= GIANT CISTERNA MAGNA = ENLARGED CISTERNA MAGNA

= large retrocerebellar CSF space with normal vermis + cerebellar hemispheres

Prevalence: 1% of all brains

May be associated with: infarction, inflammation, infection (esp. CMV), chromosomal abnormalities (esp. trisomy 18)

Size of normal cisterna magna:

3–8 mm measured in midsagittal plane from posterior lip of foramen magnum to caudal margin of inferior vermis

• intact vermis + normal 4^th ventricle
• paired linear echoes that join + descend toward base of posterior fossa = dural folds = inferior attachment of tentorium
• NO enlarged posterior fossa, NO mass effect on cerebellum

DDx: arachnoid cyst