Radiation, contrast and imaging safety, as public bodies state it

Most people who are booked for a scan have the same quiet worry: is this safe? The honest answer depends on which scan it is, because each one makes its picture from a different kind of energy, and only some of them use radiation. This page sets out what public health and radiology bodies say about radiation dose, contrast agents and the safety checks that happen before you are scanned. It describes general guidance, not your situation. Your own doctor and imaging team know your history and have the final word.

Which scans use ionizing radiation

An X-ray is, in the words of RadiologyInfo.org, "a form of energy, like light and radio waves", with enough energy to pass through the body. As it passes through bones, tissues and organs differently, it lets a radiologist (a doctor trained to read images) build a picture. Computed tomography, or CT, is a type of X-ray scan: the National Institute of Biomedical Imaging and Bioengineering (NIBIB) notes that all X-rays produce ionizing radiation, a form of radiation with the potential to cause biological effects in living tissue.

Nuclear medicine works differently. A small amount of radioactive material, called a radiotracer, is injected, swallowed or inhaled, and a special camera records the energy it gives off. Patients are exposed to radiation from the tracer itself, and RadiologyInfo.org says the doctor will use the lowest dose possible to obtain the highest quality images. You can read how these scans are done on the nuclear medicine page.

MRI and ultrasound do not use X-rays. NIBIB describes MRI as differing from CT in that it does not use the ionizing radiation of X-rays, and describes diagnostic ultrasound as generally regarded as safe, without ionizing radiation. Each has its own safety questions, covered below. For a comparison of what the three most common scan types show and how they feel, see X-ray, CT and MRI compared.

What a dose figure means

When radiation passes through the body, some of it is absorbed. That absorbed amount contributes to your radiation dose, and radiation that passes straight through does not. Doctors use a measure called "effective dose" when they talk about risk to the whole body, and its unit is the millisievert (mSv). Effective dose takes into account how sensitive different tissues are, so an exam that includes more sensitive organs will carry a higher figure.

The useful trick is comparison with the natural background radiation we all live with. RadiologyInfo.org states that the average person in the U.S. receives an effective dose of about 3 mSv per year from natural radiation, including cosmic radiation, and that this varies according to where you live. On that basis it says one adult chest X-ray (0.1 mSv) is about the same as 10 days of natural background radiation.

Those figures come with warnings from the source itself. The values can vary greatly with the size of the patient, the reason for imaging and the technology used, and the page notes that doses given to children differ significantly from adult doses. It also quotes the International Commission on Radiological Protection as saying effective dose has severe limitations when used to assess the exposure of patients. In plain terms, a dose chart is a rough guide for comparison, not a personal measurement.

Weighing benefit and risk

NIBIB states that, when used appropriately, the diagnostic benefits of X-ray scans significantly outweigh the risks, and that the risk of developing cancer from X-ray exposure is generally considered to be very small. It also says the risk from ionizing radiation increases with the number of exposures over an individual's lifetime. Both statements sit together on purpose: a single scan is a small matter, and the running total is a reason for doctors to ask whether each scan is needed.

RadiologyInfo.org explains that hospitals and imaging centres apply the principle known as ALARA, As Low As Reasonably Achievable, which means making every effort to decrease radiation risk. It adds that a person is at risk too if the doctor cannot accurately diagnose an illness or injury. The same page describes media stories linking CT scans to cancer and notes limits in those studies, such as the lack of direct dose measurements and of the reason for each scan, while saying the studies have usefully raised awareness of minimizing exposure.

The practical advice in the source is modest. It is worth knowing why you need an exam, and which exam will best answer the medical question at the lowest radiation dose, since many imaging procedures have no or very low radiation dose. If you have frequent X-ray exams and change healthcare providers, RadiologyInfo.org suggests keeping a personal record of your X-ray history, which can help a new doctor make an informed decision. A list of questions to ask before a scan can help you start that conversation.

Contrast agents

Some scans use a contrast agent (also called contrast material or contrast medium). RadiologyInfo.org is clear that these are not dyes that permanently discolour internal organs, but substances that temporarily change the way X-rays or other imaging tools interact with the body, making certain structures look different from their surroundings. The materials it discusses do not produce radiation themselves.

