Colloid cyst of the third ventricle: radiologic-pathologic correlation.

We describe a patient with a large colloid cyst of the third ventricle and resultant hydrocephalus. An unenhanced brain CT demonstrated the central portion of the mass to be of increased density relative to the peripheral portion. On T1-weighted images, the central portion of the mass was of increased signal intensity relative to brain, whereas the periphery was isointense. On T2-weighted images, the central portion was markedly hypointense to brain, while the peripheral portion was isointense. These imaging findings corresponded to an actual pathological difference between the central and peripheral portions of the mass. The central portion was solid, whereas the periphery was liquid. Since no histological difference was identified between the central and peripheral portions of the mass, we hypothesize that the differences in densities on CT and signal characteristics on MR were related to the state of hydration in the mass where the central portion possibly had a higher protein concentration than the periphery.