Specific Imaging Findings Active multiple sclerosis (MS) plaques cause breakdown of the blood–brain barrier (BBB) and therefore enhancement on postcontrast MR images. Two enhancement patterns exist: uniform, reflecting the onset of a new lesion, and ringlike, indicating reactivation of an older lesion. Uniform enhancement may be faint, irregular, and with ill-defined margins. Peripheral ring-like enhancement is usually incomplete (“open”); however, it may form a full circle. There is no associated mass effect and no edema surrounding the enhancing area (except for tumefactive MS, which is discussed separately). Hyperacute plaques (first 24 h) may show restricted diffusion due to acute inflammation. On DWI acute lesions frequently show a ring pattern with hyperintense rim, while the center of the lesion has high ADC values. The diffusion findings and contrast enhancement, however, frequently do not coexist. Although MS plaques can be found throughout the brain, they have a predilection for periventricular white matter, corpus callosum, juxtacortical regions, optic radiations, as well as infra-tentorial locations. The enhancing lesions also follow this distribution. Ovoid appearance with orientation perpendicular to the ventricular surface is highly characteristic, but only on axial images. Microbleeds may be seen within MS plaques on T2* MR imaging, primarily with SWI. The detection rate of enhancing lesions is increased by delayed imaging, triple dose of contrast and use of magnetization transfer. The higher contrast dose may have a role in cases of diagnostic doubt following the standard dose.
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