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Overview of Imaging Tests

by Hakan Ilaslan, MD

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 the disorder is diagnosed. Most imaging tests are painless, relatively safe, and noninvasive (that is, they do not require an incision in the skin or the insertion of an instrument into the body).

Imaging tests may use the following:

  • Radiation, as in x-rays, computed tomography (CT), and radionuclide scanning

  • Sound waves, as in ultrasonography

  • Magnetic fields, as in magnetic resonance imaging (MRI)

  • Substances that are swallowed, injected, or inserted to highlight or outline the tissue or organ to be examined (called contrast agents or dyes)

Risks of Radiation in Medical Imaging

Imaging tests that use radiation, usually x-rays, are a valuable tool in diagnosis, but exposure to radiation has some risks.

Different diagnostic tests require different amounts of radiation, but most of them use low doses that are generally considered safe. For example, the radiation dose from one chest x-ray is more than 100 times lower than the average yearly dose of radiation from the environment. However, if people have many diagnostic tests that use low doses or several tests that use high doses, they may be exposed to a relatively large amount of radiation. Exposure to radiation is cumulative, regardless of the interval between tests. Such exposure increases the risk of cancer and sometimes damages tissues.

Did You Know...

  • Exposure to radiation is cumulative, regardless of the interval between tests.

Imaging tests are only one source of exposure to radiation. Exposure to radiation in the environment (from cosmic radiation and natural isotopes—see Radiation Injury) can be relatively high, particularly at high altitudes. When traveling by airplane, exposure to environmental radiation is increased.

When planning diagnostic tests, doctors consider a person’s total (lifetime) exposure to radiation—the person’s total radiation dose. However, the benefit of a diagnostic test often outweighs the potential risks.

Comparing Radiation Doses For Different Tests*

Imaging Test

Number of Chest X-Rays Needed to Get the Same Dose

Time Needed to Get the Same Dose From the Environment

Chest x-ray (from front to back)


2.4 days

An x-ray series of the lumbar spine


84 days

CT of the head


243 days

Coronary artery angiography during cardiac catheterization


1.15–2.3 years

CT of the abdomen


3.3 years



30–60 days

*These doses account for how much radiation is delivered and how susceptible the body part exposed to radiation is to radiation damage.

CT = computed tomography.

In the United States, about 15% of all imaging tests are CT, but up to 70% of radiation exposure from imaging tests comes from CT. The radiation dose for CT can be hundreds of times the dose for most plain x-rays. However, with newer techniques, doses for some CT scans can be much lower than they have been with older techniques.

Still, even when CT is done using older techniques, the risk is low for adults, and health is unlikely to be affected.

However, the risk due to exposure is higher in certain situations:

  • During infancy

  • During early childhood

  • During pregnancy (particularly early)

  • For certain tissues, such as lymphoid tissues (part of the immune system), bone marrow, blood, the testes, the ovaries, and the intestines

To minimize risks, doctors do the following:

  • Use tests that do not require radiation, such as ultrasonography or MRI, when possible

  • Recommend diagnostic tests that use radiation, particularly high doses (as in CT) and particularly in young children, only when such tests are necessary

  • Take precautions to limit radiation exposure during tests (for example, shielding vulnerable parts of the body, such as the thyroid gland or a pregnant woman’s abdomen) when possible

Modern techniques and equipment have significantly lowered the radiation doses used in imaging tests.

Radiation risk during infancy and early childhood

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:

  • Using the lowest possible dose to make the diagnosis (for example, sometimes low-resolution scans, which use less radiation, can be used)

  • Limiting exposure to the smallest possible area of the body

  • Limiting the number of scans done

Radiation risk during pregnancy

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

  • When during the pregnancy a test is done

  • Which part of the mother’s body is x-rayed

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.

Contrast Agents

During imaging tests, 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

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:

  • Blood vessels

  • The interior of the gastrointestinal, biliary, or urinary tract

  • Blood flow in organs

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:

  • For blood vessels: Usually contrast agents that contain iodine (iodinated contrast agents)

  • For the gastrointestinal tract: Contrast agents that contain barium or 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.

Generally, radiopaque contrast agents are very safe.

Side effects of 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 vary in severity:

  • Mild, such as nausea, flushing, or itching

  • Moderate, such as a rash, vomiting, or chills

  • Severe and life threatening (anaphylactoid—see Anaphylactoid Versus Anaphylactic), 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 asthma

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

  • Impaired kidney function

  • Dehydration

  • Age over 70

  • Diabetes

  • Heart failure

  • High blood pressure (hypertension)

  • Multiple myeloma

  • 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.

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

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.

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