Retrocerebellar arachnoid cyst
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A rather defined retro-cerebellar CSF intensity cyst is noted, with thin few anterior septae seen. It follows CSF signal in all pulse sequences. No restriction on DWI/ADC. This is associated with mild mass effect manifested as indentation of the posterior aspects of both cerebellar hemispheres.
Normal supra and infratentorial ventricular system, basal cisterns and cortical sulci. The mamillopontine distance (10mm).
Ultra-thin 3D sagittal T2 weighted images (CISS) demonstrates that this lesion is bounded by a very thin membrane, best seen bulging across the midline towards the right.
CSF flow dynamic study revealed flow within the retro-cerebellar cyst, denoting communication within the subarachnoid space.
Differential diagnosis is Dandy-Walker malformation, where there are vermian hypoplasia, cystic dilatation of the 4th ventricle and upward displacement of the torcula which are not present in our case. Another differential is Blake pouch cyst with which hydrocephalus is usually present.