Barium- and Iodine-based materials are used as contrast media in X-Ray scans. Why?

The production of X-ray images depends on the differences between the X-ray absorbing powers of various tissues. This difference in absorbing power is called contrast and is directly dependent on tissue density. To artificially enhance the ability of a soft tissue to absorb X-rays, the density of that tissue must be increased. The absorption by targeted soft tissue of aqueous solutions of barium sulfate and iodized organic compounds provides this added density through the heavy metal barium and the heavy nonmetal iodine.