If your doctor has referred you for an imaging test like an X-ray, ultrasound, computed tomography (CT) scan, or magnetic resonance imaging (MRI) scan, the order may be for it to be done with a contrast agent (sometimes casually called a contrast dye, though that is inaccurate since it does not actually dye your insides). Given intravenously, swallowed, or administered via an enema before an imaging exam, these materials help distinguish organs, blood vessels, or various other structures in the body from surrounding tissues, thereby helping make abnormalities (including tumors) more easily visible. But should you be concerned about side effects or even long-term risks from these materials?
Most people don’t experience any adverse reactions from contrast materials, and for those who do, they are generally mild, transient, and harmless. Side effects include nausea, vomiting, flushing (feeling of warmth), headache, and itching; severe reactions are very rare.
Problems for kidneys?
In a small subset of people with pre-existing kidney failure, a contrast material used in MRI, the heavy metal gadolinium, can cause a condition called nephrogenic systemic fibrosis that results in a thickening of the skin, organs, and other tissues. Adverse reactions involving multiple organ systems in people with normal kidney function have also been reported to the Food and Drug Administration (FDA), but this is rare, and cause-and-effect has not been established.
While the risks are unproven and at worst slight, since most gadolinium is eliminated from the body by the kidneys within 24 hours, small amounts of it can be retained by the body for years, including in the brain. In a drug safety communication issued in 2018, the FDA affirmed that the benefit of gadolinium-based contrast agents outweighs the risk when imaging tests with the agent are needed but recommended that doctors take this retention characteristic into consideration, particularly in people who have multiple doses in their lifetime, pregnant women, children, and those with inflammatory conditions—and advised that they minimize its use as much as possible.
Another contrast material that has been of concern for people with impaired kidney function is iodine, used to enhance X-rays and CT images. It has been linked to an increased risk of developing contrast-induced nephropathy, a condition in which already-impaired kidney function worsens. But much of the research was done on agents no longer used; more recent studies have not confirmed a risk. In 2020, the American College of Radiology and the National Kidney Foundation issued a consensus statement that the risk of acute kidney impairment from iodinated contrast material in people with reduced kidney function has been “overstated,” though it acknowledged that the true risk for people with severe kidney disease “remains uncertain.”
According to Mario Corona, M.D., a nephrologist and member of our editorial board, “Because contrast-induced kidney damage can occur, the decision to use iodinated contrast material always deserves a careful risk/benefit analysis.”
What about radioactivity?
Positron emission tomography (PET) scans—and rarely some CT and MRI scans—add radioactive tracers to contrast materials (under the umbrella of nuclear medicine) to provide information that other imaging tests don’t. This helps in diagnosing or tracking conditions ranging from heart disease and cancer to bone disorders and intestinal bleeding. These tests expose patients to small doses of radiation (and make them radioactive for a short time afterward), but the tracers are usually out of the body within 60 hours. According to the National Institute of Biomedical Imaging and Bioengineering, an arm of the National Institutes of Health, concern about cancer formation from even low levels of radiation is legitimate if there is an accumulation of nuclear medicine imaging in one patient. But “this risk is accepted to be quite small in contrast to the expected benefit derived from a medically needed diagnostic imaging study.” That is, if your doctor orders this type of imaging, do not forgo it because of radiation concerns.
Minimizing risks
Contrast materials—which also include barium sulfate used with some X-rays and CT scans—come with a very slight risk of allergic reaction, with symptoms ranging from hives and red skin to swelling of the throat and difficulty breathing. It’s essential to tell the staff at the imaging facility if you have a history of allergies or asthma so they will be ready with an antidote if needed. And whatever contrast agent you are given, the Radiological Society of North America recommends increasing fluid intake afterward to help speed elimination of the substance from the body.
