Figure 7a. Severe ventriculomegaly in a fetus at 30 weeks gestation. (a) Oblique axial US scan shows a 34-week head size and massively enlarged lateral ventricles. It is difficult to assess the cortical mantle. The posterior fossa appears to be small, and there is a dilated fourth ventricle (arrow), but the cerebellum is difficult to visualize ultrasonographically. (b) Sagittal and (c) coronal RARE MR images (single shot/60) demonstrate a definite rim of smooth cortical tissue (arrowheads in b), malformed brain stem (arrow in b), and Dandy-Walker malformation (M). These findings make hydranencephaly unlikely and suggest a cerebrocerebellar malformation, such as Walker-Warburg syndrome, with aqueductal stenosis. This type of information is important in counseling patients prior to delivery.