General Imaging Radiation FAQs

How much ionizing radiation exposure do I receive naturally (also known as background radiation)?

Naturally occurring sources of ionizing radiation include cosmic rays, radon, naturally occurring radioactive materials found in the ground (such as uranium) and in certain foods (such as bananas and brazil nuts). Since the origin of the earth, naturally occurring sources of radiation have continuously been a source of exposure to all life forms. There are certain places on earth where the average annual exposure to background is much higher than normal. In the U.S. the highest background radiation exposure are in states that have the greatest amounts of radioactive material in the ground and have higher exposure to cosmic radiation (such as Colorado). The lowest background radiation exposure is in states that are close to or below sea level without concentrated amounts of radioactive material in the ground (such as Florida).

An interesting fact is that the highest incidence of cancer is found in the 7-9 states that have lowest amount of background radiation; while the lowest incidence of cancer is found in the 7-9 states that have the highest amount of background radiation.

How does radiation exposure from medical imaging procedures compare with the amount of background radiation?

Some medical imaging procedures have zero radiation exposure (such as ultrasound and MRI). Some medical imaging procedures have radiation exposure that is much less than the U.S. annual average (3.1 mSv). Examples include dental x-rays, conventional x-rays (such as extremities, chest and head), and screening mammography.

Some medical imaging procedures have radiation exposure that is about the same amount as the U.S. annual average (3.1 mSv). Examples include head/brain CT, simple fluoroscopy procedures, and some nuclear medicine procedures (such as lung, kidney, liver, and brain scans). Some medical imaging procedures have radiation exposure that is more than the U.S. annual average (3.1 mSv). Examples include CT scans of chest, abdomen and pelvis, interventional (catheter based) angiography, PET/CT and some nuclear medicine procedures (such as heart perfusion scans).

Is the millisievert (mSv) the best unit to measure a patient's radiation dose?

Yes, the mSv is widely accepted as the most appropriate unit of dose when discussing risk of cancer due to radiation exposure from medical imaging procedures. This unit is also called the effective dose because it takes into account the different sensitivities of various organ systems to cancer. It allows us to describe the average risk to a patient from the medical imaging performed on different parts of the human body. It is comparable to the older unit of millirem (mrem) that was previously used for radiation doses in the US.

What are the risks associated with medical imaging procedures that expose patients to radiation?

Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods (such as annually) are too low to be detectable and may be nonexistent [1]. For radiation doses greater than 100 mSv, a slight increase risk of cancer has been proven scientifically. No other health risks are associated with radiation exposure from diagnostic imaging procedures in the low dose range (< 100 mSv). Examples of procedures are CT, PET, nuclear medicine, and fluoroscopy.
For single procedures resulting in doses below 50 mSv or for multiple procedures over short time periods resulting in a total dose of 100 mSv , "predictions of hypothetical cancer risk in patient populations exposed to such low doses are highly speculative and should be discouraged [2]." Predictions at low doses used for typical medical imaging procedures may be more harmful than good. It may cause patients to refuse medical imaging procedures, placing them at substantial risk by not receiving the clinical benefits of the prescribed procedures.

Because of the lack of scientifically proven data at doses less than 50 - 100 mSv, the call center and its associated web site do not provide patients with estimates of cancer risk from diagnostic medical imaging procedures.

To obtain a calculated radiation dose estimate for a specific procedure, contact the facility that performed the procedure and ask for the radiation safety specialist.

What are the risks associated with high radiation therapy doses?

In medicine, high doses of radiation are often used to treat patients who have been diagnosed with certain types of cancer. At high doses, the radiation prevents the cancer cells from reproducing, which causes the tumor to shrink and sometimes disappear. This field of medicine is called radiation therapy or radiation oncology. A majority of cancer patients are treated with radiation therapy at some point during their disease. A radiation oncologist may use radiation to cure cancer or to relieve a cancer patient's pain. Examples include external beam, intensity modulated radiation therapy, image guided radiation therapy, high dose rate and low dose rate brachytherapy, radioactive implants, radioembolization, stereotactic radiosurgery, gamma knife, and proton beam.

For information regarding the risks from high radiation dose or therapeutic doses of radiation, contact your physician directly.

What is the risk to patients who have multiple CTs and/or other medical imaging procedures over the course of their lifetime?

For radiation doses greater than 100 mSv, a slight increase risk of cancer later in life has been scientifically proven. When compared to the large cancer incidence rate in humans from all known causes, which is 25% to 33% (one in three to one in four), the number of predicted radiation induced cancers is very small.

Although the odds of developing cancer from 50 - 100 mSv is very small, the risk increases the more radiation exposure you receive and the younger you are at the time of exposure.

Because of the lack of scientifically proven data at doses less than 50 - 100 mSv, the call center and its associated web site do not provide patients with estimates of cancer risk from diagnostic medical imaging procedures.

Patient specific radiation dose estimates can be calculated by a radiation safety specialist for patients that contact the facility that performed the procedure.

