Mega cisterna magna
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Prominent cisterna magna appearing as a wedge shaped area of CSF density in the midline posterior fossa region resting on the inner table of the squamas portion of the occipital bones. Its apex is seen pointing at the cerebellar vermis. A fold of the arachnoid membrane is seen crossing inside the cistern.
A mega cisterna magna is a controversial subject. In general, however, the term refers to non-pathological enlargement of the retro-cerebellar CSF space and 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 virtually are not seen in any similar lesion like a retrocerebellar arachnoid cyst and Dandy-Walker malformation.