Where is this mass likely to originate from?
Given its size it is hard to tell, however it appears to be below enlarged internal cerebral veins, raising the possibility that it is arising form the pineal region.
What feature should you be looking for to make the diagnosis of a teratoma (not necessary, but very helpful if present)? Is it present in this case?
Fat would clinch the diagnosis, but unfortunately it does not appear to be present.
What is the hyperintense material on T1 sequences superiorly attributable to?
The intrinsic areas of high T1 are attributable to blood (they are low on T2). Review of the CT with soft tissue windows looking for fatty tissue or fat attenuated T1 non-contrast MRI (demonstrating the hyperintense material to remain hyperintense) would confirm this.
MRI confirms the findings, with a large heterogeneous mass with variable signal and blood layering both within cystic components of the mass as well as in the occipital horns.