The velum interpositum is the potential subarachnoid space between the fornix and its attached choroid above and the choroid forming the roof of the 3rd ventricle inferiorly, and is an anterior extension of the quadrigeminal plate cistern just located superior to the pineal gland. If this potential space is simply prominent, it is known as cavum velum interpositum. However, if there is mass effect such as inferior displacement of the internal cerebral veins or the pineal gland, arachnoid cyst is the most likely explanation.
Dr.Sumer K Sethi, MD
Sr Consultant Radiologist ,VIMHANS and CEO-Teleradiology Providers