Mega cisterna magna needs to be distinguished from other causes of an enlarged retrocerebellar CSF space:
Posterior fossa arachnoid cyst
Epidermoid cyst: often shows a heterogeneous/dirty signal on FLAIR and restricted diffusion
Dandy Walker malformation: vermis not intact
Blake's pouch cyst: usually a hydrocephalus is present
Pilocytic astrocytoma: if very posterior and predominantly cystic.