fig 7.

28-year-old man with a prepontine arachnoid cyst, corpus callosum anomaly, hydrocephalus, ventriculoperitoneal shunt, and headaches.

A, T2-weighted axial image (2000/80/1) shows a hyperintense cyst (arrow) within the prepontine cistern, pushing the crura cerebri and the optic chiasm apart.

B, Midsagittal T2-weighted section (2000/80/1) shows defined margins of the cyst (straight black arrows), except for the narrowing between the basilar artery and skull base (white arrow). Note callosal dysplasia (curved arrow). The bottom of the third ventricle is elevated.

C, Midsagittal SSFP image (20/25/1) reveals broad-based signal void within the cyst (black arrow) probably due to transmission of pulsations from the basilar artery. A continuous jet phenomenon between the prepontine cistern with pronounced signal void (straight white arrow) on the one hand and the cyst on the other hand is not demonstrable, making communication unlikely. Flow void within the fourth ventricle was absent (curved arrow) owing to occlusion of the aqueduct. Membranectomy was not performed, because the patient had no evidence of visual impairment.