ARRT Ultrasound Practice Test

Downward displacement of the cerebellar vermis, fourth ventricle, and brainstem through the foramen magnum is a hallmark finding of Arnold-Chiari II malformation. This hindbrain herniation produces the characteristic banana sign on fetal ultrasound, due to an abnormally curved cerebellum, and the lemon sign, which reflects frontal skull contour changes. These features often occur with spinal neural tube defects such as myelomeningocele, making Chiari II a classic association. Other conditions have distinct imaging patterns. Dandy-Walker malformation shows an enlarged posterior fossa with cystic dilation of the fourth ventricle and vermian abnormalities, not downward herniation through the foramen magnum. An arachnoid cyst is a CSF-filled space that doesn’t produce the hindbrain herniation pattern typical of Chiari II. Hydranencephaly involves near-complete loss of cerebral hemispheres and lacks this hindbrain displacement scenario.