Ask The Doctor: Iodine in CT ... Health Article

Mary Pickett, M.D., is a lecturer for Harvard Medical School and an assistant professor of medicine at Oregon Health & Science University in Portland, OR. At OHSU, she practices general internal medicine and teaches medical residents and students.

Question:

Is there an alternative to iodine as a contrast medium in a CT scan?

Answer:

Your question is about radiology "contrast" dye, and some of our readers may need to know what that is before we get to your specific question. To have a computed tomography scan (CT scan) done, you may need to have an injection of "contrast" or "dye." This liquid is visible on X-ray pictures or CT scans, so once it is injected it will cause your blood vessels to be visible to the radiologists. This makes your anatomy very clear. "Contrast" also can highlight areas with inflammation because blood flows more generously to inflamed areas. The reason that contrast dye is visible on X-rays is because it contains highly concentrated iodine.

During the last 25 years, we have learned a lot about radiology contrast dye. Up to 12 percent of people who got contrast dye in its earliest versions had a reaction of some kind. Improvements in the dye have lowered the risk for a significant reaction to closer to one in 100. Typical symptoms of a contrast reaction could include nausea and vomiting, rash, fever, headache, or dizziness.

Some reactions to contrast dye are allergic, but the trigger for the allergy has not been precisely identified. Doctors have noticed that older dye preparations that have a high concentration of iodine are more likely to trigger a reaction. Contrast dye has been adjusted in a variety of ways in order to make it less likely to cause a reaction. Iodine has not been removed from contrast dye, but the dye is now prepared so that the iodine is chemically "hidden" from your immune system. Your immune system does not detect the iodine easily since each iodine particle is packaged inside a complex salt. The newer dye is called "non-ionic" or "low-osmolar" contrast.

Both new dyes and old dyes are in use today. The new dyes are about 20 times more expensive, so they are not used in all cases. A person who had a serious allergic reaction to a "conventional" contrast dye (old version of dye) only has about a one in 100 chance of reacting to non-ionic (low-osmolar) contrast dye.

There are other options if you have had a serious allergy to contrast dye, in addition to having your doctors choose the new dye. It is common for people who had a reaction to take medicine prior to a CT scan, such as an antihistamine and prednisone. In some cases, bubbles of carbon dioxide gas can be substituted for the contrast dye. Finally, a different study (such as an MRI scan) may be used in place of a CT scan, so that contrast dye containing iodine is not needed. MRI scans use a type of dye called gadolinium, which is unrelated to the dye used in CT scans.