Does the government regulate radiation exposure to patients?

Regulations require that each mammography and CT machine is checked annually by a qualified medical physicist to ensure that the unit operates correctly and that the radiation output is within the regulated limits. The amount of radiation dose to patients is not regulated for most scans. This allows the radiation dose to be specific to each person's body habitus and results in the optimum image quality.

Larger patients may need more output of x-rays to create a good image, but this does not necessarily mean they receive a larger dose.

Should patients inquire if they have concerns over radiation exposure? What questions should they ask?

Yes, ask about the benefits that will be gained from the procedure. If you have had multiple CTs in the past, inform your physician, and ask about alternative imaging modalities, such as MRI, ultrasound, or conventional radiography. If this involves your child, ask for a referral to a medical imaging facility that has a radiologist who is specially trained in pediatric radiology. When you arrive at the imaging facility, ask whether the technologists are registered (ARRT - American Registry of Radiological Technologists).

If I have been treated with radiation for cancer in the past, or currently have cancer, then will medical imaging, such as CT, make it worse?

No. Low dose radiation from medical imaging does not affect known cancer. Patients who have received radiation therapy are at no greater risk of getting cancer from medical imaging procedures than patients who have not received radiation therapy.

If you are having frequent medical imaging procedures that are typically above 3 mSv each in any one year, or if you are changing health care providers, then it is a good idea to keep your own record and track your radiation dose in mSv. This could inform your doctor and help to maximize the benefit and minimize the risk to you from multiple CT scans.

Is there a test that can show if you have had too much radiation?

No, there is no means of testing for lifetime radiation exposure from medical imaging procedures.

What is contrast or contrast material? Why is it used? What are the risks?

Contrast, contrast agents, or contrast media is any internally administered substance that has a different opacity (e.g. stands out) from soft tissue on x-ray and CT exams. Contrast or contrast materials, such as barium or iodine, are often used to produce pictures of major blood vessels throughout the body.

You should inform your physician of any medications you are taking and if you have any allergies, especially to barium or iodine contrast materials. If you have a history of allergy to x-ray contrast material, your radiologist or referring physician may advise that you take special medication for 24 hours before the exam to lessen the risk of allergic reaction. The risk of serious allergic reaction to contrast materials that contain iodine is extremely rare, and radiology departments are well-equipped to deal with them. Another option is to undergo a different exam that does not call for contrast material injection. Also inform your doctor about recent illnesses or other medical conditions.

If you are breastfeeding at the time of the exam, you should ask your radiologist or referring physician how to proceed. It may help to pump breast milk ahead of time and keep it on hand for use after contrast material has cleared from your body, about 24 hours after the test. Contrast is not a form of radiation and it is not a dye.

What is the radiation dose from interventional radiology procedures (catheter based such as cardiac cath, angiography, and angiograms)?

Interventional radiologic procedures use diagnostic-type imaging equipment to assist a physician in the treatment of a patient's condition. These procedures frequently provide favorable medical results with minimal recovery time. In some cases these procedures avoid the need for conventional surgery or improve the prospects for a favorable outcome from surgery. Catheter based interventional procedures may exposure a patient to doses equal to or greater than CT scans depending on the complexity of the procedure. In very rare cases, a patient may have skin reddening or even skin damage as a result of the procedure.

What is the radiation exposure from ultrasound and MRI?

Diagnostic medical ultrasound is an imaging technique that uses high frequency sound waves to view soft tissues, such as muscles and internal organs. Ultrasound is not a source of ionizing radiation and no risk is known to exist for this type of imaging procedure.
Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to obtain images. MRI procedures are not associated with exposure to ionizing radiation.

What medical imaging procedures trigger radiation detectors, such as those used at the USA border with Canada?

In nuclear medicine, a trace amount of radioactive material is administered to the patient, then scanners are used to image the radiation emitted from the patient. For most common nuclear medicine procedures (including PET), about 90% of the residual radioactive material is gone within 48 to 72 hours. If a patient crossed the border within 3 days of a nuclear medicine scan, then the residual trace amount of radiation may trigger the very sensitive radiation detectors used to monitor the vehicles. It is recommended that patients carry documentation that they have had a nuclear medicine procedure and appropriate contact information if they plan to cross the border soon after the scan.

What is radon and what is the risk from exposure to radon?

Radon is a naturally occurring radioactive gas found in soils, rock, and water throughout the U.S. Radon gas has been shown to cause lung cancer in uranium miners, and is considered a threat to public health by the Environmental Protection Agency because it can collect in homes, primarily in the basement. The EPA has set a recommended limit on the concentration of radon, which is 4 picocuries per liter of indoor air. Radon is the largest source of exposure to naturally occurring radiation.

You cannot see, feel, smell, or taste radon. Testing your home is the only way to know if concentration of radon exceeds the recommended limits. The EPA recommends testing for radon in all rooms below the third floor.

The National Radon Hotline: 800-767-7236