The risks from radiation are higher in infants and young children because children live longer, giving cancers more time to develop. Also in children, cells are dividing more rapidly, and rapidly dividing cells are more susceptible to damage by radiation.

The risk of cancer resulting from radiation is difficult to determine. Some experts estimate that about 18 of every 10,000 1-year-olds who have a CT scan of the abdomen eventually develop cancer caused by the radiation. This scan uses one of the highest doses of radiation in medical imaging. Also, one study suggested that for every 10,000 CT scans of the head done in children under 10 years old, the radiation exposure would cause one case of leukemia and one brain tumor during the decade after that CT scan.

When children require diagnostic tests, parents should talk to the doctor about the risks and about possible use of tests that do not require radiation. If tests that use radiation are necessary, parents can help minimize the risks by asking about the following:

Pregnant women should be aware that radiation from imaging tests has risks for the fetus. If women need to have an imaging test, they should tell their doctor whether they are or may be pregnant. Doctors also consider whether the woman may be pregnant and not know it. However, x-rays, if necessary, can be done in pregnant women. During diagnostic tests, the examiner protects the fetus from exposure to radiation by covering the woman’s abdomen with a lead apron.

The risk to the fetus depends on

During pregnancy, the time when risk is greatest is when organs are being formed, during the 5th to 10th weeks of pregnancy. At this time, radiation can cause birth defects. Earlier during pregnancy, the most likely problem to develop is a miscarriage. After the 10th week, miscarriages and significant birth defects are less likely.

X-rays of parts of the mother's body that are far away from the fetus, such as the wrists and ankles, expose the fetus to less radiation than x-rays of closer parts, such as the lower back. Also, x-rays of smaller body parts, such as fingers and toes, require less x-ray energy than x-rays of larger body parts, such as the back and pelvis. Because of these facts, plain x-rays that do not involve the abdomen have little risk, regardless of when they are done, particularly if a lead shield is worn over the uterus. Thus, if x-rays are necessary (for example, to evaluate a broken bone), the benefit usually outweighs the risk.

A radiopaque contrast agent absorbs x-rays and thus appears white on x-rays. It is typically used to show the following:

Usually, the contrast agent is injected into a vein (intravenous contrast), taken by mouth (oral contrast), or inserted through the anus (rectal contrast). With some tests, the contrast agent is injected into an artery through a catheter or into a joint through a needle.

The contrast agent used depends on what type of test is done and which body part is being evaluated:

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.

Generally, radiopaque contrast agents are very safe.

Side effects of iodinated contrast agents occur in a few people. They include

Allergic-type contrast reactions vary in severity:

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:

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) due to use of a iodinated contrast agent may occur in people with certain conditions:

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.

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.

Paramagnetic contrast agents change the magnetic properties of particles in a way that increases the contrast between different tissues, making the images clearer. These agents usually contain gadolinium and are used for MRI.

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

In this disorder, 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.