Figure 53-2 Computed tomography scan depicting normal (left) and compressed (right) basal cisterns. The basal, or perimesencephalic, cerebrospinal fluid space consists of the interpeduncular cistern (anterior), the ambient cisterns (lateral), and the quadrigeminal cisterns (posterior). In the right panel, in a patient with diffuse cerebral swelling (as a result of sagittal sinus thrombosis), the cisterns have been obliterated. (Courtesy of Ivan Petrovitch, M.D.)