MR reveals large supratentorial arachnoid cyst in the quadrigeminal cistern, which was measured 32 x 25 x 20 mm. The cyst is isointense to CSF in all sequences. There is no diffusion restriction. Cyst is compressing the tectum, narrowing the aqueduct, and causing hydrocephalus.
Lateral ventricles and third ventricle were dilated.