What Is the Contrast Dye Used in CT Scans?

Contrast is used in CT scans to increase the visibility of various internal tissues in the body. The density of blood vessels, the kidney and ureters, and other organs can be similar to the surrounding tissues, making them hard to visualize on a radiograph.

How Contrast Improves Images

Images using X-rays are formed by the number of x-ray photons which hit a special "detector" in a CT machine. The more X-ray photons that hit the detector, the darker the image will appear. Soft tissues are not dense so more photons hit the detector, resulting in a darker radiographic image without much contrast.

The iodine or barium in contrast dye is very dense and "absorbs" X-ray photons at a greater rate than the body's soft tissues. The body part with the contrast inside will appear lighter on the radiographic image than the surrounding tissue.

The images at the right demonstrate the difference in visibility of the abdominal organs with and without contrast.

The top image shows the abdomen without contrast. The less dense tissues of the liver, kidneys and other organs appear dark, while the denser bones of the vertebra and ribs appear light.

The bottom image shows the abdomen after an IV contrast injection. Notice how the kidneys, veins, and arteries appear much more visible when compared to the first image without contrast.

CT contrast, also referred to as contrast "dye," is introduced into a patient's body intravenously (IV), orally, or rectally.

Intravenous Contrast

IV contrast is a liquid that is injected into a person's circulatory system. The IV contrast is stored in a sterile container. Tubing is connected to the container, and then the contrast is rapidly pushed through the sterile tubing, through an IV needle, and then into a vein.

Intravenous CT contrast contains very small amounts of iodine. X-rays pass through the surrounding tissue, but they are absorbed by the iodine molecule in the contrast. This provides the different levels of dark and light in a CT image.

Oral Contrast

Oral contrast will be either a barium sulfate solution or Gastrografin, an iodine-based solution. Both are either swallowed or used as an enema. Barium sulfate, a natural mineral, is a thick, chalky solution, whereas the iodine-based solution is a clear liquid that is lemon-flavored.

The barium, like iodine, is denser than the body's tissues. Barium sulfate acts in a similar way to iodine molecules. It also absorbs more X-ray photons than surrounding tissues, thus providing the contrast needed to see the anatomy of the digestive tract.

The contrast, when taken orally, will be drunk 2-12 hours before the scheduled CT scan.

Reactions to IV contrast vary from the mild to life threatening. Certain people are more susceptible to serious reactions to IV contrast. People who are at higher risk include those who have:

• had a past reaction to IV contrast
• are asthmatic
• have a prior history of allergic reactions to things other than contrast dye
• have a history of heart disease
• have a history of kidney disease
• are older
• are diabetic

People with these prior conditions will be carefully screened to ensure they are healthy enough to receive IV contrast. Blood will be drawn and analyzed, and an antihistamine may be prescribed prior to the next IV injection.

A person who is taking glucophage or metformin must abstain from it for 48 hours after receiving CT IV contrast. They must also return to their doctor to have blood drawn and their creatinine levels checked.

A reaction to IV contrast is not a true allergy. According to the website U.S. Pharmacist, "Reactions to contrast media are not a true allergy, but rather a pseudoallergy... There is no allergic antibody present that causes the reaction.... Contrast media act to directly release histamine and other chemicals from mast cells.... The higher the iodine concentration, the greater the risk of an adverse reaction."

If you have a reaction to the contrast, that means you are allergic to iodine.

According to the website CT Scan Info, "'Iodine allergy' doesn't really exist. Iodine is a substance essential to life and is found throughout your body and within the thyroid hormone."

Iodine is not an allergen. Iodine is a natural element and is essential for the proper functioning of the thyroid. Iodine is found in seafood, dairy products, and some vegetables. If you had a reaction to the contrast, you are allergic to a substance in the contrast itself.

Is it true that if you are allergic to seafood, you cannot have intravenous CT contrast?

Seafood allergies are not related to the iodine found naturally in seafood. People who are allergic to seafood can eat other foods with iodine with no ill effects.

When a person is allergic to shellfish, the body's allergic reaction is to tropomyosin, a protein found in muscles, not the iodine itself.

People who are sensitive to topical solutions containing iodine cannot have IV contrast.

A sensitivity to these topical solutions do not contraindicate the use of IV contrast. According to the website U.S. Pharmacist, "Sensitivity to Betadine and other iodine-containing solutions is unrelated to reactions to iodinated radiographic contrast agents."