Sonographic characteristics of the cavum velum interpositum.

BACKGROUND AND PURPOSE Differential considerations for pineal region CSF collections include both true cysts and normal cystlike anatomic variations. Our purpose was to determine the sonographic characteristics of pineal region fluid spaces that reflect the presence of a normal persistent cavum velum interpositum (CVI). METHODS Eighteen neonates and infants who had sonographic findings of "cysts" in the pineal region were examined prospectively with conventional sonography and color Doppler sonography to evaluate the shape of the fluid collection and its anatomic relationship with the color-coded internal cerebral veins. Subsequent MR images were obtained in eight of these infants to determine the exact nature of the cystlike collections. RESULTS The cystlike spaces in the pineal region were of an inverted helmet shape in 14 subjects and roundish in four. All were situated inferior or slightly anteroinferior to the splenium of the corpus callosum and 2.5 to 4 mm away from the quadrigeminal plate. The internal cerebral veins were either inferior (n = 12) or inferolateral (n = 6) to the cystlike spaces at sonography. Subsequent MR studies confirmed eight of these cystlike spaces to be the posterior portion of the CVI. CONCLUSION The CVI may appear as a cyst in the pineal region on neonatal sonograms. Usually, it has a characteristic inverted helmet shape and is situated beneath the fornices and above the internal cerebral veins.