Colloid Cyst

            Colloid cysts originate from primitive neuroepithelium within the roof of the anterior third ventricle. They are positioned just posterior to the foramina of Monro between the columns of the fornix. Histologically, they consist of a thin, fibrous capsule with an epithelial lining. The cysts contain a mucinous fluid with variable amounts of proteinaceous debris, blood components, and desquamated cells.

            Colloid cysts are smoothly marginated spherical lesions without surrounding brain reaction. Two signal patterns have been reported on MR scans and correlated with their CT features. Those that are low density on CT are isointense on T1-weighted images and hyperintense on T2-weighted images, probably indicating a fluid composition similar to CSF. Most colloid cysts are isodense or slightly hyperdense on CT. The MR counterpart is a high signal capsule and a hypointense center on T2-weighted images. The signal characteristics of the fluid depend on the protein content of the cyst fluid and is similar to that observed in sinonasal secretions. Endnote

            Dilatation of the lateral ventricles is a common finding, and the enlargement may be unequal owing to asymmetric positioning of the cyst at the foramina of Monro. The expanding cyst also enlarges the anterior third ventricle, but the posterior third, aqueduct, and fourth ventricle should be normal. Following contrast infusion, colloid cysts may show ring enhancement, due to either enhancement of the cyst wall or choroid plexus draped around the cyst.  

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