Radiation Basics

What is a Contrast Agent

Contrast agents, also known as "dyes," are used to highlight specific organs, blood vessels, or tissues and make them easier for the radiologist to determine the extent of disease or injury.
The most common contrast agents that are used include:

  • Iodine
  • Barium sulfate
  • Gastrografin
  • Gadolinium

These may be administered in three different ways:

  • By injection into the vein (intravenous)
  • By mouth
  • Through the rectum

Intravenous Contrast

Intravenous contrast is used to highlight blood vessels and to enhance the structure of organs like the brain, spine, liver, and kidney. The contrast agent (usually an iodine compound or gadolinium) is clear and is injected through a small needle taped in place (usually on the back of the hand) during a specific period in the imaging exam.

Once the contrast is injected into the bloodstream, it circulates throughout the body. When the test is finished, the kidneys and liver quickly eliminate the contrast from the body.

Iodine is considered to be a safe contrast agent but some individuals can have an allergic reaction that can be severe or fatal. Because iodine contrast increases the visibility of target tissues on the images, the benefits are considered to outweigh the risks but discuss this option first with your doctor to determine if this is the best choice for you.

Gadolinium is often used as a contrast agent for MRI tests. While it is considered largely free of side effects, some may experience side effects that are mild to severe. As with iodine mentioned above, it is always best to discuss the possible risks with your physician.

Oral Contrast

Oral contrast is used to highlight gastrointestinal (GI) organs in the abdomen and pelvis. If oral contrast will be used during an examination, the patient will be asked to fast for several hours before administration.

Two types of oral contrast are used:

Barium sulfate, the most common oral contrast agent, resembles a milk shake in appearance and consistency. The compound, available in various flavors, is prepared by mixing with water.

Gastrografin is a yellowish, water-based drink mixed with iodine. It can have a bitter taste.

When oral contrast has been requested by the doctor, patients usually drink the equivalent of three or four 12-ounce drinks.
After the contrast is swallowed, it travels to the stomach and gastrointestinal tract. Like intravenous iodine, barium and gastrografin highlight the organs that have responded to the contrast as white areas.

In general, both barium and gastrografin contrast are safe and pass through the gastrointestinal tract but some patients may experience mild to severe side effects. Discuss the benefits versus risks with your physician.

Rectal Contrast

Rectal contrast is used when enhanced images of the large intestine and other lower GI organs are required. The same types of contrast used for oral contrast are used for rectal contrast, but in different concentrations.

Rectal CT contrast is usually administered by enema. When the contrast is administered, the patient may experience mild discomfort, coolness, and a sense of fullness. After the CT is complete, the contrast is drained and the patient may go to the bathroom.

The preparation for rectal contrast is similar to oral contrast, in that the patient should be fasting for several hours before the test. In addition, the patient will be required to use an enema to cleanse the colon the night before the examination.