Arachnoid Cyst

Axial T1W MR image shows a large T1 hypointense extra axial lesion in the left parietal region (arrow). 

Coronal T2GRE and T2W images demonstrate homogeneous T2 hyperintensity of the mass that follows CSF signal. There is compression of the adjacent gyri.  

Facts: Arachnoid Cyst

• Congenital lesion arising during development from splitting of arachnoid membrane
• Contains fluid usually identical to CSF
• Do not communicate with ventricles or subarachnoid space
• Lined with meningothelial cells 
• Incidence: 5 in 1,000 autopsies. 1% of intracranial masses
• Predominant male
• Mostly asymptomatic

Imaging:

• Extraaxial cystic mass with pressure effect to underlying brain
• Signal intensity follows CSF in all sequences
• No restricted diffusion
• Most common location (from highest to lowest frequency: Sylvian fissure (almost 50%), cerebellopontine angle, supracollicular, vermin, sella/suprasella

Reference:

Greenberg MS. Handbook of Neurosurgery, 7th edition. 2010