Though, physicians do not prefer contrast agents, they are necessary in certain situations. Contrast dye can provide important and valuable information that cannot be obtained otherwise. Contrast agents used in radiology include dyes that are widely used to map the GI tract, dyes that can be injected into the spinal canal, and other agents. Most contrast agents, including barium and iodine-based dyes are used intravenously for x-rays and CT scans. Though they are safe, there may be rare complications. Questions concerning the use of contrast agents include when to use them, which of them to use, and what dose to use. The only facts that are significant in the medical community are that sometimes contrast agents provide physicians with information that they could not have obtained without them. Most x-ray dyes can be injected through relatively thin butterfly needles or small plastic tubes that are not difficult to insert. Materials of certain atomic numbers interact with radiation in some familiar ways. Barium, for example, is very similar to calcium in its atomic makeup and looks dense on an x-ray just like bones that contains calcium. So when barium is introduced into the GI (digestive) tract, you can see the inner line of the organ or organs. Iodine is similar and it is not quite as dense, but unlike barium, iodine is highly acceptable. By convention, radiologists assign the dense barium and less dense iodine to the whitest parts of the gray scale.