Congenital Cystic Malformations of the Posterior Fossa: How Can Fetal Magnetic Resonance Imaging Contribute to the Diagnosis?

Congenital cystic malformations of the posterior cranial fossa encompass Dandy-Walker malformation, Blake's pouch cyst, and arachnoid cyst, which are usually associated with increased cerebrospinal fluid in the posterior fossa. Although isolated cerebellar vermis hypoplasia courses with a normal posterior fossa volume and is not associated with a cyst, the smaller volume of the cerebellar vermis results in a relative increase in cerebrospinal fluid compared to the posterior fossa volume and is considered a differential diagnosis. Furthermore, mega cisterna magna, which is an anatomical variant, is also a differential diagnosis of these congenital malformations. Usually, these malformations are diagnosed through screening prenatal ultrasound. However, fetal magnetic resonance imaging (MRI) can confirm, change, or complement questionable findings and identify associated malformations. Therefore, fetal MRI plays an essential role in evaluating fetuses with suspected brain malformations. Manual MRI segmentation can facilitate the understanding of these congenital cystic posterior fossa malformations. Three-dimensional (3D) segmentation offers a spatial view of the abnormalities, as well as their relationships with adjacent anatomical structures, allowing a better multidisciplinary team approach. The aim of this article is to review the embryology of the posterior cranial fossa, discuss the fetal MRI protocol to evaluate the cystic posterior fossa malformations, and to illustrate the key MRI features of these malformations.