During imaging tests Overview of Imaging Tests Imaging tests provide a picture of the body’s interior—of the whole body or part of it. Imaging helps doctors diagnose a disorder, determine how severe the disorder is, and monitor people after... read more , contrast agents may be used to distinguish one tissue or structure from its surroundings or to provide greater detail.
Contrast agents include
Radiopaque contrast agents (sometimes inaccurately called dyes): Substances that can be seen on x-rays
Paramagnetic contrast agents: Substances that are used in magnetic resonance imaging Magnetic Resonance Imaging (MRI) In magnetic resonance imaging (MRI), a strong magnetic field and very high frequency radio waves are used to produce highly detailed images. MRI does not use x-rays and is usually very safe... read more (MRI)
Radiopaque Contrast Agents
A radiopaque contrast agent absorbs x-rays and thus appears white on x-rays. It is typically used to show the following:
The interior of the gastrointestinal, biliary, or urinary tract
Blood flow in organs
Usually, the contrast agent is injected into a vein (intravenous contrast), artery (angiography Angiography In angiography, x-rays are used to produce detailed images of blood vessels. It is sometimes called conventional angiography to distinguish it from computed tomography (CT) angiography and magnetic... read more ), taken by mouth (oral contrast), inserted through the anus (rectal contrast), or injected into a joint using a needle.
The contrast agent used depends on what type of test is done and which body part is being evaluated:
For blood vessels: Usually contrast agents that contain iodine (iodinated contrast agents)
For the gastrointestinal tract: Contrast agents that contain barium or iodine (gastrografin)
Before a test that uses a contrast agent, people may be asked to refrain from eating for several hours and from drinking for 1 hour. After the test, drinking extra fluids for the rest of the day is recommended.
When some contrast agents are injected, people may feel a warm sensation throughout the body. Other contrast agents may cause a cold sensation at the injection site. Contrast agents taken by mouth may have an unpleasant taste.
Side effects of radiopaque contrast agents
Generally, radiopaque contrast agents are very safe, particularly when given by mouth or into the rectum.
Side effects of injected iodinated contrast agents occur in a few people. They include
An allergic-type reaction
Kidney damage, especially when people have a preexisting kidney problem or when large amounts of contrast is used
Allergic-type contrast reactions
Allergic-type contrast reactions vary in severity:
Mild, such as nausea, flushing, or itching
Moderate, such as a rash, vomiting, or chills
Severe and life threatening (anaphylactoid Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more ), such as a swollen throat that interferes with breathing, wheezing, very low blood pressure, or an abnormal heart rate
At the first sign of a reaction, the contrast agent is stopped. Mild or moderate reactions are treated with the antihistamine diphenhydramine, given intravenously. Severe reactions may be treated with oxygen, fluids given intravenously, epinephrine, or other drugs, depending on the type of reaction.
Allergic-type contrast reactions are most likely to occur in people who have one of the following:
Have many other allergies
Have previously had allergic-type reactions after a contrast agent was used
If people have had several severe reactions to iodinated contrast agents, an imaging test that does not require this contrast agent should be done instead. If an iodinated contrast agent must be used, drugs (diphenhydramine and a corticosteroid) may be given before the test to prevent a reaction. People who previously have had a reaction to a contrast agent should tell their doctor before an imaging test is done.
Kidney damage (contrast nephropathy Acute Tubular Necrosis Acute tubular necrosis is kidney injury caused by damage to the kidney tubule cells (kidney cells that reabsorb fluid and minerals from urine as it forms). Common causes are low blood flow to... read more ) due to use of a iodinated contrast agent may occur in people with certain conditions:
Impaired kidney function
Age over 70
High blood pressure (hypertension)
Use of drugs that can damage the kidneys
In over 99% of people, the kidney damage causes no symptoms and goes away within 1 week or so. Fewer than 1% have lasting damage, and only a very few of them require kidney dialysis Dialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more .
If tests that require radiopaque contrast agents must be used in people at risk of kidney damage, people are given fluids intravenously before and after the agent is given. A low dose of the contrast agent is used if possible. People who have had impaired kidney function for a long time may be given acetylcysteine the day before and the day the contrast agent is given. Doctors withhold certain oral antihyperglycemic drugs (for example, metformin) for 48 hours after IV radiopaque contrast agents are given. They do this to avoid any accumulation of these drugs in the event that contrast agents impair kidney function.
Did You Know...
Paramagnetic Contrast Agents
Paramagnetic contrast agents change the magnetic properties of particles in a way that increases the contrast between different tissues, making MRI images clearer. These agents usually contain gadolinium.
Side effects of paramagnetic contrast agents
Usually, no side effects occur. However, in a few people who have severe kidney disease or who are undergoing dialysis, these agents may cause a life-threatening disorder called
Nephrogenic systemic fibrosis
In nephrogenic systemic fibrosis, the skin, connective tissue, and organs thicken. Red or dark patches may develop on the skin. The skin may feel tight, movement is difficult and limited, and organs may malfunction. This disorder is now very rare because doctors use gadolinium paramagnetic contrast agents in people with kidney problems only when necessary, and they use the lowest dose and safest agent possible. Doctors also consider using other imaging tests in people with severe kidney problems.