Not every CT scan requires the use of contrast. In some cases, a pharmaceutical contrast agent may be used after your referring doctor and radiologist have agreed that it is needed for diagnostic purposes. There are different forms of contrast agents, the most common of which is iodine, and can be administered orally, rectally, or intravenously. Using contrast increases the visibility of organs and structures with a high amount of blood vessels, and helps identify masses or infection. Many physicians recommend using contrast for abdomen or pelvis CT scansfor example, in case of evaluating kidney stone infection. CT scans of the brain may also be accompanied by contrast in order to find malignancy or abscess. For gastrointestinal CT scans, an oral contrast agent containing barium sulfate or gastrografin are likely to be used.