Imaging (Radiology) Tests

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Questions about radiation risk from imaging tests

In large doses, radiation can cause serious tissue damage and increase a person’s risk of later developing cancer. The low doses of radiation used for imaging tests might increase a person’s cancer risk slightly, but it’s important to put this risk into perspective. In this section we will answer some of the more common questions people have about radiation risks linked to imaging tests.

How much radiation is the average person exposed to in day-to-day life?

We are constantly exposed to radiation from a number of sources, including radioactive materials in our environment, radon gas in our homes, and cosmic rays from outer space. This is called background radiation and it varies across the country.

The average American is exposed to about 3 mSv (millisieverts) of radiation from natural sources over the course of a year. (A millisievert is a measure of radiation exposure.) Much of this exposure is from radon, a natural gas with levels that vary from one part of the country to another.

Because the earth’s atmosphere blocks some cosmic rays, being at a higher altitude increases a person’s exposure. For example, people living in the higher parts of New Mexico and Colorado are exposed to more radiation per year than people living at sea level. And a 10-hour airline flight increases cosmic ray exposure by about 0.03 mSv.

How much does an imaging test expose a person to radiation?

Often not much, but it depends on the imaging test used. A single chest x-ray exposes the patient to about 0.1 mSv, which is about the radiation dose people are exposed to naturally over the course of about 10 days. A mammogram exposes a woman to 0.4 mSv, or about the amount of exposure a person would expect to get in about 7 weeks.

Some other imaging tests have higher exposures. A lower GI series using standard x-rays exposes a person to about 8 mSv. A CT scan of the abdomen (belly) and pelvis exposes a person to about 10 mSv; this goes up to 20 mSv if the test is done twice (once with and once without contrast). A CT colonography exposes you to about 10 mSv of radiation. Keep in mind that these are estimates, and studies have found that the amount of radiation you get can vary a great deal.

If you have concerns about the radiation you may get from a CT scan, or any other imaging test, check with the facility that will do the test. (Remember that MRI and ultrasound exams do not expose you to radiation.) You may also want to keep a “medical imaging history” to track your own history of imaging tests and share it with your health care providers. (One can be found online at See the “To learn more” section.)

The best advice at this time is to get only the imaging tests that are needed and try to limit your exposure to all forms of radiation. If you do need to have a test that will expose you to some radiation, ask if there are ways to shield the parts of your body that aren’t being imaged. For example, a lead apron can sometimes be used to protect parts of your chest or abdomen from getting radiation, and a lead collar (known as a thyroid shield or thyroid collar) can be used to protect your thyroid gland.

How much does the extra radiation increase a person’s cancer risk?

Researchers have estimated that radiation exposure from the average diagnostic x-ray may increase cancer risk very slightly (likely on the order of hundredths to thousandths of one percent). Of course, this can be affected by the type of test done, the area of the body exposed, the person’s age and gender, and other factors.

Radiation experts say that the risk levels represented in imaging tests are only very small additions to the 1 in 5 chance we all have of dying from cancer. It’s hard to know just how much radiation exposure from imaging tests might increase a person’s cancer risk. Most studies on radiation and cancer risk have looked at people exposed to very high doses of radiation, such as uranium miners and atomic bomb survivors. The risk from low-level radiation exposure is not easy to calculate from these studies. We do know that children are more sensitive to radiation and should be protected from it as much as possible.

Because radiation exposure from all sources can add up over a lifetime, and radiation can, indeed, increase cancer risk, imaging tests that use radiation should only be done for a good reason. In many cases, other imaging tests such as ultrasound or MRI may be used. But if there’s a reason to believe that an x-ray, CT scan, or nuclear medicine scan is the best way to look for cancer or other diseases, the person will most likely be helped more than the small dose of radiation can hurt.

Last Medical Review: 03/16/2015
Last Revised: 09/15/2015