The cavum velum interpositum (CVI) is a thin, triangular-shaped cerebrospinal fluid (CSF)-filled space between the lateral ventricles that lies below the fornices and above the third ventricle. It is a normal variant. When larger than 1 cm 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.