If you’ve suffered a serious injury or your doctor suspects cancer, a CT scan could be lifesaving. But since 1980, the number of those tests done each year has grown from fewer than 3 million to more than 80 million. And recent research suggests that roughly a third of them serve little if any medical purpose.

Researchers estimate that at least 2 percent of all future cancers in the United States — about 29,000 cases each year, resulting in 15,000 deaths annually — are likely to come from the radiation used in CT scans alone. Here’s Consumer Reports’ head-to-toe advice for when CT scans and X-rays are needed — and when they’re not.

A headache

The problem: A severe headache may make you worry about a brain tumor, which may make you think you need a CT scan. But that’s rarely true. Most people who seek medical help for head pain have tension headaches or migraines. Although those can be painful, your doctor should be able to diagnose the problem with a medical history and a neurological exam.

When you might need a CT scan: An imaging test might be appropriate if you have an abnormal neurological exam or your doctor can’t diagnose your headache based on your symptoms and an exam. But in most of those cases, you need an MRI, not a CT. That test, which does not expose you to radiation, better detects serious causes of headaches, such as aneurysms and tumors.

A knock on the head

The problem: The number of emergency-room visits for head injuries in young children has almost doubled in the past decade, according to the Centers for Disease Control and Prevention. But about a third of the scans done as a result are unneeded, the American Academy of Pediatrics says.

When you might need a CT scan: In most cases, a neurological exam, including questions about the injury and symptoms, can determine whether your child has a minor concussion. Expect a CT scan only after a car crash or a serious accident such as a fall from a bike without a helmet or if the child experiences worrisome symptoms such as confusion, loss of consciousness, weakness or tingling on one side of the body, or loss of hearing or vision.

Lung-cancer screening

The problem: Lots of ads pitch CT scans for lung-cancer screening. And it would be great if there were a safe, effective way to detect the disease early, because it’s still the leading cause of cancer deaths in men and women. But research shows that the test is worthwhile only in people with very high lung-cancer risk.

When you might need a CT scan: Lung-cancer screening makes sense for current or former smokers between the ages of 55 and 80 who smoked the equivalent of a pack a day for 30 years.

Breast-cancer screening

The problem: Mammograms don’t expose women to much more radiation than a chest X-ray. And they can prevent breast-cancer deaths. But some doctors recommend them more often than necessary or encourage women to start young. That can needlessly expose women to radiation and raises the possibility of false alarms.

When you need a mammogram: Women age 50 to 74 should get screened every two years. Women of other ages should talk with a doctor about whether testing makes sense for them based on their risk factors.

Heart-disease screening

The problem: Doctors can use CT scans to assess your risk of heart disease by measuring the buildup of calcium in your coronary arteries. Or they may look for blocked arteries with a test called CT angiography. Some doctors say the results can help them decide how to treat people at moderate heart-attack risk; for example, whether they should prescribe drugs for cholesterol levels that are borderline high. But doctors can usually accomplish the same thing by looking at risk factors such as blood pressure and family history. And the test exposes you to a lot of radiation.

When you might need CT angiography: The test may be an option if your doctor strongly suspects a pulmonary embolism, a potentially fatal event that occurs when a blood clot travels from your legs or elsewhere in your body to your lungs.

Colon-cancer screening

The problem: Some doctors recommend CT colonography, also known as virtual colonoscopy, over the regular version because it allows patients to avoid having a tube inserted into the colon. But it requires the same pretest prep, and abnormal results must be confirmed by a regular colonoscopy. And CT colon­ography is not as accurate.

When you might need CT colonography: The test might make sense if you cannot or will not undergo a colonoscopy.

One test to avoid at all costs

Whole-body CT scans expose you to much more radiation than regular CT scans. One study calculated that for every 1,250 45-year-old adults who have the exam, one will die of cancer as a result. And it’s unclear whether any lives will be saved.

Questions to Ask Your Doctor

●Why is the imaging test necessary? You should never refuse a test if it’s needed. But they’re often not. Ask why the test is being done, how the results will affect your treatment and what will happen if you skip it.

●Is there a safer alternative? Ultrasounds or MRIs, which don’t expose you to radiation, can sometimes be used instead.

●Is your doctor credentialed? Ask whether the radiology facility is accredited by the American College of Radiology, whether the CT technologist who administers the test is credentialed and whether the person interpreting the studies is a board-certified radiologist or pediatric radiologist.

●Has your doctor invested in a CT scanner or radiology clinic? Studies show that physicians who own scanners use imaging much more than those who refer their patients to other radiology centers. So ask your doctors whether they are referring you to a machine or an imaging center that they have a financial interest in.

●What’s the right dose for me? The smaller or thinner you are, the lower the radiation dose you need.

●Do you have a prior scan? If you recently had an X-ray or a CT scan of the same area, ask whether your doctor can use that instead.

Copyright 2015. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.