Arachnoid cyst

10-year-old male with giant arachnoid cysts. Transverse axial T2-weighted (A), FLAIR (B), and parasagittal T1- (E) and T2-weighted (F) MRIs show large, well-circumscribed, bilateral frontal extra-axial masses that displace cortex and compress the ventricles. On all sequences, including trace diffusion (C) and ADC (D), arachnoid cysts are isointense compared with cerebrospinal fluid. The wall of the arachnoid cyst consists of flattened but otherwise normal arachnoid cells.