What distinguishes backscatter machines from millimeter wave scanners?

If you judge solely by name, you might assume that 'advanced imaging technology machines' are tools for doctors to detect tumors or medical conditions. In fact, the term is a euphemism used by the U.S. Transportation Security Administration (TSA) to refer to the body scanners at airports designed to detect weapons, explosives, or other threats carried by passengers.

As of November 2012, the TSA's website stated that there were 800 advanced imaging technology machines installed across 200 U.S. airports. These machines fall into two categories based on the electromagnetic radiation they use. Backscatter machines, which make up about 30% of installations, send low-energy X-rays to reflect off the passenger's body. Millimeter wave (mmw) scanners, on the other hand, emit radiation similar to microwaves. Both types can see through clothing to generate a 3-D image of the individual inside the machine.

When the TSA began rolling out the scanners in 2010, they quickly sparked a flurry of questions from passengers, pilots, and public health officials. How much radiation do these machines emit? Is the exposure significant enough to potentially raise cancer rates? And could TSA agents see personal details we'd prefer they didn’t?

The European Union has responded to these concerns with clarity: it prohibits any body scanners that utilize X-ray technology. This ban aligns with legislation in several European nations that restricts exposure to X-rays, allowing them only for medical purposes. In contrast, the TSA and manufacturers of the scanners, such as Rapiscan (for backscatter) and L-3 Communications (for millimeter wave), continue to reassure the public about the safety of these devices. They have also introduced software aimed at safeguarding passenger privacy by creating generic outlines or blurring certain parts of the image.

Nevertheless, a significant number of people remain doubtful that airport scanners, in any form, are entirely safe. Additionally, many find it difficult to grasp how these machines function or how they differ from one another. To help clear up these concerns, we will compare and contrast the two technologies based on a variety of factors, beginning with the type of energy they emit.

X-rays or Millimeter Waves?

Both scanner types emit energy as electromagnetic radiation, which consists of energy waves formed by electric and magnetic fields. These waves travel through space in various sizes, or wavelengths. Backscatter machines emit low-energy X-rays, with a wavelength of about 0.0000000001 meters, or 0.0000001 millimeters. Millimeter wave scanners, in contrast, produce microwaves with wavelengths ranging from 0.001 meters (1 millimeter) to 0.01 meters (10 millimeters). Therefore, mmw scanner waves are much larger, making them less likely to affect tiny structures like proteins and nucleic acids in the human body.

The design of a backscatter machine includes two radiation sources to capture images of both the front and back of the person without creating blind spots. These radiation sources are housed in rectangular structures resembling large industrial freezers, facing each other with a gap wide enough to fit a person.

A millimeter wave scanner, on the other hand, resembles a large, hexagonal telephone booth. Two of its six panels serve as an entrance and exit, while the other four feature transparent glass or plastic. Inside the structure are two stacks of disc-shaped transmitters, each enclosed in a protective shell called a radome, which rotates 180 degrees around a central axis.

Now that we understand the general design of the scanners, let's explore how they generate images of the person being scanned.

Backscatter machines utilize rotating collimators to produce X-rays, which pass through a slit and hit the passenger inside. These X-rays go through the clothing, reflect off the person's skin, and return to the detectors mounted on the machine. The radiation also bounces off hidden weapons, explosives, or other concealed items near the skin. By detecting and analyzing this reflected radiation, known as backscatter, the machine can generate an image of the individual and any organic or inorganic objects they may be carrying.

Millimeter wave scanners operate with small, disc-shaped transmitters that generate an image. Each transmitter sends out a pulse of energy that travels as a wave to the person standing in the scanner, passing through their clothing, bouncing off their skin or hidden items, and returning. Since a single disc scans a small section of the person, the machine contains two sets of discs that rotate around the individual. This setup allows the device to capture a full, 360-degree image, from head to toe, both front and back.

Peering Beneath Your Clothing: Backscatter and Millimeter Wave Images

Both types of scanners depend on software to transform reflected electromagnetic energy into images. The configuration of the software plays a key role in determining how detailed the final image will be. For instance, a backscatter machine with the simplest version of the software displays a full-body silhouette resembling a chalk drawing, offering some insight into the person's body shape. However, when privacy algorithms are applied, the software blurs these details and highlights only potential threats.

Millimeter wave scanners can also generate images that reflect a person's unique topography, but the result resembles a rough graphite sketch. Since their inception, the TSA has equipped these machines with Automated Target Recognition (ATR) software, which produces a standard outline of a person—uniform for everyone—and marks any areas that may require further inspection. This occurs only if the scanner detects something potentially suspicious; otherwise, the display simply shows "OK" with no image.

The scanning process for passengers is essentially the same in both machines. They are required to remove all items from their pockets, including belts, jewelry, lanyards, and cell phones. Then, they step onto a small ramp, stand in the center of the machine, raise their arms in a bent position, and remain still as the device scans them. The primary difference lies in the time it takes to complete the scan: Backscatter machines require about 30 seconds, while mmw scanners take approximately 10 seconds.

Another notable difference, potentially more significant than the 20-second time gap, is that backscatter machines rarely trigger false alarms. A study conducted in Britain found their false alarm rate to be around 5 percent [source: Grabell and Salewski]. In contrast, millimeter wave scanners tend to be more prone to errors. They can be misled by clothing folds, buttons, and even sweat beads. In a German study, mmw scanners had a false positive rate of 54 percent, meaning half of the individuals scanned required a pat-down, which revealed no concealed items or weapons [source: Grabell and Salewski].

Privacy and Safety Concerns of Advanced Imaging Technology

Now we address the most controversial and widely debated issue surrounding whole-body scanners: their safety. The core question revolves around whether the scanner uses ionizing radiation. Ionizing radiation has enough energy to remove electrons from atoms, which can disrupt the structure of biological molecules like proteins and nucleic acids. X-rays are a type of ionizing radiation, while radio waves, visible light, and microwaves are not.

Backscatter machines utilize X-rays, so the concern centers around their intensity and exposure duration. The manufacturers claim that a single scan exposes a person to extremely low levels of radiation. For example, a Rapiscan executive remarked, "You would have to go through [a backscatter] scanner 1,000 times to equate to one medical X-ray. You get twice as much radiation when eating a banana than when going through the scanner" [source: Paur].

However, other studies have raised more concerning findings. One study by the Marquette University College of Engineering revealed that backscatter X-rays can penetrate the skin and reach deeper tissues. Another study by researchers from Columbia University Medical Center predicted that 1 billion backscatter scans per year could result in 100 radiation-induced cancers in the future.

Millimeter wave scanners, on the other hand, do not carry these risks because they use non-ionizing radiation. To date, no safety concerns have been identified with this type of scanner.

Another major concern is privacy. Both types of scanners can produce images that expose private details about travelers. However, the TSA has made significant efforts to safeguard the privacy of individuals being scanned. For example, backscatter machines are equipped with software that includes a privacy algorithm to blur genitalia and faces while focusing on potential threats.

Most millimeter wave machines, though not all, employ automated target recognition (ATR) software that generates a generic outline of every individual, with any suspicious areas marked for attention. If no abnormalities are detected, the software simply displays the word 'OK' without showing an image. In cases where ATR software is absent, a security operator located remotely monitors the image and communicates wirelessly with the agent operating the machine.

It is generally believed that neither type of machine retains images; they are supposedly erased as soon as the security team completes its review. However, there have been instances, such as reports of U.S. marshals in Florida, where thousands of images were downloaded and stored [source: McCullagh].

That's all the information we have. You can now consider yourself well-versed in the workings of advanced imaging technology machines.