If you drive through certain Los Angeles neighborhoods, you’ll see as many billboards advertising body scans as you do burger joints. Or maybe you’ve seen the ads on TV or online: “Protect your health! Get a body scan now!”

Can whole-body CT scans really do that and what are the risks? And are DEXA scans a good way to check body composition?

While technologies vary, most of these high-tech checkups use computed tomography (CT) scans to examine the entire body or specific parts, such as the heart and lungsto try to detect dangerous diseases in earlier and more curable stages.

During the 15- or 20-minute scan, you lie inside a donut-shaped machine while an imaging device rotates around you and transmits radiation. The technique combines multiple x-ray images and, with the help of a computer, produces cross-sectional views of your body. By examining the views, a doctor can look for early signs of abnormalities.

The scans aren’t cheap: Full-body scans cost between $500 and $1,000 per scan and are usually not reimbursed by insurance. And the question of how useful these scans really are is a topic of debate among medical experts.

Proponents promote scans as a smart part of a routine Physical exam. But if you’re healthy and don’t have worrisome symptoms, a scan usually isn’t warranted, says Arl Van Moore, MD, a radiologist and clinical assistant professor of radiology at Duke University Medical Center in Durham, North Carolina, who is also spokesperson for the American College of Radiology (ACR).

According to the ACR’s official position, there is not enough evidence to recommend screening for those who do not have symptoms or a family history suggesting a disease. But Van Moore sees a possible exception. “There may be a benefit for people at high risk of lung cancer, such as current smokers or those with a long history of smoking,” he says.

For healthy people, scans can cause excessive worry, for example, finding something that turns out to be benign. Additionally, the amount of radiation exposure, especially with frequent scans, is another concern. If scans are done too frequently, radiation exposure can actually increase the number of cancer long-term cases, according to a 2004 report in the journal Radiology.

The American College of Preventive Medicine says that whole-body scans “are not very good at detecting cancer in people without symptoms” and that the radiation received from these scans can increase the risk of cancer.

Before scheduling a body scan, talk to your doctor about your general health risks and how a scan may or may not help you. In particular, ask yourself:

• What’s your story? Do you have a personal or family history of lung diseaseheart diseases or specific cancers?
• Did you inhale? Are you a long-time smoker?
• If so, for how long? Even if you have give up smokingFor how many years were you an active smoker?

This is a different type of scan, called DEXA (dual energy x-ray absorptiometry). You may have heard of DEXA scans to check bone density and see if you have osteoporosis or osteopenia. It uses low-level x-rays to check your body composition, such as how much body fat you have and where it is located on your body.

There are several ways to measure body fat. Experts have told WebMD in the past that DEXA scanning is a “very good technique” and “one of the most accurate methods out there.” And researchers have called it the “gold standard” for checking body composition—specifically, bones, fat, and muscle. But insurance doesn’t cover it unless you have a DEXA scan to detect bone density. The cost of a DEXA scan varies, starting around $75 in some cases.