What is the differential of an enlarged posterior fossa 'CSF-like' space?
The main differential is between a mega cisterna magna, an epidermoid cyst and an arachnoid cyst. Cerebellar malformations (e.g. Dandy-Walker malformations) need to also be considered.
Which sequence(s) is most helpful in distinguishing between an arachnoid cyst and an epidermoid cyst?
DWI/ADC will show no restricted diffusion in arachnoid cysts (and mega cisterna magna) whereas epidermoid cysts will show intermediate restriction.
MRI through the posterior fossa demonstrates a large right sided extra-axial CSF intensity mass lesion. It follows CSF on all sequences, including FLAIR and DWI/ADC. There is significant mass effect on the adjacent cerebellar tissue and remodelling and expansion of the adjacent skull is evident.
High resolution T2 images (FIESTA) demonstrates that this lesion is bounded by a very thin membrane, best seen bulging across the midline towards the left.
Sagittal images demonstrate upward bowing and thinning of the corpus callosum suggesting hydrocephalus.