There are several kinds. Iodine-based compounds and barium sulfate are used for X-ray and CT; gadolinium is the key component of the contrast material most often used in MRI; and for ultrasound, tiny gas microbubbles can be injected to show blood flow. NIBIB adds that iodine-based agents are injected into the bloodstream to help show blood vessels, while barium-based compounds are swallowed to image the digestive system.

The source describes contrast materials as safe drugs. Reactions ranging from mild to severe do occur, but severe reactions are very uncommon, and radiology departments are equipped to deal with them. For iodine-based material, mild reactions include nausea, headache, itching or flushing, and a small percentage of patients get a delayed rash hours to days later. Gadolinium is less likely than iodine-based material to cause an allergic reaction.

The questions staff ask beforehand

If contrast is planned, RadiologyInfo.org says to tell your doctor about any previous allergic reaction to contrast material, allergies to food, drugs, dyes, preservatives or animals, and any history of heart disease, diabetes, kidney disease or thyroid problems. Kidney function gets particular attention. The source says that if you have severe kidney disease you may be at increased risk of worsening kidney function from iodinated contrast, while adding that the benefits of a contrast-enhanced scan often outweigh the risks in ensuring a proper diagnosis. For gadolinium, a rare complication called nephrogenic systemic fibrosis, a thickening of the skin and other tissues, can occur in patients with kidney disease, and the contrast may be withheld in some patients with severe kidney disease.

The same page notes that tiny traces of gadolinium may be retained in organs, including the brain, after contrast-enhanced MRI, that no negative effects are known, and that doctors may take this into account when choosing an agent. Expect some sensations too. Injected iodine-based contrast can bring a warm, flushed feeling and a metallic taste for a few minutes; gadolinium usually brings coolness at the injection site for a minute or two. After iodine-based or barium contrast it is a good idea to drink more fluids, and if you have not been sedated, no recovery period is needed.

On breastfeeding, RadiologyInfo.org reports that manufacturers advise mothers not to breastfeed for 24 to 48 hours after intravenous contrast, but that both the American College of Radiology and the European Society of Urogenital Radiology say the available data suggest it is safe to continue. A parent who is still concerned can talk to the radiologist about the options. Pregnancy is covered on the children and pregnancy page.

MRI safety checks

MRI uses a powerful magnetic field, radio energy and a computer, and RadiologyInfo.org says the electromagnetic fields produce no known tissue damage when applied correctly. The risk is the magnet itself, which can pull on objects made of ferromagnetic metals such as iron with great force. That is why you fill in a screening form, change into a gown and remove metal items, including watches, jewellery, phones and clothing with metallic threads or fasteners, which can heat up and burn during a scan.

The screening form asks about cardiac pacemakers, implanted medication pumps, certain cochlear implants, vascular clips, bullets or metallic fragments and metal near the eye. The source stresses that some newer devices are acceptable for MRI, but staff must know the exact type, so give the name of the device and its manufacturer. In unusual cases the exam may be cancelled, for example when a ferromagnetic aneurysm clip is present. You will wear earplugs because the scanner is loud, you can talk with the technologist by intercom, and you will be given a call button. The same page says that one out of every twenty people may require a mild sedative to remain calm in the scanner.

Ultrasound safety

NIBIB describes diagnostic ultrasound as generally regarded as safe, with no ionizing radiation, while noting that it can produce some biological effects under specific settings. For that reason the U.S. Food and Drug Administration requires that diagnostic ultrasound devices operate within acceptable limits, and the FDA and many professional bodies discourage casual use, such as keepsake videos, and recommend scanning only when there is a true medical need. More on how scans are done is on the ultrasound page.

The short version

Radiation dose depends on the scan: X-ray, CT and nuclear medicine use ionizing radiation, while MRI and ultrasound do not. Dose charts are rough comparisons, and public bodies describe the benefit of a needed scan as outweighing its small risk, which is why imaging teams work to keep doses as low as reasonably achievable. Contrast agents and MRI both come with screening questions, and answering them fully is the most useful thing you can do on the day.