Alarm bells sounded in my brain when I read a study in Archives of Internal Medicine (AIM). The authors estimated that the radiation from CT scans performed in the US in a single year could lead to 29,000 future cases of cancer — 66% of them in women.

Why I was worried: Because a friend had recently undergone several CT scans after we’d been in a car crash together… and a few months earlier, I’d let the ER doctor do a CT on my teenaged son when he developed sudden and excruciating head pain. In hindsight, I wondered if those tests might have done my loved ones more harm than good.

To find out more, I spoke with Richard Semelka, MD, vice chairman for quality and safety in the department of radiology at University of North Carolina Hospitals and one of the first internationally recognized radiology experts to speak out on the potential hazards of medical radiation. He told me that, in many cases, the immediate benefits of a CT scan — which pairs X-rays with computers to produce three-dimensional images — do outweigh its long-term risks. But: Far too often, CT scans are done even when there is no compelling reason or when a safer test could be used instead. “Ideally, physicians would do thorough risk/benefit analyses before ordering any tests, but this rarely happens. Given radiation’s risks, patients must consider whether they really need that CT or not,” Dr. Semelka cautioned.

Here’s what you need to know to protect yourself from the dangers of medical radiation…

When To Say Yes

There is no question, Dr. Semelka said, that in certain types of emergencies or when you may have certain serious diseases, a CT scan is invaluable. If your doctor recommends a CT scan, don’t hesitate if you have…

Possible internal injuries — for instance, when an accident may have caused significant trauma to the brain, organs or blood vessels.

Facial trauma. The test could help map future surgical reconstruction.

Possible symptoms of a potentially deadly problem, such as a heart attack, stroke, aneurysm, blood clot, severe pneumonia or interstitial lung disease (inflammation of the tissue that surrounds the lungs’ tiny air sacs).

A suspected complex bone problem — for instance, a major fracture of a complex location, such as the hip — if a traditional X-ray has not provided sufficient information.

When To Ask About Other Options

In situations other than the above, before you agree to a CT scan, Dr. Semelka suggested first discussing these points…

Vulnerability of tissues to be scanned. In the AIM study, the areas most sensitive to radiation were the abdomen and pelvis and, to a lesser degree, the chest and head. So you can assent without much worry to a CT of the leg or arm….but before scanning the torso or head, your doctor should clearly explain why the test is necessary.

Alternate ways to diagnose — based on your symptoms, medical history, risk factors, etc. — and the best test for confirming that particular problem.

For instance: Suppose you have severe abdominal or pelvic pain. If your doctor suspects kidney stones, a CT is most appropriate. If an inflammatory condition such as appendicitis seems probable, a CT may be the easiest way to make the diagnosis — but often magnetic resonance imaging (MRI) can reveal the problem without exposing the patient to radiation. And although CT scans often are done to look for cancer, many malignancies (such as some abdominal cancers) are shown as well or even better with MRI.

Previous radiation exposure. Radiation risk is cumulative, so be sure that your current doctor is aware of any CT scans you’ve had in the past — and whenever possible, provide copies of those results. If another scan is suggested within a year of a previous CT, ask if it is safe to wait before repeating the test.

Amount of radiation involved. Ask the doctor how many images will be taken and whether that number can be reduced without compromising accuracy. Also, request a referral to a radiology facility that uses up-to-date equipment — many newer CT machines use significantly less radiation than older models. If your doctor is not sure about the quality of a local facility’s equipment, ask him or her to find out.

Your age. The younger you are, the more likely it is that a CT scan is not worth its risks.

Examples: A young woman’s breasts are more sensitive to radiation than an older woman’s, so a CT of the lungs is potentially more harmful…and a heart CT generally is less appropriate for a younger woman because her risk for heart disease is low.

Shields. Using an AttenuRad breast shield during a chest CT reduces radiation delivered to the breasts by up to 57% without significantly changing the quality of the images. Shields also are available to protect the eyes, gonads and thyroid gland from radiation when other areas are being scanned. When you schedule your CT, ask whether shields are provided — and if they are not, find a center where they are.

Protection in a pill? I also asked Dr. Semelka about the dietary supplement BioShield-Radiation (888-606-8883, www.bioshieldpill.com), which purportedly protects against radiation by providing multiple antioxidants to combat free radicals. He said that the information available on this product is too preliminary to recommend it, though research is ongoing. If you want to try this product, discuss it with your doctor.

When To Say No Thanks

Tell your doctor that you prefer to skip a CT scan if none of the above situations apply and if you…

Could instead have a safer test — one that does not use radiation.

Examples: Gallstones are easily seen with ultrasound. MRI generally is best in cases of soft-tissue injury, such as a torn ligament or tendon… injury to the spinal cord, such as disk herniation… a suspected brain tumor… and sometimes for complex fractures in joints.

Are pregnant. Radiation can harm the fetus.

Suspect that a doctor or hospital is guarding against a lawsuit — and that’s the primary reason for the CT recommendation. “Many physicians worry that if they miss anything, they could get sued for malpractice. Covering all bases with a CT scan is a defensive weapon against potential legal action,” Dr. Semelka explained.

Say to your doctor, “I understand that there are no guarantees with health care. Please weigh my concerns about radiation and my likelihood of having this condition you are looking for, then suggest which tests you think that I should undergo.”