This presentation will discuss assessment and disorders of midline structures of the fetal brain including:

· Corpus callosum
· Cavum veli interpositi.
· Posterior fossa structures such as Blake's pouch and cerebellar vermis.

Midline structures require special non-standard views and are more difficult to evaluate than structures such as the ventricles which are visible on standard axial views. Sagittal trans-fontanelle views and 3D ultrasound can allow more detailed assessment of midline structures and conditions. These include development of the corpus callosum, cavum veli interpositi, and the posterior fossa, especially the vermis.

Agenesis of the corpus callosum is readily seen but surprisingly infrequently detected. Midline cysts such as the cavum veli interpositi are very common and only on rarely clinically significant. The posterior fossa, especially the vermis, remains a problem. However, sagittal scanning and 3D are starting to differentiate significant disorders from benign variants such as Blake's pouch cyst. MRI is proving useful.