Case 43 Answer: Cavum Velum Interpositum Cyst

This is a benign incidental finding - see write up: http://radiopaedia.org/articles/cavum-velum-interpositum. Not to be confused with a cavum septum pellucidum or cavum septum vergae. Here is a nice pictoral comparison of the 3 areas.

From the references:

There are two potential spaces between the leaflets of the septum pellucidum in adults. The cavum septum pellucidum (CSP), which is anterior, is separated from the more posterior cavum vergae by the anterior columns of the fornix. During development, these spaces obliterate postero-anteriorly: the cavum vergae followed by the cavum septum pellucidum

A cavum velum interpositum (CVI) is a normal variation where there is a dilated CSF space involving the velum interpositum, which extends below the splenium of the corpus callosum and columns of the forinces and above the internal cerebral veins. It is triangular in shape, the apex pointing anteriorly reaching as far forward as the foramina of Monro ^1,4-5.

When larger than 1cm in axial transverse measurement, with outwardly bowed margins and positive mass effect, the term cyst of the velum interpositum or cavum velum interpositum cyst should be used.