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 oedema is usually visible on T2/FLAIR images.
Pathology
Microscopic appearance
Histopathologically there are zones of fresh haemorrhage and/or compact coagulative necrosis (hypo/isointense) alternating with zones of oedema, fibrin-rich necrosis or foamy histiocytic infiltration (hyperintense).
Differential diagnosis
balo concentric sclerosis (usually showing only mild perifocal oedema) in an immunocompetent patient