How Backscatter X-ray Systems Work

Old-school X-ray machines use X-ray tubes to bombard your body with high-energy X-rays, which are a type of electromagnetic radiation. Like visible light, X-rays are made of photons, which are elementary particles that make up all electromagnetic radiation. For more detailed information on the subject of radiation, see our article How Radiation Works.

The X-rays pass through you and then are recorded onto film or a computer sensor on the other side. Because denser parts of your body (such as bones) partially block the X-rays, the resulting image displays lighter and darker areas corresponding to the relative density of your body parts. For example, your skin and flesh appear darker, while denser materials like bone appear white.

In short, the X-ray energy is imprinted on the transparent, radiographic film that your doctor sees. The result is a monochromatic picture that your doctor can use to pinpoint a broken bone or other issues.

It's impossible to use this kind of X-ray machine for airports, where efficiency is paramount and there's no time to develop film for every passenger. Instead, airports use related technology called dual-energy transmission X-ray systems to screen your carry-on items, which pass through the machine on a conveyor belt.

These dual-energy systems emit powerful rays that are then filtered in a way that lets security personnel view differences in density. Skilled operators are trained to use visual cues that tell them whether items are metallic, organic or non-organic. Their eyes also pick up on items that might indicate a potential danger. You can read more about the details of dual-energy X-rays systems in How Airport Security Works.

Backscatter X-ray machines (sometimes called soft X-ray scanners) are more sophisticated than medical X-ray and dual-energy X-ray systems. Even the machine layout is different. With traditional X-ray machines, the X-ray tube and imaging sensor sandwich the subject. But with backscatter scanners, the imaging sensor is placed on the same side of your body next to the X-ray tube.

Backscatter X-rays are much weaker than those your doctor employs. These rays don't go through your flesh and bones. Instead, they penetrate your clothing and about an inch into your body, where your tissues scatter and ricochet the rays back toward the sensor.

The sensor records those scattered rays, creating a picture that looks a lot like a naked human body. If that body happens to be surreptitiously carrying a dubious or irregular object, authorities will know. Security officials will also notice if those irregular objects are love handles, however, a touchy subject that has set off privacy debates that we'll review later.

A scanner that can render a nude image of your body while you're still clothed might sound like an insidious invention straight out of a science fiction movie. But the technology is very real.

Traditional X-rays create images by blasting powerful rays all the way through a human body. These rays are strong enough to cause health problems if you receive too many high doses, which is why your doctor uses X-rays sparingly. It's also why you wear a lead jacket when you're exposed to X-rays at the dentist's office. The jacket deflects the X-rays before they come into contact with your body.

In contrast, backscatter machines use less powerful X-rays. The radiation dose is around 0.02 to 0.03 microsieverts, or about the same as one hour of exposure to natural environmental radiation (called background radiation) [source: Health Protection Agency]. These X-rays don't barge through you by sheer power. Instead, they smack into your body and scatter.

Backscatter X-rays interact differently depending on the type of material they come in contact with. Each different type of material, be it organic or non-organic, causes the X-rays to scatter at different intensity levels, providing a lot of contrast in the resulting two-dimensional image. Security personnel monitoring the scanner can use these images to detect suspicious objects squirreled away under a person's clothing or perhaps hidden inside body cavities.

If you're a physics buff, there's a more technical way to think about how backscatter X-rays work. As the X-ray collides with atoms in your body, the photons in the X-ray beam scatter. During this process, those photons also push electrons out of some of the atoms, resulting in ions, and sometimes slower moving photons, too - this effect is why X-rays fall into the category of ionizing radiation.

The scanner uses the energy signatures of those ions and sluggish photons to produce images that identify organic material, like your arms and legs, or nonorganic objects, such as weapons or that belt buckle you forgot to remove before screening. The differences are often easy to see, even to the untrained eye.

If this sounds like a pretty high-tech, expensive prelude to your weekend trip to grandma's house, it is. Backscatter scanners aren't cheap. They cost more than $100,000 per unit [source: EPIC].

The TSA currently operates around 500 backscatter scanners in close to 100 airports and is planning to deploy at least 1,000 total scanners. With that many scanners in place, nearly 70 percent of travelers will be screened using the new technology [source: Los Angeles Times].

In order to successfully roll out hundreds more of these scanners, security officials will have to counter privacy and health concerns. Keep reading to see how these scanners work in detail -- and why some groups are working to keep these machines out of our airports.

