Imaging description

Colloid cysts are congenital intracranial masses, derived from embryonic foregut, with an outer fibrous capsule and inner single-layer mucin-producing epithelium, containing mucoid and gelatinous material [1]. They are classically located in the anterior third ventricle, posterior and inferior to the crus of the fornix, which wraps around the cyst. Because of their unique location, they have the potential to impede the flow of CSF at the foramen of Monro, causing obstructive hydrocephalus [1,2].

On CT scan, colloid cysts are seen as oval or rounded anterior third ventricular masses measuring 1 mm to 3cm. They may be hyperdense (Figs. 32.1A, 32.2A, 32.5A), isodense (Fig. 32.3A), or hypodense (Fig. 32.4A), depending on hydration state and proteinaceous content [3,4]. However, up to two-thirds display mild to moderate hyperdensity. On T1-weighted MR imaging, the colloid cyst may be isointense (Fig. 32.2B) or hypointense (Fig. 32.3B), but up to two-thirds appear to be hyperintense (Figs. 32.1B, 32.5B). On T2-weighted MR imaging, there may be hypointense (Figs. 32.1C, 32.2C) or hyperintense (Fig. 32.3C) [3,4]. On post-contrast imaging, there may be mild patchy enhancement of the capsule of the colloid cyst, but the content of the cyst does not exhibit any significant enhancement (Fig. 32.3B). When it is associated with cyst apoplexy, a part of the cyst may appear hyperdense on CT scan and hyperintense on T1-weighted MR imaging (Fig. 32.4A), and may be associated with rapid clinical deterioration [5]. A double density appearance of colloid cyst on CT and MRI due to different consistency of proteinaceous content is also described (Fig. 32.4) [6].