Epidermoid cyst

45-year-old with an epidermoid cyst of the cerebellopontine angle (CPA). MRIs show a slightly lobulated, T2-hyperintense (A), T1-hypointense (B), and nonenhancing (C) extra-axial mass (*) in the left CPA cistern. In this case, the mass is isointense compared with vitreous humor in the eye on both T1- and T2-weighted images; in most cases, epidermoids are slightly hyperintense compared with CSF on T1-weighted images. On trace diffusion-weighted (D) and apparent diffusion coefficient (E) images, the contents of the cyst (keratin, solid crystalline cholesterol) have reduced diffusivity, which helps to distinguish epidermoid from arachnoid cysts.