CT contrast is used to better visualize internal organs and the structures around them. "CT dye",as it is often referred to, is a substance that is opaque to x-rays. When administered, it allows greater visualization of the organ or tissue it fills. There are two kinds of contrast material used in a CT scan with contrast: oral contrast and intravenous or IV contrast. Both are often used at the same time when performing abdominal CT scan.
Drinking oral contrast material, shown in this picture with sweetening agents added, is needed when it's necessary to see the stomach, small bowel and colon. Oral contrast is composed of a mineral called barium sulfate (BaSO4). Barium is a metallic element that is chemically similar to calcium but more reactive. It is a common, naturally occurring mineral.
CT Contrast is sometimes administered by enema, depending on the need, as a suspension of fine particles in a water-based solution. This is not usually common in CT except in special cases when sections of colon need to be better visualized. The compound works due to barium's relatively high atomic number (Z = 56), since large nuclei absorb X-rays much better than smaller nuclei.
The Barium used in CT is much more dilute than the barium used in general radiology. Barium for CT is about 2% of the concentration strength that is used in general radiology so if you've had a procedure in general radiology that required full strength barium, you will need to wait up to 1 week or more until it is completely passed out of your colon before you can have a CT of the abdomen.
Intravenous or IV CT contrast is administered in the vein so you may have to have an IV started before the scan. IV CT contrast material (pictured here) is a substance that is opaque to x-rays. When administered, it brightens internal organs, arteries, veins and tissues as it courses through them. For some exams it is essential and some exams cannot be done properly without it. For many studies it is important for increasing diagnostic accuracy and best detection of subtle cancers or other diseases and conditions. Your referring doctor with your radiologist and the CT Technologist will know best when to use or omit IV contrast for your exam.
The IV Contrast/Iodine Allergy Myth
Some people that have a history of allergy to shellfish, a history of a previous reaction to IV iodinated contrast, or a history of a reaction to a topical iodine solution, have been told that they have "iodine allergy."
This is actually not correct. "Iodine Allergy" doesn't really exist. Iodine is a substance essential to life and is found throughout your body and within the thyroid hormone. The reactions experienced in each of these situations do not have to do with the presence of the iodine.
Instead, in each case, the reaction is to another substance. In the case of shellfish, the reaction is to tropomyosin, a muscle protein. In the case of iodinated contrast, it is the other portion of the contrast molecule, not the iodine. In the case of topical iodine solution, it is to other substances in the solution, not the iodine itself.
Shellfish allergy is not a contraindication to iodinated IV contrast because the allergy is entirely unrelated. This is a longstanding myth even within the medical community and is difficult to dispel. To some degree, asthma will increase the possibility of a contrast reaction. Sensitivity to Betadine® and other Iodine-containing solutions is unrelated to reactions to iodinated radiographic contrast agents.
The American College of Radiology hand book on IV contrast states the following: "The predictive value of specific allergies, such as those to shellfish or dairy products, previously thought to be helpful, is now recognized to be unreliable. A significant number of health care providers continue to inquire specifically into a patient’s history of “allergy” to seafood, especially shellfish. There is no evidence to support the continuation of this practice."
If necessary, and as a precaution, a steroid preparation may be given prior to the injection of IV contrast to alleviate allergic reactions from the other substances in the IV contrast.
But the oral CT contrast you drink to visualize your stomach and intestines and colon is an inert mineral that is naturally found in the earth and does not cause allergic reactions.
However, a substance called Gastrograffin or Gastroview can be used in CT scans for the abdomen. It does have a small amount of iodine in it and can sometimes be used as a substitute for barium. A small amount of approximately 20-40 cc's (an ounce) in 1000cc's (a litre) of water can be mixed and used instead of barium.