But many experts are concerned about an explosion in the use of higher radiation—dose tests, such as CT and nuclear imaging. Over 80 million CT scans are performed in the United States each year, compared with just three million in There are good reasons for this trend. CT scanning and nuclear imaging have revolutionized diagnosis and treatment, almost eliminating the need for once-common exploratory surgeries and many other invasive and potentially risky procedures. The benefits of these tests, when they're appropriate, far outweigh any radiation-associated cancer risks, and the risk from a single CT scan or nuclear imaging test is quite small.
But are we courting future public health problems? The radiation you get from x-ray, CT, and nuclear imaging is ionizing radiation — high-energy wavelengths or particles that penetrate tissue to reveal the body's internal organs and structures.
Ionizing radiation can damage DNA, and although your cells repair most of the damage, they sometimes do the job imperfectly, leaving small areas of "misrepair. We're exposed to small doses of ionizing radiation from natural sources all the time — in particular, cosmic radiation, mainly from the sun, and radon, a radioactive gas that comes from the natural breakdown of uranium in soil, rock, water, and building materials. How much of this so-called background radiation you are exposed to depends on many factors, including altitude and home ventilation.
But the average is 3 millisieverts mSv per year. A millisievert is a measure of radiation exposure; see "Measuring radiation. Exposure to ionizing radiation from natural or background sources hasn't changed since about , but Americans' total per capita radiation exposure has nearly doubled, and experts believe the main reason is increased use of medical imaging.
If you mention the measurement of radiation, many people will recall the classic Geiger counter with its crescendo of clicks. But Geiger counters detect only the intensity of radioactive emissions.
Measuring their impact on human tissues and health is more difficult. That's where the sievert Sv and millisievert mSv come in.
These units, the ones most commonly used in comparing imaging procedures, take into account the biological effect of radiation, which varies with the type of radiation and the vulnerability of the affected body tissue. Taking these into account, millisieverts describe what's called the "equivalent dose.
We've long known that children and teens who receive high doses of radiation to treat lymphoma or other cancers are more likely to develop additional cancers later in life. But we have no clinical trials to guide our thinking about cancer risk from medical radiation in healthy adults.
Most of what we know about the risks of ionizing radiation comes from long-term studies of people who survived the atomic bomb blasts at Hiroshima and Nagasaki. These studies show a slightly but significantly increased risk of cancer in those exposed to the blasts, including a group of 25, Hiroshima survivors who received less than 50 mSv of radiation — an amount you might get from three or more CT scans.
See "Imaging procedures and their approximate effective radiation doses. The atomic blast isn't a perfect model for exposure to medical radiation, because the bomb released its radiation all at once, while the doses from medical imaging are smaller and spread over time. Still, most experts believe that can be almost as harmful as getting an equivalent dose all at once.
Source: Mettler FA, et al. Most of the increased exposure in the United States is due to CT scanning and nuclear imaging, which require larger radiation doses than traditional x-rays.
Become a Gold Supporter and see no ads. Log in Sign up. Articles Cases Courses Quiz. About Recent Edits Go ad-free. Edit article. View revision history Report problem with Article. Citation, DOI and article data. Bell, D. Gray SI unit. Reference article, Radiopaedia. URL of Article. Terminology One gray is a large unit and is usually used with a prefix, e. History and etymology In July , the International Commission on Radiation Units and Measurements ICRU recommended that a specific name, the gray, be adopted for the SI unit 'joule per kilogram' when used for absorbed radiation dose.
Sekiya M, Yamasaki M. Louis Harold Gray November 10, July 9, : a pioneer in radiobiology. The new special names of SI units in the field of ionizing radiations. Sinclair Wynchank. Louis Harold Gray.
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