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The estimates below are based on an average adult.

To put these numbers into perspective, we are all exposed to some radiation just by being on the planet. Radiation exposure is measured in millisieverts (mSv). The average American is exposed to about 3 mSv of background radiation from natural sources over the course of a year.

X-ray

Chest x-ray: A chest x-ray exposes you to about 0.1 mSv of radiation. This is about the amount you get from natural sources in 10 days.

Standard mammogram: A standard screening mammogram exposes you to 0.28 mSv of radiation. This is about the amount you get from natural sources in just over a month (34 days).

3D mammogram: A 3D screening mammogram exposes you to 0.34 mSv of radiation. This is about the amount you get from natural sources in 42 days.

Lower GI series: A lower GI series using x-rays of the large intestine exposes you to around 6 mSv of radiation. This is about the amount you get from natural sources in 2 years.

Fluoroscopy: This imaging test uses x-rays to make real-time moving images. It exposes you to different amounts of radiation depending on how long it is used.

CT scan

Abdomen and pelvis: A CT scan of the abdomen (belly) and pelvis exposes you to around 7.7 mSv of radiation. This is about the amount you get from natural sources in 2.6 years.

Low-dose CT: This lung cancer screening test exposes you to around 1.5 mSv of radiation. This is about the amount you get from natural sources in 6 months.

PET scan

A PET/CT exposes you to about 22.7 mSv of radiation. This is equal to about 8 years of exposure from natural sources.

MRI and ultrasound

MRI and ultrasound exams do not expose you to radiation.

How does radiation from imaging tests affect the risk of cancer?

In large doses, radiation can cause serious tissue damage and increase your risk of later developing cancer.

Imaging tests use the lowest dose of ionizing radiation possible. But even with low doses of radiation, there is still concern about whether these tests increase the risk of cancer.

Radiation doses used for screening and diagnostic tests have decreased over the years. Because radiation doses are lower than they used to be, it’s unclear how past research on cancer risk applies to current imaging tests.

Tracking the effect of radiation is also complicated because an individual person’s radiation exposure depends on many factors. This includes the type of test done, the area of the body exposed, and the person’s body size, age, and sex. All of these affect the amount of radiation absorbed by the body.

Current testing guidelines

Health care professionals must follow the as low as reasonably achievable (ALARA) principle.

This means they must use the lowest possible dose of radiation that allows them to get the clearest images they need to diagnose a problem or monitor response to treatment.

Being exposed to a single radiation dose of 50 mSv or a lifetime dose of 100 mSv has not been linked to health risks. These numbers are the upper limits in the guidelines health care professionals follow.  

Studying cancer risk from imaging tests

It can be hard to study cancer risks from imaging tests, because these tests use lower doses of radiation than most other types of exposure. For example, the exposure received by atomic bomb survivors is significantly higher. A lot of studies used these exposures to study cancer risk.

To find even a small risk of cancer from imaging tests, researchers must study hundreds of thousands or millions of people. This type of study takes decades, because cancers take that long to develop after radiation exposure.

In the past, studies that found an increased risk of cancer after imaging tests often involved people who had many imaging tests or high-dose procedures.

For example:

• Studies of women who were imaged many times with fluoroscopy as a teen or young woman during treatment for tuberculosis have found an increased risk of breast cancer years later.
• Teens and young women who had many x-rays of the spine to monitor scoliosis were found to have an increased risk of breast cancer later in life.
• Some studies have suggested a higher risk of meningioma (a brain tumor that is most often benign) due to certain dental x-rays done in the past when radiation doses were higher. 

A recent study looked at the risk of blood cancers in millions of children and adolescents who did and did not have imaging tests. It suggested a slight but notably higher risk of blood cancers in those who were exposed to radiation from tests.

How many x-rays and other tests can I safely get?

Most studies have not been able to detect an increased risk of cancer among people exposed to low levels of x-rays or gamma rays. For example, people living at high altitudes do not have higher cancer rates even though they are exposed to more natural background radiation from cosmic rays than people living at sea level.

Most scientists and regulatory agencies do agree that even small doses of gamma and x-ray radiation can increase cancer risk. But this is likely a very small amount of increased risk.

In general, the lower the radiation amount used, the smaller the increase in risk. But there is no threshold below which this kind of radiation is thought to be totally safe.

What about radiation from imaging tests and children?

Children are more sensitive to radiation than adults. Because of this, health care providers should be careful to reduce their exposure to imaging tests that use radiation.

Childhood exposure to radiation from imaging tests is of special concern because:

• Children are expected to live longer than adults, so they have more time to develop problems from radiation exposure.
• Radiation impacts young, growing and developing bodies differently than it impacts adults.
•  Children are more sensitive to radiation.

This means that for a young child, the risk of developing a radiation-related cancer could be several times higher than for an adult who gets the same imaging test.

The full risks from these tests are not known for sure. But to be safe, most doctors recommend that children only get these tests when:

• The test is the best way to get the information needed
• The minimum amount of radiation needed to get the image is used

AND

• The benefits outweigh the risks of exposure

Questions to ask your child’s health care team

Parents can and should ask questions before any imaging tests are done.

Consider asking these questions:

• Why does my child need an imaging test?
• What type of imaging test do you think they need?
• Does it use radiation?
• Are there other options that do not use radiation but give you the same information?

Tracking your child’s imaging record

You may also want to keep a medical imaging record. You can use this to track your child’s history of imaging tests and share it with their health care providers. A version for children can be found online through the .

What can I do if I’m worried about radiation from imaging tests?

At this time, the best advice is to get only the imaging tests you need and try to limit your exposure to all forms of radiation.

If you have concerns about the radiation you might get from a CT scan, PET scan, x-ray, or any other imaging test that uses radiation, talk to your health care provider.

Ask:

• Is this test needed? If so, is it the best one to use?
• What will be learned from the test?
• Are there ways to shield parts of my body not being imaged to prevent exposure?

Protection during imaging

A lead apron can be used to protect parts of your chest or abdomen from getting radiation. A lead collar can be used to protect your thyroid gland. This is known as a thyroid shield or thyroid collar.

Talk to your health care team about the options available to you. 

Keeping track of imaging tests

You may also want to keep track of your imaging history and share it with your health care providers. This could help prevent repeat tests from being ordered. English and Spanish examples of imaging records for adults can be found online at .