Does this confirm the diagnosis? How?
Yes. This is a velum interpositum cyst. The location (above the internal cerebral veins and below the fornices) is typical, as is the fact that it does not reach any further anterior than the foramen of Monro.
Name the boundaries of the velum interpositum.
The velum interpositum is formed by an invagination of pia matter forming a triangular membrane the apex of which points anteriorly bounded superiorly by the columns of the fornices and hippocampal commisure (psalterium) reaching as far forward as the foramen of Monro. Inferiorly it is bounded by the internal cerebral veins and tela choroidea of the third ventricle and inferolaterally by the thalamus. Posteriorly, the narrow base of the triangle abuts the splenium of the corpus callosum.
True or false: Velum interpositum cysts are associated with neuropsychiatric abnormalities and behavioural disturbances?
False. Such a link has however been suggested for cavum septum pellucidum et vergae.
There is a large, well circumscribed mass midline in the pineal region which follows CSF signal on all sequences. There is no solid component.
The fornices are splayed and displaced superiorly, and the internal cerebral veins inferiorly.