What is the CSF intensity space / structure in the pineal region?
A cavum velum interpositum.
What is the key relation in distinguishing it from a cavum vergae?
A cavum velum interpositum is located below the fornices whereas a cavum vergae is located above them.
What is the key relation in distinguishing it from a pineal cyst or other pineal region cystic mass?
A pineal cyst (as is the case with the pineal) is below the internal cerebral veins, whereas a cavum velum interpositum is above them.
A cystic space is seen in the region of the pineal gland, displacing the fornices upwards and the internal cerebral veins downwards. The pineal gland can be seen as separate from this region, below the internal cerebral veins - best seen on sagittal images. Features are characteristic of a cavum velum interpositum cyst.
On axial imaging, it is triangular in shape. Incidental note is also made of a large cerebellomedullary cistern (mega cisterna magna).