What is the abnormality and what is the differential?
Expansion of the suprasellar cistern, with CSF density and no calcification. The differential includes an arachnoid cyst, a Rathke's cleft cyst, a cystic craniopharyngioma (although absence of calcification would be unusual) and an epidermoid cyst.
CT is essentially normal, however careful examination reveals expansion of the suprasellar cistern, with CSF density and no calcification.