CTs, once rare, are now routine. One in 10 Americans undergo a CT scan every year, and many of them get more than one. This growth is a result of multiple factors, including a desire for early diagnoses, higher quality imaging technology, direct-to-consumer advertising and the financial interests of doctors and imaging centers. CT scanners cost millions of dollars; having made that investment, purchasers are strongly incentivized to use them.

While it is difficult to know how many cancers will result from medical imaging, a 2009 study from the National Cancer Institute estimates that CT scans conducted in 2007 will cause a projected 29,000 excess cancer cases and 14,500 excess deaths over the lifetime of those exposed. Given the many scans performed over the last several years, a reasonable estimate of excess lifetime cancers would be in the hundreds of thousands. According to our calculations, unless we change our current practices, 3 percent to 5 percent of all future cancers may result from exposure to medical imaging.

We know that these tests are overused. But even when they are appropriately used, they are not always done in the safest ways possible. The rule is that doses for medical imaging should be as low as reasonably achievable. But there are no specific guidelines for what these doses are, and thus there is considerable variation within and between institutions. The dose at one hospital can be as much as 50 times stronger than at another.

A recent study at one New York hospital found that nearly a third of its patients undergoing multiple cardiac imaging tests were getting a cumulative effective dose of more than 100 millisieverts of radiation — equivalent to 5,000 chest X-rays. And last year, a survey of nuclear cardiologists found that only 7 percent of stress tests were done using a “stress first” protocol (examining an image of the heart after exercise before deciding whether it was necessary to take one of it at rest), which can decrease radiation exposure by up to 75 percent.

In recent years, the medical profession has made some progress on these issues. The American College of Radiology and the American College of Cardiology have issued “appropriateness criteria” to help doctors consider the risks and benefits before ordering a test. And the insurance industry has started using radiology benefit managers, who investigate whether an imaging test is necessary before authorizing payment for it. Some studies have shown that the use of medical imaging has begun to slow.