n. a diagnostic technique used from 1918 until the mid-1980s to examine the cerebral ventricles and subarachnoid space of the brain by injecting air into the cerebrospinal fluid. Because of the difference in opacity between the air and brain tissues, the air appears as a dark shadow on the resulting radiographic image (X-ray), which is known as a pneumoencephalogram. The development of computed tomography and magnetic resonance imaging has eliminated the need for this test.