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Radiation Dose in X-Ray and CT Exams

What are x-rays and what do they do?

X-rays are a form of energy, similar to light and radio waves. X-rays are also called radiation. Unlike light waves, x-rays have enough energy to pass through your body. As the radiation moves through your body, it passes through bones, tissues and organs differently, which allows a radiologist to create pictures of them. The radiologist views these images on photographic film or on monitors similar to a computer display.

X-ray examinations provide valuable information about your health and help your doctor make an accurate diagnosis. X-rays are sometimes used to help place tubes or other devices in the body or to treat disease.

See the X-ray, Interventional Radiology and Nuclear Medicine Radiation Safety page for more information.

Measuring radiation dosage

Because radiation can pass through the body, radiation dose is measured according to the amount of radiation received by the whole body. The scientific unit of measurement for whole body radiation dose, called "effective dose," is the millisievert (mSv). Other radiation dose measurement units include rad, rem, roentgen, sievert, and gray.

Doctors use "effective dose" when they talk about the risk of radiation to the entire body. Risk refers to possible side effects, such as the chance of developing a cancer later in life. Effective dose takes into account how sensitive different tissues are to radiation. If you have an x-ray exam of tissues or organs that are more sensitive to radiation, your effective dose will be higher. Effective dose allows your doctor to assess your risk and compare it to more familiar sources of exposure, such as natural background radiation.

Naturally-occurring "background" radiation

We are exposed to natural sources of radiation all the time. According to recent estimates, the average person in the U.S. receives an effective dose of about 3 mSv per year from natural radiation and cosmic radiation from outer space. These natural "background doses" vary according to where you live.

People living at high altitudes such as Colorado or New Mexico receive about 1.5 mSv more per year than those living near sea level. A coast-to-coast round trip airline flight is about 0.03 mSv due to exposure to cosmic rays. The largest source of background radiation comes from radon gas in our homes (about 2 mSv per year). Like other sources of background radiation, the amount of radon exposure varies widely depending on where you live.

To put it simply, the amount of radiation from one adult chest x-ray (0.1 mSv) is equal to 10 days of natural background radiation.

Effective radiation dose in adults

Here are some comparisons of background radiation and effective radiation dose in adults for several radiology procedures described on this website.

ABDOMINAL REGION

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Computed Tomography (CT)–Abdomen and Pelvis 10 mSv 3 years
Computed Tomography (CT)–Abdomen and Pelvis, repeated with and without contrast material 20 mSv 7 years
Computed Tomography (CT)–Colonography 6 mSv 2 years
Intravenous Pyelogram (IVP) 3 mSv 1 year
Barium Enema (Lower GI X-ray) 8 mSv 3 years
Upper GI Study with Barium 6 mSv 2 years

BONE

 
Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Spine X-ray 1.5 mSv 6 months
Extremity (hand, foot, etc.) X-ray 0.001 mSv 3 hours

CENTRAL NERVOUS SYSTEM

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Computed Tomography (CT)–Head 2 mSv 8 months
Computed Tomography (CT)–Head, repeated with and without contrast material 4 mSv 16 months
Computed Tomography (CT)–Spine 6 mSv 2 years

CHEST

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Computed Tomography (CT)–Chest 7 mSv 2 years
Computed Tomography (CT)–Lung Cancer Screening 1.5 mSv 6 months
Chest X-ray 0.1 mSv 10 days

DENTAL

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Dental X-ray 0.005 mSv 1 day

HEART

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Coronary Computed Tomography Angiography (CTA) 12 mSv 4 years
Cardiac CT for Calcium Scoring 3 mSv 1 year

MEN'S IMAGING
 
Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Bone Densitometry (DEXA) 0.001 mSv 3 hours

NUCLEAR MEDICINE
 
Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Positron Emission Tomography–Computed Tomography (PET/CT) 25 mSv 8 years

WOMEN'S IMAGING

Procedure Approximate effective radiation dose Comparable to natural background radiation for:
Bone Densitometry (DEXA) 0.001 mSv 3 hours
Mammography 0.4 mSv 7 weeks

Note for pediatric patients: Pediatric patients vary in size. Doses given to pediatric patients will vary significantly from those given to adults. For more information on radiation safety in pediatric imaging, visit http://www.imagegently.org/Roles-What-can-I-do/Parent.

* The effective doses are typical values for an average-sized adult. The actual dose can vary substantially, depending on a person's size as well as on differences in imaging practices.

Please note that this chart attempts to simplify a very complex topic. If you have questions about radiation risk, talk to your medical physicist and/or radiologist and ask about the benefits and risks of radiologic care.

The International Commission on Radiological Protection (ICRP) Report 103 states: "The use of effective dose for assessing the exposure of patients has severe limitations that must be considered when quantifying medical exposure," and "The assessment and interpretation of effective dose from medical exposure of patients is very problematic when organs and tissues receive only partial exposure or a very heterogeneous exposure which is the case especially with x-ray diagnostics." In other words, effective dose is not always the same for everyone. It can vary based on a person's height and weight and the area of the body being examined.

Benefit versus risk

The risk associated with medical imaging procedures refers to possible long-term or short-term side effects. Most imaging procedures have a relatively low risk. Plus, hospitals and imaging centers practice ALARA (As Low As Reasonably Achievable). This means they make every effort to decrease radiation risk. It is important to remember that a person is at risk if the doctor cannot accurately diagnose an illness or injury. Therefore, it could be said that the benefit from medical imaging (an accurate diagnosis) is greater than the small risk that comes with using it. Talk to your doctor or radiologist about any concerns you may have about the risks of a procedure.

For more discussions about benefit versus risk, see the Benefits and Risks section

Additional Information and Resources

If you are interested in doing research on the use of effective dose, here are a few resources:

The Alliance for Radiation Safety in Pediatric Imaging's "Image Gently" Campaign:

http://www.imagegently.org/Roles-What-can-I-do/Parent

This page was reviewed on April 20, 2018

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