Pilocytic astrocytoma

Pilocytic astrocytoma in a 5-year-old who presented with obstructive hydrocephalus. Sagittal noncontrast CT (A) shows a large, low-density cerebellar mass (*) that compresses the brainstem and causes third and lateral ventriculomegaly. Transependymal edema, suggesting acute hydrocephalus, is demonstrated on the transverse axial FLAIR (B) MRI. Transverse axial T2- (C), T1- (D), and postcontrast T1-weighted images in transverse axial (E) and coronal (F) planes show a cystic cerebellar mass with a mural nodule (arrows). Enhancement is present within the mural nodule as well as part of the cyst wall (arrowheads).