Radiation Dose

What are x-rays and what do they do?

X-rays are a form of energy – like 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. This allows a radiologist to create images of them. The radiologist is a specially trained doctor who can examine these images on a computer display. X-rays allow the radiologist to see the structures in your body in very fine detail.

X-ray exams provide valuable information about your health and help your doctor make an accurate diagnosis. Your doctor may use x-rays to help place tubes or other devices in your body or to treat disease.

See Safety in X-ray, Interventional Radiology and Nuclear Medicine Procedures for more information.

Measuring radiation dosage

When radiation passes through the body, some of it is absorbed. The x-rays that are not absorbed are used to create the image. The amount the patient absorbs contributes to the patient's radiation dose. Radiation that passes through the body does not contribute to this dose. 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 considers how sensitive different tissues are to radiation. If you have an x-ray exam that includes tissues or organs that are more sensitive to radiation, your effective dose will be higher. Effective dose allows your doctor to evaluate your risk and compare it to common, everyday 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, which includes 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 about the same as 10 days of natural background radiation that we are all exposed to as part of our daily living.

Effective radiation dose in adults

Here are some approximate comparisons of background radiation and effective radiation dose in adults for several radiology procedures described on this website. These values can vary greatly, depending on the size of the patient and the type of imaging technology being used. Manufacturers of imaging technology continue to make improvements towards reducing radiation exposure while maintaining image quality.

ABDOMINAL REGION	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Computed Tomography (CT)–Abdomen and Pelvis	7.7 mSv	2.6 years
	Computed Tomography (CT)–Abdomen and Pelvis, repeated with and without contrast material	15.4 mSv	5.1 years
	Computed Tomography (CT)–Colonography	6 mSv	2 years
	Intravenous Urography (IVU)	3 mSv	1 year
	Barium Enema (Lower GI X-ray)	6 mSv	2 years
	Upper GI Study with Barium	6 mSv	2 years
BONE	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Lumbar Spine	1.4 mSv	6 months
	Extremity (hand, foot, etc.) X-ray	Less than 0.001 mSv	Less than 3 hours
CENTRAL NERVOUS SYSTEM	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Computed Tomography (CT)–Brain	1.6 mSv	7 months
	Computed Tomography (CT)–Brain, repeated with and without contrast material	3.2 mSv	13 months
	Computed Tomography (CT)–Head and Neck	1.2 mSv	5 Months
	Computed Tomography (CT)–Spine	8.8 mSv	3 years
CHEST	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Computed Tomography (CT)–Chest	6.1 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
	Panoramic X-ray	0.025 mSv	3 days
	Cone Beam CT	0.18 mSv	22 days
HEART	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Coronary Computed Tomography Angiography (CTA)	8.7 mSv	3 years
	Cardiac CT for Calcium Scoring	1.7 mSv	6 months
	Non-Cardiac Computed Tomography Angiography (CTA)	5.1 mSv	Less than 2 years
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) Whole body protocol	22.7 mSv	7.6 years
WOMEN'S IMAGING	Procedure	Approximate effective radiation dose	Comparable to natural background radiation for:
	Bone Densitometry (DEXA)	0.001 mSv	3 hours
	Screening Digital Mammography	0.21 mSv	26 days
	Screening Digital Breast Tomosynthesis (3D Mammogram)	0.27 mSv	33 days

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.

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

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

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, how the procedure is performed, and the body area being exposed to radiation.

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. Hospitals and imaging centers apply the principles of 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, which is 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 given procedure.

For more discussions about benefit versus risk, see the Radiology Benefits and Risks section on the Patient Safety page.