Radiography of the spinal cord to detect possible lesions after an injection of a contrast medium into the subarachnoid space (situated beneath the middle of the three membrane arachnoids that cover the brain and spinal cord).

The arachnoid is named for its delicate, spider-web-like filaments that extend from its under surface through the cerebrospinal fluid in the subarachnoid space to the pia mater or the innermost membrane surrounding the central nervous system.