Asymptomatic giant arachnoid cyst.

A 51-year-old male bricklayer without cerebral symptoms underwent whole-body FDG PET for suspicion of mediastinal sarcoidosis. PET/CT incidentally demonstrated a voluminous right frontal arachnoid cyst with normal glucose metabolism in the adjacent cortex. MRI demonstrated compression of the ipsilateral cerebral parenchyma and precentral, angulate, callosal, and superior frontal sulci. Functional bilateral finger-thumb paradigm MRI revealed right hemispheric reorganization of cortical motor activation with prominent activation between the inferior frontal gyrus and the postcentral gyrus.