Copyright R. Zamenhof, 2013.
An implementation of the x-ray backscatter technique that is a modification of the one described in this blog has been developed by American Science and Engineering. Rather than designed for detecting threat objects on airline travelers, which this narrative is focused on, this new implementation is scaled-up to examine the contents of cargo containers; for example, shipping containers and trucks. The backscatter system is scaled-up by using much higher energy x-rays, plus other modifications, to enable the x-rays to penetrate into much larger objects. Instead of conventional "diagnostic" x-ray machines, this device uses linear accelerators, the same technology that is used for radiation treatment of cancer, and which produces x-rays of approximately 10-20 times higher energy than the airport threat detection devices described in this blog. The picture above shows a truck that attempted to cross the border to the United States. A simple physical examination suggested that the truck contained bananas. However, the center of the cargo area contained a compartment within which 20 or so illegal aliens were attempting to cross the border undetected. The x-ray backscatter image showed the human cargo very clearly.
As Northwest Flight 253 made its final approach to Detroit airport on Christmas Day 2009, a terrorist carrying an unusual form of plastic explosive almost succeeded in killing its 300 passengers and crew.
Because of that incident, U.S. and worldwide airport security efforts were rapidly ramped up. One approach was the installation of so-called body scanners at airports. What are these devices, how do they work, are they effective, and are they safe?
Principles of Operation of Airport Scanners
The body scanners deployed in airports in the U.S. and Europe generally use either x-ray transmission imaging or x-ray backscatter imaging. Over 1,500 of them are now deployed in U.S. airports, with the number rapidly growing.
X-Ray Transmission Imaging
X-ray transmission imaging is probably the most familiar form of x-ray imaging—widely used in medicine as well as for many security applications. It is the type of imaging that produces the familiar chest x-ray. X-ray transmission imaging is effective for detecting guns, bombs, and other threat objects that are made of dense metallic materials. However, plastic explosives, such as Semtex and C4 (frequently used by terrorists), or drugs, are poorly depicted by transmission imaging since they have similar x-ray properties to biological tissue and consequently are poorly visualized against the background image of the body.
Backscatter imaging involves sending a narrow x-ray beam into the body and detecting only those x-rays that scatter in the backward or sideways directions from tissues and threat objects that reside within the superficial 1-2 inches of the body. The x-ray beam is rapidly scanned and the position of the beam on the body at any moment in time is accurately known. The total scattered x-ray signal from the detectors at that same moment in time correlates with the backscattering property of the tissues and/or other objects over an area equivalent to the diameter as the x-ray beam—I.e., approximately 1-2 mm—and a depth up to about 2 inches.
Backscatter imaging has a number of advantages for security applications. Because backscattered x-rays need to pass through only a few inches of the body–-an inch or two on their way in, then an inch or two on their way out–-fewer x-rays are needed than in transmission imaging, where the x-rays have to penetrate through the entire thickness of the body before they can be detected. Consequently, the radiation dose to the body in backscatter imaging is also more than 100x lower than in transmission imaging. In addition, because backscatter detectors can be made very much larger in capture area than transmission detectors—perhaps 1,000x larger--this further reduces the necessary radiation dose to the body by approximately 1000x.
Advantages of Backscatter Imaging for Threat Detection
For equal densities, plastics, plastic explosives, drugs, and soft tissues of the body produce more backscattered x-ray signal than metals, so the former are more clearly depicted in backscatter than in x-ray transmission imaging. The reverse is true in x-ray transmission imaging. For example: Certain weapons, such as some models of the German Glock handgun, are manufactured with a large amount of plastic material to reduce weight. Many models of Glock handguns have plastic hand grips, which are very difficult to see in transmission images but are clearly depicted in backscatter images. Similarly, plastic explosives, even when located among a complicated background of metallic objects, can be clearly seen in backscatter images but are essentially invisible in transmission images.
Possible Health Risks
What are the health risks from x-ray backscatter imaging body scanners? The effective radiation dose from one x-ray backscatter body scan is equal to about 11 nano-Sievert (0.0011 mrem in old-fashioned units). This is equivalent to about 3-4 minutes of natural background dose; in other words, a traveller standing in line for a backscatter body scan would probably receive more radiation dose from natural background radiation than from the scan itself! Therefore, it would require a traveler to have approximately 455,000 body scans in one year to reach the 5 milli-Sievert (500 mrem) annual radiation dose limit set by U.S. Federal government and State regulations for the general public.
