used together (hence the term "double-contrast" barium enema) air can be used as a contrast material because it is less radio-opaque than the tissues it is defining. In the picture it highlights the interior of the colon. An example of a technique using purely air for the contrast medium is an air arthrogram where the injection of air into a joint cavity allows the cartilage covering the ends of the bones to be visualized. Before the advent of modern neuroimaging techniques, air or other gases were used as contrast agents employed to displace the cerebrospinal fluid in the brain while performing a pneumoencephalography. Sometimes called an "air study", this once common yet highly-unpleasant procedure was used to enhance the outline of structures in the brain, looking for shape distortions caused by the presence of lesions. Carbon dioxide Carbon dioxide also has a role in angioplasty. It is low-risk as it is a natural product with no risk of allergic potential. However, it can be used only below the diaphragm as there is a risk of embolism in neurovascular procedures. It must be used carefully to avoid contamination with room air when injected. It is a negative contrast agent in that it displaces blood when injected intravascularly. Discontinued agents Thorotrast Thorotrast was a contrast agent based on thorium dioxide, which is radioactive. It was first introduced in 1929. While it provided good image enhancement, its use was abandoned in the late 1950s since it turned out to be carcinogenic. Given that the substance remained in the bodies of those to whom it was administered, it gave a continuous radiation exposure and was associated with a risk of cancers of the liver, bile ducts and bones, as well as higher rates of hematological malignancy (leukemia and lymphoma). Thorotrast may have been administered to millions of patients prior to being disused. Nonsoluble substances In the past, some non water-soluble contrast agents were used. One such substance was iofendylate (trade names: Pantopaque, Myodil) which was an iodinated oil-based substance that was commonly used in myelography. Due to it being oil-based, it was recommended that the physician remove it from the patient at the end of the procedure. This was a painful and difficult step and because complete removal could not always be achieved, iofendylate's persistence in the body might sometimes lead to arachnoiditis, a potentially painful and debilitating lifelong disorder of the spine. Iofendylate's