18 F-FDG Brain PET/MRI in Granulomatous Primary Angiitis of the Central Nervous System.

An 18-year-old man presented with encephalopathy, headache, tremor, and left hemiparesis. 18 F-FDG brain PET/MRI revealed pronounced hypometabolism in the right cerebral hemisphere corresponding to extensive T2/FLAIR signal abnormality, with accompanying miliary enhancement and microhemorrhages in this region. The differential diagnosis favored an autoimmune or inflammatory origin, rather than an infectious or neoplastic etiology. Brain biopsy demonstrated nonnecrotizing granulomatous inflammation affecting the vessel walls, without evidence of glial neoplasm, lymphoma, or infection. Treatment with corticosteroids was subsequently initiated, with favorable clinical response.