Concentric target sign (cerebral toxoplasmosis)
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The concentric target sign is a typical sign of cerebral toxoplasmosis. It is seen on T2 weighted MR imaging as a deep parenchymal lesion showing a series of concentric rings with hyperintense and hypointense/isointense signals alternatingly. Strong perifocal edema is usually visible on T2/FLAIR images.
Histopathologically there are zones of fresh hemorrhage and/or compact coagulative necrosis (hypo/isointense) alternating with zones of edema, fibrin-rich necrosis or foamy histiocytic infiltration (hyperintense).
balo concentric sclerosis (usually showing only mild perifocal edema) in an immunocompetent patient