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.

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:
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
Radiography (X-ray)-Lower GI Tract 8 mSv 3 years
Radiography (X-ray)-Upper GI Tract 6 mSv 2 years
Radiography (X-ray)-Spine 1.5 mSv 6 months
Radiography (X-ray)-Extremity 0.001 mSv 3 hours
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
Computed Tomography (CT)-Chest 7 mSv 2 years
Computed Tomography (CT)-Lung Cancer Screening 1.5 mSv 6 months
Radiography-Chest 0.1 mSv 10 days
Intraoral X-ray 0.005 mSv 1 day
Coronary Computed Tomography Angiography (CTA) 12 mSv 4 years
Cardiac CT for Calcium Scoring 3 mSv 1 year
Bone Densitometry (DEXA) 0.001 mSv 3 hours
Positron Emission Tomography – Computed Tomography (PET/CT) 25 mSv 8 years
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.

* 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 the above chart attempts to simplify a highly complex topic for patients' informational use. Patients with radiation dose questions should consult with their radiation physicists and/or radiologists as part of a larger discussion on 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."

Reviewed February 08, 2017

For more information about this and other radiology procedures, please visit Radiologyinfo.org