This case illustrates the typical radiological features of an cerebral arachnoid cyst which are benign, congenital, intra-arachnoidal space-occupying lesions that contain a clear, colorless fluid identical to CSF ^1,2. They are thought to arise during fetal development of the subarachnoid space by rupture of the webbed arachnoid or by splitting of the arachnoid membrane during its delamination from the overlying dura ^2,3. Arachnoid cysts represent about 1% of intracranial mass lesions ^1-4.

There is strong predilection for intracranial arachnoid cysts in the middle cranial fossa (anterior to the temporal lobes), followed by frontal convexity, posterior fossa and the suprasellar region ^2,3. Less common locations are within the interhemispheric fissure, over the cerebral convexity, the choroidal fissure, cisterna magna, quadrigeminal cistern, and the vermian fissures ^1,4.

Imaging characteristics on CT include ¹:

• sharply demarcated extra-axial cyst that can displace or deform adjacent brain matter
• scalloping of the adjacent calvaria
• no identifiable internal architecture
• no contrast enhancement

MRI characteristic of an arachnoid cyst include ¹:

• the cyst typically has the same signal intensity as CSF on all sequences
• displacement of surrounding structures
• no contrast enhancement

Case courtesy of Dr. Frank Gaillard