A 24 year-old woman had an MRI done for symptoms consistent with common migraine.
(Left) Axial T2-weighted MRI; (Right) Sagittal T1-weighted with gadolinium. 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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