Partially sequestered fourth ventricle: CT and MR diagnosis of an unusual entity.

Encystment of the fourth ventricle, due to occlusion of the aqueduct as well as the foramina of Magendie and Luschka has been described previously. Partial sequestration such as that encountered in two cases described is a less common entity. In these two cases, the aqueduct of Sylvius was occluded, but the basal foramina (Magendie and Luschka) were patent. We discuss this partial sequestration or communicating hydrocephalus of the fourth ventricle on the basis of MR and CT scan findings. Although computed tomography following water soluble cisternography did diagnose the entity indirectly MRI proved to be a superior tool. It permitted direct visualization of the basal foramina noninvasively.