CT scan and gas encephalography in diagnosis of arachnoid cyst and epidermoid cyst of Galen's cistern.

The CAT scan is insufficient to make the differential diagnosis between an arachnoid cyst and an epidermoid cyst in the cistern of Galen. Gas encephalography is necessary. This procedure reveals arachnoid cysts of the commmunicating type, but is insufficient in distinguishing between an epidermoid cyst and a non-communicating arachnoid cyst. In this situation, histograms of the absorption values in each section of the lesion may be helpful for the diagnosis.