What is by far the most likely diagnosis of the pineal region abnormality?
Incidental large pineal cyst.
What is the main differential diagnosis, which results in these often needing imaging follow-up?
Cystic pineocytoma can appear very similar, although usually some enhancing components are present.
Look carefully at the vessels. What do you see? What is this known as?
A large vessel passes from the cavernous internal carotid artery on the right, to the superior basilar artery. This is known as a persistent trigeminal artery.
A well-circumscribed non-enhancing ovoid pineal mass measures 2.0 x 1.7 x 1.1 cm and distorts the upper tectum, mildly narrowing the upper aqueduct, with no associated signal abnormality or obstructive hydrocephalus.
Note is made of a right trigeminal artery, which is moderately tortuous, but without overt sacular aneurysm. Corresponding hypoplasia of the basilar artery proximal to the trigeminal artery is also shown.