Contast is the name given to any substance used to show up a part of the body more clearly. For example barium can be given orally (barium swallow) or rectally (barium enema) to show up the bowel.
X rays are used to follow the contrast through the body. Iodine based dyes can be injected to show up the kidneys (intravenous urogram or IVU) and blood vessels.
Contrast agents are also commonly used in CT and MRI scans.
