A 33 year-old woman presented had a MRI done for complaints of dizziness.
(Left) Axial T1-weighted MRI; (Right) Axial T2-weighted image. Note the large cystic lesion in the posterior fossa (yellow arrow). This is an arachnoid cyst, a common congenital malformation. The most common locations are the sylvian fissure and middle cranial fossa (as in this case above), suprasellar cistern, quadrigeminal cistern, cerebellopontine angle, posterior infratentorial midline, cerebral convexity and interhemispheric fissure. Most are asymptomatic and are found as an incidental finding. Rarely, they can enlarge and present as a mass lesion with focal neurologic signs, seizures or hydrocephalus.
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