Middle Cranial Fossa Arachnoid Cyst

Arachnoid cysts are relatively common benign cysts within the subarachnoid space that contain CSF but do not communicate with the remainder of the CSF space. As a result they follow CSF on ALL sequences, including FLAIR and DWI. Beware however of CSF flow artefact due to pulsation.
Most frequently (50-60%) they are located in the middle cranial fossa, with cerebellopontine angle accounting for 10%.
They are extremely well circumscribed, with an imperceptible wall, and displace adjacent structures (e.g. blood vessels). When large and over time they can exert a remodelling effect on the bone.
Although the vast majority are sporadic, they are seen with increased frequency in mucopolysaccharidoses
For an amazing intraoperative image of the same case visit Radiopaedia.org here.
Reference:
1. StatDx.com, Amirsys
Credit: Dr Frank Gaillard
http://www.radpod.org