On histologic examination, acute MS plaques show partial or complete destruction and loss of myelin with sparing of axon cylinders. They occur in a perivenular distribution and are associated with a neuroglial reaction and infiltration of mononuclear cells and lymphocytes. The perivascular demyelination gives the appearance of a finger pointing along the axis of the vessel.
Active demyelination is accompanied by transient breakdown of the blood-brain barrier.
Chronic lesions show predominantly gliosis. MS plaques are distributed throughout the white matter of the optic nerves, chiasm and tracts, the cerebrum, the brain stem, the cerebellum and the spinal cord. Specific signal intensities of MS lesions will vary depending on the magnetic field strength, the pulse sequence parameters, and partial volume effects. Occasionally, acute plaques may have a thin rim of relative T2 hypointensity or T1 hyperintensity. The T1 hyperintensity has been attributed to free radicals, lipid-laden macrophages, and protein accumulations.