MRI demonstrates a CSF intensity space distorting the optic chiasm and pituitary infundibulum (pushing them forwards and upwards). A thin membrane can be seen (best on sagittal and axial T2 weighted images) invaginating upwards into the third ventricle, splaying the septum pellucidum and even bulging into the left foramen of Munro. The lateral ventricles are dilated, in keeping with obstructive hydrocephalus, clearly long standing (note large size and lack of transependymal oedema).