angular, or confluent. Rarely, giant plaques can demonstrate mass effect. Usually, there is marked asymmetry in distribution of plaques. Lesions are most commonly located in a periventricular location and in the deep white matter, especially adjacent to the atria, occipital horns, and body of the lateral ventricles. Approximately 30% of patients have brain stem and cerebellar lesions which often involve the middle cerebellar peduncles. Diffuse or focal atrophy of the corpus callosum is present in 40% of patients. This finding is most prominent in patients with long-standing and extensive disease and is best visualized in the sagittal plane. 5-10% of lesions occur in the gray matter, either in the cortex or basal ganglia. Recently it has been suggested that lesions which appear round on axial images and linear on coronal images, with an orientation perpendicular to the lateral ventricle, are highly specific for MS.

MS plaques have prolonged T1 and T2 relaxation times, having low signal intensity on T1-weighted images (Film 1, top row) and high signal intensity on T2-weighted images (Film 1, scan 4). T1-weighted inversion recovery (IR) provides excellent contrast between low-intensity lesions and normal surrounding white matter but does not demonstrate periventricular or gray matter lesions well. Some lesions are only seen on T2- weighted images, and the lesions with long T1 relaxation times tend to appear larger on the T2-weighted image. Periventricular lesions may be difficult to distinguish from CSF on a long-TR, long-TE sequence. Therefore T2-weighted images with an intermediate TE (with CSF isointense with surrounding brain) is the most sensitive MR technique to detect MS plaques (Film 1, scan 3). Scanning is usually performed in the axial plane with thin sections (5 mm) to minimize partial volume averaging effects. MS lesions may progress, resolve or remain stable when serially studied over time, and one must be certain that apparent changes are real and not artifactual secondary to technique or positioning because it is difficult to prescribe exactly the same section locations on different examinations.