Imaging tests that use radiation, usually x-rays, are a valuable tool in diagnosis, but exposure to radiation has some risks (see also Radiation Injury).
Different diagnostic tests require different amounts of radiation (see table Comparing Radiation Doses), but most of them use low doses that are generally considered safe. For example, the radiation dose from one chest x-ray is comparable to 10 days of exposure to naturally occurring background radiation. However, exposure to radiation is cumulative, regardless of the interval between tests. This means that 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. The greater the cumulative dose, the greater the risk of cancer and sometimes tissue damage.
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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 potential benefit of a diagnostic test often outweighs the potential harms.
In the United States, about 15% of all imaging tests are computed tomography (CT), but up to 70% of radiation exposure from all imaging tests comes from CT. The radiation dose for CT can be hundreds of times the dose for most x-rays. However, with newer techniques, doses for most CT scans can be much lower than 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 radiation 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 magnetic resonance imaging (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.
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, 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 potential benefit usually outweighs the potential harm.
