(Images 1 and 2)
(Images 3 and 4)
(Images 1 and 2)
(Images 3 and 4)
Click on Images for Enlarged View
Findings: Image #1 is an axial T2 weighted image through the head demonstrating an approximately 1 to 1.5 cm lesion located at approximately the region of the roof of the third ventricle between the foramen of Munro. The lesion demonstrates signal intensity equivalent to CSF.
Image #2 is a T1 weighted axial image at the same level as Image #1. This image demonstrates predominantly isointense lesion.
Image #3 is a post contrast enhanced axial image through the same level demonstrating no evidence for enhancement of this lesion.
Image #4 is a post contrast enhanced T1 weighted coronal image which again demonstrates the lesion to be located in the region of the roof of the third ventricle.
Diagnosis: Colloid cyst.
Discussion: The colloid cyst is an endodermally derived lesion with a unique location at the junction of the foramen of Munro. The cyst is generally located in the anterior third ventricle attached to the third ventricular roof positioned between the foramen of Munro. The exact etiology of the colloid cyst is uncertain, but it is presumed to represent endodermal tissue with morphology similar to bronchi or nasal cavity epithelium. The colloid cyst is generally a smooth spherical, well delineated cystic lesion ranging in size from a few mm up to 3 to 4 cm. These are lesions that are most often found in adults, generally between the third and fifth decades of life. There is no sex predilection. The symptoms caused by colloid cysts are generally related to obstructed hydrocephalus due to the precarious position of the colloid cyst at the junction of the third and lateral ventricles. The most common presenting symptom is headache with less common presentations of vertigo, memory deficits, behavioral disturbances, and diplopia.
Colloid cysts are most likely hyperdense on computed tomography with a small percentage demonstrating iso to hypodensity. The location of the lesion is the most characteristic clue to its diagnosis. In addition, enhancement is nearly always absent. The MRI signal characteristics are somewhat variable, but the most common appearance is a hyperintense lesion on T1 with iso or hypointensity on T2 weighted images. However, colloid cyst can display virtually any signal intensity on any pulse or sequence. Due to the variable signal characteristics and sometimes variable density on CT, the most pathognomonic characteristic of this lesions is its well established and unique location at the junction of the foramen of Munro at the roof of the third ventricle.
References:
Osborn A. Diagnostic Neuroradiology. Mosby, St. Louis. 1994;642-645.
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