D.D.Arachnoid cyst / Epidermoid cyst by diffusion

If the lesion appears similar to the intensity of CSF in DWI---The lesion is arachnoid cyst.

If the lesion appears more brighter than CSF in DWI---The lesion is epidermoid cyst.

A Epidermoid cyst in prepontine cistern with typical restricted diffusion, rules out Arachnoid cyst.   

Arachnoid on diffusion W.I.

On MRI;
Arachnoid cysts have typical imaging findings as in these cases seen as a sharply demarcated cyst.
Extra axial, does not communicate with ventricular system.
Isointense to Csf on all pulse sequences.
Complete suppression of fluid signal on FLAIR.