Is this mass well circumscribed or invasive?
Well circumscribed, without evidence of invasion into the adjacent brain.
Given all the imaging findings (hyperdense on CT, iso-intense to grey matter on T1 and T2, vivid enhancement, well circumscribed, restricted diffusion) and the fact it is probably separate (above) the pineal gland, what is your preferred diagnosis?
You should be favouring a meningioma.
A large well circumscribed mass is located in the pineal region. It is isointense to grey matter on both T1 and T2 weighted imaging with a central region of lower T2 signal (possibly due to a vascular core). It demonstrates vivid contrast enhancement. Restricted diffusion is seen throughout most of the mass, sparing the central region.
It severely distorts the midbrain, compressing the aqueduct with marked obstructive hydrocephalus. Best seen on sagittal post contrast T1 images, a small nodule to tissue is seen immediately below the mass which represents the tectal plate, and given the calcification on CT possibly possibly also a displaced pineal gland.
The internal cerebral veins are located above and the basal veins of Rosenthal below and lateral to the mass.