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. That's where the sievert (Sv) and millisievert (mSv) come in. Measuring their impact on human tissues and health is more difficult. But Geiger counters detect only the intensity of radioactive emissions. If you mention the measurement of radiation, many people will recall the classic Geiger counter with its crescendo of clicks. CT alone accounts for 24% of all radiation exposure in the United States, according to a report issued in March 2009 by the National Council on Radiation Protection and Measurements.

The proportion of total radiation exposure that comes from medical sources has grown from 15% in the early 1980s to 50% today. (A millisievert is a measure of radiation exposure see 'Measuring radiation.')Д®xposure to ionizing radiation from natural or background sources hasn't changed since about 1980, but Americans' total per capita radiation exposure has nearly doubled, and experts believe the main reason is increased use of medical imaging. But the average is 3 millisieverts (mSv) per year. How much of this so-called background radiation you are exposed to depends on many factors, including altitude and home ventilation. 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. 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.' The result is DNA mutations that may contribute to cancer years down the road. 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. But are we courting future public health problems? Exposure to ionizing radiation on the rise 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. 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. Over 80 million CT scans are performed in the United States each year, compared with just three million in 1980.

But many experts are concerned about an explosion in the use of higher radiationвЂ“dose tests, such as CT and nuclear imaging. For most women, there's very little risk from routine x-ray imaging such as mammography or dental x-rays. Patients want to know if radiation from mammograms, bone density tests, computed tomography (CT) scans, and so forth will increase their risk of developing cancer. There's always questions about radiation exposure from medical imaging.

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