Why is this not a cavum vergae cyst?
Because it is located posteroinferior to the fornices, which it displaces superiorly. Cavum vergae cysts are located above the fornicies, often splaying them, but displacing them posteroinferiorly.
What anatomical structure enables you to confidently say that this is not a cystic pineal lesion?
The internal cerebral veins and vein of Galen and depressed inferiorly, whereas pineal lesions elevate these structures.
In the midline an intraventricular cyst is present which measures ~4.5cm in diameter and bulges into the posterior body of the lateral ventricles. It is located inferior to the fornices which are displaced superiorly, and above the internal cerebral veins. There is no solid component, no calcification, and the content follow CSF on all sequences. No enhancing component. The occipital and temporal horns remain somewhat prominent, more so on the left, suggesting that this cyst results in some impairment to normal CSF outflow.
Scattered throughout both cerebral hemispheres are a number of focal regions of T2 hyper intensity, which are non-specific but are suggestive of chronic small vessel ischaemic change.