Contrast and Dyes

Generally, physicians would rather not use contrast agents, but they are necessary in certain situations. Contrast dye can provide important information that could not have been obtained otherwise. Contrast agents used in radiology include dyes that are used to image the GI tract, dyes that can be injected into the spinal canal, and other agents. Most contrast agents, including barium and iodine-based dyes used intravenously for x-rays and CT scans are safe, but there may be rare complications.

Questions concerning the use of contrast agents include when to use them, which of them to use, and what dosage to use. The only facts that are uncontested in the medical community are that

  1. Sometimes contrast agents provide physicians with information that they could not have obtained without them.
  2. This information is often very important.

How Contrast Agents Are Used

Materials of certain atomic numbers interact with radiation in predictable ways. Barium, for example, is very similar to calcium in its atomic makeup and looks dense on an x-ray—just like bones that comprised of calcium. So when barium is introduced into the GI (digestive) tract, you can seen the outline (actually the "inline") of the organ(s).

Iodine is similar. It isn't quite as dense, but unlike barium, iodine is intravenously acceptable. By convention, radiologists assign the dense barium and less dense iodine to the whitest parts of the gray scale. Wherever this dye appears in any concentration on the images, it looks white like the bones.

Frequently Asked Questions about Contrast Agents

  • Why do radiologists use contrast agents? Contrast agents improve our images and allow radiologists to see things that they might not have been able to see without them. Dyes do not necessarily make every study better, but they help with many.
  • Is "dye" the wrong word? It's not exactly wrong, but it's not as accurate as contrast agent or contrast material.
  • Does a patient have any choice over which contrast agent is used? Radiologists are trained to perform imaging procedures and use contrast agents. Talk to your radiologist if you have concerns.
  • I am supposed to have an imaging test, but now I have a cold. Should I cancel or should I go? In most cases, a minor cold does not affect a radiological procedure or test.
  • The preparation says that I am to take nothing by mouth. Does that include my medications? Usually not. Talk to your doctor and take your medications with as little water as possible. Calcium supplements should be taken after the test.

Questions about Barium

  • Can anything be done about the taste of the barium? Barium usually is more tolerable cold. Unfortunately, what is unpleasant about the substance is what makes it effective.
  • Barium is used for upper GI series, barium enemas, CT scans; can it also be used intravenously? Never, never, never. Though inert, insoluble barium in the vascular system acts just like a blood clot and can cause very serious damage.
  • Is barium safe? Fortunately, barium is as inert as a substance can be. Except for its slightly unpleasant taste and its propensity to become impacted if left around in the large bowel for a few days, it is the perfect contrast agent. It is also well tolerated.
  • Are there any complications to barium? While barium is a terrific agent in the gastrointestinal tract, it can cause serious complications when it gets into places it doesn't belong—like the peritoneal space (through a perforation in the stomach or duodenum) or into the sigmoid colon (from diverticulitis).
  • Are there alternatives to barium? Yes, very good ones. If a perforation is suspected in the upper tract, the radiologist may want to use a water-soluble iodine-based agent. Leakage of this material is of no major concern because it is just reabsorbed back into the vascular system and then excreted by the kidneys. Similarly, in the large bowel, if a perforation is suspected, the radiologist may choose to use a water-soluble agent rather than barium.
  • Is the barium used for an upper GI (UGI) series and for CT scans the same? Yes and no. The agent is still barium, but the density is very different. The UGI barium is thick and dense, so it can be seen on conventional x-ray and fluoroscopy; while the agent used in CT is quite a bit less dense (and easier to tolerate). The thin barium is too thin to be seen well enough on x-rays; the thick barium, on the other hand, is so dense that it can ruin CT images.

Questions about Iodine

  • How common are serious reactions to the contrast material? The chances that any given patient will have a reaction are very, very small. Technicians must obtain "informed consent" before the procedure, so they must discuss all possible risks with the patient.
  • How much of an ordeal will the injection be? Most x-ray dyes can be injected through relatively thin butterfly needles or small plastic tubes (catheters) that are not difficult to insert.
  • What's the most common adverse reaction? Hives—a rash composed of small wheals of that are raised and itch—are most common. Hives may be solitary, in a bunch, or very rarely all over the body. Most cases do not require treatment and subside on their own. Widespread hives may be treated with a small dose of antihistamine and severe cases (rare) may require epinephrine.
  • Are there different kinds of intravenous dyes? Ther are many different kinds and different brands. There are two main categories of iodinated dye for intravenous injection, high osmolarity contrast agents and low osmolarity contrast agents, so called HOCA and LOCA, respectively.
  • I have heard that you can be pre-medicated with steroids and antihistamines to prevent adverse reactions. Does that work? Yes, pre-medication may reduce the incidence of reactions. However, most radiology departments do not routinely pre-medicate patients. Talk to your radiologist, nurse, or technologist about pre-medication.
  • If I had a reaction before, is it more likely that I'll have a reaction now? Yes, but much greater for HOCA than LOCA. The problem is that the second reaction can be worse than the first.
  • I am an asthmatic, so can I still have the dye? Guidelines and risk figures vary. Many radiologists are hesitant to use any form of iodinated dye, HOCA or LOCA, in people with uncontrolled asthma. Imaging without contrast, ultrasound, and/or MRI scan may be preferred if similar information can be provided using these tests. If contrast is necessary, decisions are made on a case-by-case basis.
  • I have renal disease, so can I have the contrast? It depends. Iodinated dye is slightly toxic to the kidneys as it passes through. This usually is not a major concern, as long as the kidneys are functioning well. If the kidneys are not functioning well, contrast dye may cause the organs to shut down. To prevent this circumstance, the patient must be well hydrated and the test should not be performed if laboratory tests suggest significant renal impairment.
  • I take medications; are there any that interact with the dye? There are a few interactions that require caution. For example, if you are taking glucophage, you must stop taking it a few days before the test. Check with the radiology department.
  • Are fatal complications possible? The chance of having a severe anaphylactic reaction that results in death is extremely rare—less than one hundredth of one percent.
  • If not my own physician, who would respond to a severe reaction? If there is a rare, serious reaction, an organized team of physicians will address the problem. Each radiology department has a plan for emergencies and has a qualified physician is immediately available to provide emergency medical care.
  • Does it feel different, depending on which type of dye is injected? Yes, that is often the case: the LOCA and HOCA dyes feel different. As a general rule, patients injected with LOCA feel nothing, while those injected with HOCA may experience unpleasant side effects, including nausea and vomiting.
  • I've been told that my urine will change color from the dye. Is that true? No, IV dyes used in radiology are clear going and coming out.
  • I have heard that occasionally patients get strange sensations in their genitals from the dye injection. Is this true? It is uncommon, but some patients may experience a feeling of warmth in their anal and genital regions. Women, in particular, seem to be prone to this reaction.
  • Will I be able to drive after a contrast injection? The contrast material should not affect your ability to drive, but talk to your radiologist ahead of time to be sure. If you have had an antihistamine because of a minor reaction, you should not drive home by yourself.
  • Are there delayed reactions to the dye? Delayed reactions are rare and include minor problems at the site of the injection.
  • If I have a reaction after the procedure, what should I do? After the reaction has been treated, make sure that a note is added to your medical records and your list of allergies for future reference. Some people wear medical alert jewelry indicating an allergy to iodine.

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 01 May 2000

Last Modified: 23 Mar 2015