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CT Scan

When Is A Contrast Agent Required?

Last modified: 
21/03/2012 - 14:11

Contributing Author: Guy Slowik FRCS

CT contrast agents, sometimes referred to as "dyes," are used to highlight specific areas so that the organs, blood vessels, or tissues are more visible. By increasing the visibility of all surfaces of the organ or tissue being studied, they can help the radiologist determine the presence and extent of disease or injury.

Contrast agents are available in several different forms, but in general a CT contrast agent is a pharmaceutical substance. Some of the more common contrast agents used are:

  • Iodine
  • Barium
  • Barium sulfate
  • Gastrografin

Contrast agents for CT examinations are administered in four different ways:

  • Intravenous injection
  • Oral administration
  • Rectal administration
  • Inhalation-this is a relatively uncommon procedure in which xenon gas is inhaled for a highly specialized form of lung or brain imaging. The technique, xenon CT, is only available at a small number of locations worldwide and is used only for rare cases.

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) is clear, with a water-like consistency. Typically the contrast is contained in a special injector, which injects the contrast through a small needle taped in place (usually on the back of the hand) during a specific period in the CT exam.

Once the contrast is injected into the bloodstream, it circulates throughout the body. The CT's x-ray beam is weakened as it passes through the blood vessels and organs that have "taken up" the contrast. These structures are enhanced by this process and show up as white areas on the CT images. When the test is finished, the kidneys and liver quickly eliminate the contrast from the body.

Need To Know:

Is Iodine a Safe Contrast Agent?

Iodine is considered to be a safe contrast agent. It has been used for many years without serious side effects. Because iodine contrast increases the visibility of target tissues on the images, the benefits are considered to outweigh the risks.

The most common side effect of iodine is a warm or "flushed" sensation during the actual injection of the iodine, followed sometimes by a metallic taste in the mouth that usually lasts for less than a minute. No treatment is necessary for this sensation, if experienced.

Another mild reaction is itching over various parts of the body. This reaction lasts from several minutes to a few hours after the injection. When this reaction occurs, medication is usually administered to counteract the itching.

More serious allergic reactions, while uncommon, include difficulty breathing and swelling of the throat or other parts of the body. These reactions, if experienced, are treated immediately.

Newer forms of contrast help to reduce the risk of an allergic reaction. If you have had an allergic reaction to iodine or a contrast agent in the past, the physician may recommend on of these newer agents.

In some cases, CT can still provide valuable diagnostic information without the administration of a contrast agent, so the physician may decide this is the best course of action.

Oral CT 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 about 1,000 cc to 1,500 cc (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 weaken x-rays. On CT images, the organs that have "taken up" the contrast appear as highlighted white areas.

Need To Know:

Is Oral Contrast Safe?

In general, both barium and gastrografin contrast are safe and pass uneventfully through the gastrointestinal tract. Minor and temporary side effects, such as constipation, may occur.

Rectal CT 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 a Fleets Enema to cleanse the colon; it is usually used the night before the examination.

Need To Know:

Is Rectal Contrast Safe?

Rectal contrast is considered to be safe and passes through the gastrointestinal tract uneventfully. Minor and temporary side effects, such as constipation, can occur.

 

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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.