Banana sign (cerebellum)

The banana sign is one of the many notable fruit inspired signs.

This sign is seen on axial imaging through the posterior fossa of fetuses with associated conditions such as  Chiari II malformation and/or spina bifida.

In Chiari II malformation, the banana sign describes the way the cerebellum is wrapped tightly around the brain stem as a result of spinal cord tethering and downward migration of the posterior fossa contents. This results in obliteration of the cisterna magna and subsequently, the shape of the cerebellum has the appearance of a banana. The sign persists into the second and third trimesters.

The banana sign is also seen in the majority of fetuses with spina bifida. The ultrasound appearance of the banana sign is secondary to the abnormality of the cerebellar shape.  In patients with spina bifida, this appearance disappears after 24 weeks. There is frequently concurrent hydrocephalus.

Associated signs

The same condition also gives rise to the lemon sign.