Figure 9b. Arachnoid cyst in a fetus at 27 weeks gestation. (a–c) C = extraaxial collection. (a) Axial US scan demonstrates an extraaxial collection, but it is difficult to assess the involvement of the ventricular system. (b) Coronal and (c) sagittal RARE MR images (single shot/60) clearly demonstrate the extraaxial nature of this extensive arachnoid cyst with mass effect on the surrounding structures.