MRI scan shows active hydrocephalus with periventricular T2/FLAIR hyperintensity (retrograde transependymal CSF permeation) caused by a large posterior fossa cyst, distorting and occluding the fourth ventricle outlet; and associated with significant mass effect on the brainstem. The 4th ventricle intraventricular cystic lesion shows near CSF signal intensity with inhomogeneous suppression on FLAIR due to turbulent flow and no restricted diffusibility. A right sided VP shunt tube is seen within the body of the right lateral ventricle.