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Pre and post contrast multiplanar multisequence MRI demonstrates a large nonenhancing extra axial collection at the posterosuperior aspect of the posterior cranial again demonstrates no enhancement or diffusion restriction and is consistent with a large arachnoid cyst. A few thin internal septations are noted. It results in mild positive mass effect on the cerebellum and tentorium, extending anteriorly to the quadrigeminal plate cistern and resulting in marked remodelling of the occipital bone. The fourth ventricle maintains normal calibre and there is no hydrocephalus.
Extensive periventricular grey matter heterotopia is again demonstrated. A few small nodules of grey matter are also noted in the left anterior corona radiata. Although a formal T1 FSPGR study has not been performed, there are multiple areas where normal cortical gyral pattern appears disordered but there is no definite focal cortical dysplasia.