The privacy concerns are obvious: Few travelers would look forward to a backscatter screening knowing that security employees were seeing what amounts to a naked picture. That's true even though the TSA says the images are immediately deleted and that the employees doing the screening are in a remote location and not near the security checkpoint, and thus, can't see the people they're screening. With the screeners located far away from the people being scanned, the TSA hopes travelers will feel less like they are being violated visually.

This strategy is also designed to defuse arguments about racial profiling. Because the screeners can't see skin color or clothing, they aren't influenced by preconceptions about the person standing in front of the scanner.

Still, for most travelers, it's no small detail that the scanners create a nearly naked picture. To minimize the visual intrusiveness of these scans, the TSA is testing a few scanners using updated software, which projects a cartoon-like image of the person in question. Suspicious items are highlighted in a similarly generic way, letting the authorities pinpoint people who may require further screening in the form of a pat-down.

So far, backscatter scanner privacy issues haven't blown up into major problems for the TSA in the way that enhanced pat downs, or aggressive frisking, did. But there is still plenty of opposition to these so-called strip search scans.

The ACLU (American Civil Liberties Union) argues that the scanners hold potential for humiliation. Not only are subjects stripped bare, but people with private medical equipment, such as colostomy bags, may be subjected to unnecessary searches. The group says that backscatter X-ray scanners should be used only when absolutely necessary, not systematically applied to a predetermined ratio of travelers passing through security gates.

In Britain, there are concerns that the scanners may breach child pornography laws. As a result, British travelers under the age of 18 were allowed to bypass the scanners. Some religious leaders have opposed the scanners as well, saying that the resultant nude images violate their beliefs.

But privacy is just one part of the anti-scanner argument. Some people worry about the effect of scanners on human health. Find out why on the next page.

Although security and health officials say the radiation dosage from exposure to backscatter scanners is very low, not everyone feels at ease with the technology.

This is in spite of the fact that officials say you'd have to pass through the scanners around 5,000 times to receive the same dose of radiation that you'd receive if you visited your doctor for one chest X-ray [source: Los Angeles Times].

What's more, the TSA claims that you actually receive a higher dose of radiation just by flying, thanks to cosmic rays that continually bombard the Earth's atmosphere. It takes only about 60 seconds of flight time at 35,000 feet to expose you to the amount of radiation you receive from one scan [source: CBS Denver].

That hasn't stopped people from questioning whether the scanners are safe, however. Some scientists say that the ionizing radiation these machines create always presents a potential risk to human health, particularly in the form of skin cancer. But because cancer may develop years after exposure, there may be no direct tie to the initial cause.

The potential for cancer and other health problems is a reason that a few companies, such as Rocky Flats Gear and Flying Pasties, are capitalizing on travelers' fears. The latter are specially designed inserts that block scans from exposing your private parts to screeners, but the former go a step further, partially protecting you from X-ray radiation.

Due, in part, to radiation exposure concerns, the TSA has been testing millimeter wave machines. These machines are a competing technology that produces images much like those from the backscatter scanners, but instead of X-rays, they use radio waves, which may alleviate concerns about radiation exposure.

In addition to the fact that they don't emit radiation, millimeter wave scanners may have another leg up on their backscatter brethren. Instead of simpler, 2-D images, millimeter wave machines produce 3D pictures that often provide more details and better overall security effectiveness.

While simultaneously defending backscatter X-ray technology as safe, the government is taking precautions a step further with regards to privacy. In February of 2011, the Senate voted to make misuse of scanner images a felony. This legislation should deter any mischievous screener from posting the nude pictures online, which actually happened with a scanner used at a federal courthouse in Florida.

To reassure passengers who have privacy protections, the TSA is also testing millimeter wave machines with software that doesn't display anatomical details. The TSA is also configuring some millimeter wave machines so that passengers can see the scan results at the same time as security personnel, as proof that no one is digitally peering underneath their clothes.

No matter what your stand on the issue of security versus modesty, privacy advocates say the new scanner technology is just playing catch-up to the cleverest terrorists, anyway. They say that because sophisticated terrorists understand backscatter scanners, they'll hide small amounts of plastic or sheet explosives deep in body cavities where they are much harder to detect, rendering the scanners ineffective when they're most needed.

Regardless of your personal stance on X-ray scanners, you can expect to see these kinds of technologies continuing to proliferate in airports and other areas where security is a top priority.

In the same way that terrorist threats will continue to be matched by security countermeasures, you can bet that arguments about safety and privacy will continue to crop up as travelers and government officials try to find a balance between security and individual rights.