Radiation Dose in X-Ray and CT Exams

What are x-rays and what do they do?

X-rays are forms of radiant energy, like light or radio waves. Unlike light, x-rays can penetrate the body, which allows a radiologist to produce pictures of internal structures. The radiologist can view these on photographic film or on a TV or computer monitor.

X-ray examinations provide valuable information about your health and play an important role in helping your doctor make an accurate diagnosis. In some cases x-rays are used to assist with the placement of tubes or other devices in the body or with other therapeutic procedures.

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

Measuring radiation dosage

The scientific unit of measurement for radiation dose, commonly referred to as effective dose, is the millisievert (mSv). Other radiation dose measurement units include rad, rem, roentgen, sievert, and gray.

Because different tissues and organs have varying sensitivity to radiation exposure, the actual radiation risk to different parts of the body from an x-ray procedure varies. The term effective dose is used when referring to the radiation risk averaged over the entire body.

The effective dose accounts for the relative sensitivities of the different tissues exposed. More importantly, it allows for quantification of risk and comparison to more familiar sources of exposure that range from natural background radiation to radiographic medical procedures.

Naturally-occurring "background" radiation exposure

We are exposed to radiation from natural sources 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 naturally occurring radioactive materials and cosmic radiation from outer space. These natural "background" doses vary throughout the country.

People living in the plateaus of Colorado or New Mexico receive about 1.5 mSv more per year than those living near sea level. The added dose from cosmic rays during a coast-to-coast round trip flight in a commercial airplane is about 0.03 mSv. Altitude plays a big role, but the largest source of background radiation comes from radon gas in our homes (about 2 mSv per year). Like other sources of background radiation, exposure to radon varies widely from one part of the country to another.

To explain it in simple terms, we can compare the radiation exposure from one chest x-ray as equivalent to the amount of radiation exposure one experiences from our natural surroundings in 10 days.

Effective radiation dose in adults

Following are comparisons of effective radiation dose in adults with background radiation exposure for several radiological procedures described within this website.

For this procedure: * An adult’s approximate effective radiation dose is: Comparable to natural background radiation for: ** Estimated lifetime risk of fatal cancer from examination:
Computed Tomography (CT)-Abdomen and Pelvis 10 mSv 3 years Low
Computed Tomography (CT)-Abdomen and Pelvis, repeated with and without contrast material 20 mSv 7 years Moderate
Computed Tomography (CT)-Colonography 10 mSv 3 years Low
Intravenous Pyelogram (IVP) 3 mSv 1 year Low
Radiography (X-ray)-Lower GI Tract 8 mSv 3 years Low
Radiography (X-ray)-Upper GI Tract 6 mSv 2 years Low
Radiography (X-ray)-Spine 1.5 mSv 6 months Very Low
Radiography (X-ray)-Extremity 0.001 mSv 3 hours Negligible
Computed Tomography (CT)-Head 2 mSv 8 months Very Low
Computed Tomography (CT)-Head, repeated with and without contrast material 4 mSv 16 months Low
Computed Tomography (CT)-Spine 6 mSv 2 years Low
Computed Tomography (CT)-Chest 7 mSv 2 years Low
Computed Tomography (CT)-Lung Cancer Screening 1.5 mSv 6 months Very Low
Radiography-Chest 0.1 mSv 10 days Minimal
Intraoral X-ray 0.005 mSv 1 day Negligible
Coronary Computed Tomography Angiography (CTA) 12 mSv 4 years Low
Cardiac CT for Calcium Scoring 3 mSv 1 year Low
Bone Densitometry (DEXA) 0.001 mSv 3 hours Negligible
Positron Emission Tomography – Computed Tomography (PET/CT) 25 mSv 8 years Moderate
Bone Densitometry (DEXA) 0.001 mSv 3 hours Negligible
Mammography 0.4 mSv 7 weeks Very Low

Note for pediatric patients: Pediatric patients vary in size. Doses given to pediatric patients will vary significantly from those given to adults.

* 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.

** Legend:

Risk Level
Approximate additional risk of fatal cancer for an adult from examination:
less than 1 in 1,000,000
1 in 1,000,000 to 1 in 100,000
Very Low:
1 in 100,000 to 1 in 10,000
1 in 10,000 to 1 in 1000
1 in 1000 to 1 in 500
Note: These risk levels represent very small additions to the 1 in 5 chance we all have of dying from cancer.

Please note that the above chart attempts to simplify a highly complex topic for patients' informational use. The effective dose listed above may be used to estimate cancer and cancer related deaths.

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."

Additional Information and Resources

If you are interested in researching the use of effective dose further, following are a few resources:

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

This page was reviewed on June 24, 2015


View full size with caption

About this Site is produced by:
Send us your feedback

Please note is not a medical facility. Please contact your physician with specific medical questions or for a referral to a radiologist or other physician. To locate a medical imaging or radiation oncology provider in your community, you can search the ACR-accredited facilities database.

This website does not provide cost information. The costs for specific medical imaging tests, treatments and procedures may vary by geographic region. Discuss the fees associated with your prescribed procedure with your doctor, the medical facility staff and/or your insurance provider to get a better understanding of the possible charges you will incur.

Web page review process: This Web page is reviewed regularly by a physician with expertise in the medical area presented and is further reviewed by committees from the American College of Radiology (ACR) and the Radiological Society of North America (RSNA), comprising physicians with expertise in several radiologic areas.

Outside links: For the convenience of our users, provides links to relevant websites., ACR and RSNA are not responsible for the content contained on the web pages found at these links.