Most ionizing radiation generating technologies are designed with the 5 milli-Sievert (500 mrem) annual dose limit to the general public in mind. For example: patients passing through hospital corridors that happen to be adjacent to x-ray rooms, people living near the boundaries of nuclear power plants, strangers being in proximity to patients being treated with radioiodine for thyroid disease, etc., receive radiation doses that are limited under federal and state regulations to a maximum of 0.02 milli-Sievert (2 milli-rem) in any one hour. This number is mathematically linked to the maximum annual dose limits for the general public referred to above.
Put another way, based on the linear-no-threshold (LNT) model used widely for radiation risk assessment, the risk of getting a fatal cancer from one backscatter scan is approximately that of eventually dying from pollution by living in New York City for 1 minute, traveling 100 ft by car, or traveling 1 mile by jet. It would require about 90 backscatter scans to be equivalent in effective dose to one chest x-ray.
A frequent criticism of TSA’s characterization of the radiation doses delivered by x-ray backscatter scanners is that the skin receives much larger doses than the quoted values for total effective dose, since most of the radiation dose is concentrated in the skin. However, TSA’s characterization of the dose from an x-ray backscatter scan is based on the concept of effective dose, a construct used very frequently in epidemiological radiation studies. Under that concept, the risk is calculated from the partial doses received by all organs (including skin), and the corresponding “effective dose” in a form suited to LNT dose calculations is finally calculated. The definition of effective dose, therefore, already takes into account the variable doses delivered to different tissues and organs, taking into account their varying radiation sensitivities.
Non-X-Ray Threat Detection Devices: T-Wave Scanners
An alternate form of threat detection that has been recently developed for use in airports is called “millimeter wave” or “T-wave” body scanning. Instead of using x-rays, this technology use extremely high frequency radio waves in the “terahertz” range—sometimes called T-waves—that are beamed into the traveler’s body and are then differentially reflected by any additional materials that may be concealed in or on it. Although T-wave scanners produce no radiation exposure whatsoever, there are studies showing that T-wave scanners are substantially less accurate in detecting threat objects than x-ray backscatter scanners. But from a public perception perspective, T-wave scanners are an important advance in threat detection technology because of the lack of radiation dose to the traveler.
What about the issue of privacy related to body scanning for threat detection? Indeed, in addition to producing clear images of threat objects, x-ray backscatter scanning is able to provide clear images of the surface of the traveler’s body, together with quite clear depiction of his or her “private parts”. TSA claims that there are various solutions that can “depersonalize” such images. For example: images showing private parts can be automatically blurred prior to display (either the private parts or the facial features of the traveler can be blurred) and TSA staff who examine these images are located in a separate room so that they see only the images and not the individuals being scanned. There are also software applications that automatically search for and flag threat objects in an x-ray backscatter image and only display the full image to an operator if a threat object is identified. Despite these privacy maneuvers, TSA has been sued for violation of the 4th amendment, which has resulted in many backscatter scanners being removed from airports and replaced instead with T-wave scanners. This is unfortunate, since it provides much less protection against terrorist threat.
Backscatter x-ray imaging is a new technology that is aimed at detecting threat objects that would mostly not be visible using the more conventional x-ray transmission imaging approach, such as plastic explosives, drugs, and the non-metallic components of certain models of handguns.
The dose from x-ray backscatter scanning is extremely low; in fact it is virtually negligible. Based on the linear-no-threshold (LNT) model, used widely for radiation risk assessment, the risk of getting a fatal cancer from one backscatter scan is approximately that of eventually dying from pollution by living in New York City for 1 minute, traveling 100 ft by car, or traveling 1 mile by jet. It would require about 90 backscatter scans to be equivalent in effective dose to one chest x-ray. Even if someone travels frequently and receives backscatter scans during every security check, when such very low radiation doses are spread out over time their effect on the body is not linearly cumulative because the body quickly repairs minor X-ray damage when it is protracted in time. Backscatter scanning in tandem with x-ray transmission scanning (often combined in the same apparatus), appears to be the most significant development for reducing the terrorist threat at airports. However, privacy concerns and lawsuits based on 4th amendment issues have resulted in the deactivation of many backscatter scanners in the U.S. and Europe.
T-wave body scanners are an alternative technology recently developed for threat detection. T-wave scanners do not use ionizing radiation (such as x-rays), and produce no radiation dose to the subject. However, at the present time, although embraced by the public because of their total safety and lack of privacy violation (due to the poor quality of the images), T-wave scanners do not appear to have the necessary accuracy or sensitivity for adequate threat detection.
TSA has implemented various solutions to depersonalize x-ray backscatter images, that in addition to depicting threat objects clearly depict the “private parts” of a subject.
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