Contrast X-ray agents usually contain iodine or sometimes barium. These two elements have a high atomic weight, and they are wrapped up inside molecules to give them tissue-friendly characteristics. Owing to the high atomic weight of iodine or barium, the contrast medium absorbs more x-rays than the tissue by itself. Therefore, the organ containing the contrast agent, such as the kidney, stands out clearly on the developed x-ray film and appears whiter than surrounding tissues.