Abstract
<title>Abstract</title> <p> <bold>Background:</bold> Mega cisterna magna (MCM) is an enlarged retrocerebellar cerebrospinal fluid space with preserved cerebellar anatomy. It is usually asymptomatic and often detected incidentally. Knowledge of posterior fossa anatomy is important for distinguishing MCM from other cystic abnormalities. <bold>Case presentation:</bold> We present two adult cases of incidental MCM demonstrated by CT and MRI. A 64-year-old man underwent urgent non-contrast CT following a transient episode of slurred speech. CT showed no acute intracranial abnormality but revealed a 17 mm retrocerebellar CSF space. A 42-year-old woman underwent MRI for intermittent headache. MRI demonstrated a similar CSF space measuring 19 mm and following CSF signal on all sequences. In both cases, the cerebellar vermis and fourth ventricle were normal, without hydrocephalus or significant mass effect. <bold>Conclusion:</bold> These cases demonstrate the typical radiological anatomy of MCM on CT and MRI. A sound understanding of posterior fossa anatomy and embryology helps distinguish MCM from Blake’s pouch cyst, arachnoid cyst and Dandy–Walker malformation. </p>