A major limitation inherent in conventional x-ray technology is the inability to show the difference between soft tissues (e.g., blood vessels, muscles and intestines). Basically, a radiograph shows the difference between bone, air and in certain situations fat. Some soft tissues such as the pancreas are not recognized at all, this is why contrast materials have to be used often.
About thirty percent of all radiological examinations use the addition of a contrast media to the body in order to visualize different systems. Contrast media are put in a class with pharmaceuticals by the Federal Food and Drug Administration, and so it is very important to have a knowledge of agents in order to predict their effect on the patient and to be able to evaluate the advisability of using them in a procedure.
In radiology, contrast means density difference. Contrast agents usually add density to the x-ray film by absorbing more radiation than the surrounding tissue. In other words the contrast agent will make the film brighter than the neighboring areas where there is no contrast present.
Example—If you have a bottle filled with milk, you can visualize the milk clearly. You may have a problem seeing where the bottle surface is, but you would not have a problem with seeing the milk. Barium sulfate, a contrast material, is injected into the body to outline the intestine. Much in the same way as the milk is seen in the bottle, barium defines the intestine so that it can be visualized better. It is difficult to differentiate the intestine from surrounding tissue without a contrast media. (See Figure 5 and X-ray Package at end of unit.)
Contrast agents injected into the blood stream usually contain iodine. The more iodine in the compound use, the greater the radio-density of the contrast agent. Visualization of blood vessels is clearer and interpretation of the roentgenograms is enhanced. (See IVP Arteriography in X-ray Package at end of unit.)