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Overview of Imaging Tests

by Jon A. Jacobson, MD

Imaging tests provide a picture of the body’s interior—of the whole body or part of it. 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

Risks of Radiation

Radiation, usually as x-rays, is a valuable tool in diagnosis. 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 (from cosmic radiation and natural isotopes—see see Radiation Injury). 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. 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)

1

2.4 days

X-ray of the lumbar spine

35

84 days

CT of the head

100

243 days

CT of the abdomen

500

3.3 years

Mammography

15

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.

About 70% of radiation exposure from imaging tests comes from CT. The radiation dose for CT is hundreds of times the dose for most plain x-rays. Still, even when CT is done, the risk is low for adults, and health is unlikely to be affected. However, the risk is higher in certain situations: during early childhood, during pregnancy (particularly early), and for certain tissues (such as breast tissue in young women, abdominal tissue, and the thyroid gland).

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

During Early Childhood

The risks from radiation are higher in 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. About 18 of every 10,000 1-year-olds who have CT of the abdomen (which uses one of the highest doses of radiation) eventually develop cancer caused by the radiation.

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

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. However, x-rays, if necessary, are 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. The risk is greatest 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 birth defects are less likely.

The risk to the fetus also depends on what part of the mother’s body is x-rayed. X-rays of body parts 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 substances that can be seen on x-rays (called radiopaque dyes) and substances that are used in magnetic resonance imaging (called paramagnetic contrast agents).

A radiopaque dye absorbs x-rays and thus appears white on x-rays. It is typically used to show blood vessels or the interior of the gastrointestinal, biliary, or urinary tract. Usually, the dye is injected into a vein (intravenous contrast), taken by mouth (oral contrast), or inserted through the anus (rectal contrast). With some tests, dye is injected into an artery through a catheter or into a joint through a needle. The dye used depends on what type of test is done and which body part is being evaluated.

Most radiopaque dyes that are injected into a vein contain iodine (iodinated contrast agents). Dyes that contain barium are used only in the gastrointestinal tract.

Before a test that uses a dye, 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 dyes are injected, people may feel a warm sensation throughout the body. Other dyes may cause a cold sensation at the injection site. Dyes taken by mouth may have an unpleasant taste.

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.

Side Effects

Generally, radiopaque dyes are very safe. However, a few people have an allergic-type reaction or kidney damage after a dye is used, usually when it is given intravenously.

Allergic-type reactions vary in severity:

  • Mild, such as nausea, flushing, or itching

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

  • Severe and life threatening (anaphylactoid), 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, use of 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 reactions are most likely to occur in people who have many other allergies, who have asthma, or who have previously had allergic-type reactions after a radiopaque dye was used. If people have had several severe reactions to dyes, an imaging test that does not require a dye should be done instead. If a dye 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) 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 some of those require kidney dialysis.

If tests that require radiopaque dyes must be used, people are given fluids intravenously before and after the dye is given. A low dose of the dye 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 dye is given.

Paramagnetic contrast agents usually have no side effects. 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.

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  • ADRENALIN
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