Mega cisterna magna
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Mega cisterna magna appearing as a wedge-shaped area of CSF density in the midline posterior fossa region resting on the inner table of the squamous portion of the occipital bones. Its apex is seen pointing towards the cerebellar vermis. A midline fold of the arachnoid membrane is seen crossing the cistern in an AP orientation.
A mega cisterna magna is a controversial subject. In general, however, the term refers to non-pathological enlargement of the retrocerebellar CSF space, not associated with cerebellar abnormalities (normal cerebellar vermis and hemispheres).
One of the supportive findings for the diagnosis of mega cisterna magna is the presence of internal lines crossing inside the cistern representing folds of the arachnoid membrane. These folds are virtually never seen in any similar lesion like a retrocerebellar arachnoid cyst or Dandy-Walker malformation.