The recent deployment of backscatter scanning devices meant for airline passengers has caused controversies focused on both the privacy issues of the scans and the safety of the devices themselves not to mention the unpleasant alternative of an aggressive frisking. The discussion of safety issues has been clouded by two competing narratives.
On one side, there's radiation exposure that's comically low compared to what comes from simply boarding the aircraft and being lifted above a lot of the Earth's atmosphere. On the other, there are arguments that the sort of exposure generated by backscatter devices is somehow different.
To provide a better perspective on matters, we'll explain why both of these arguments are right. In a traditional medical X-ray, the imaging is based on sending a lot of high-energy X-rays at an object of interest, such as a sore ankle.
The film or sensor that creates the images is placed on the opposite side of the object from the radiation source. If the object has some materials that aren't very dense—like, say, a liver—the X-rays will pass straight through and expose the film. Denser objects, like bones, absorb more of the X-rays, and so leave an area of film underexposed compared to the surroundings.
That's why bones appear as white on a traditional black-and-white film X-ray. What those attempts did accomplish, however, was to keep travelers all over the world more frightened about the prospect of flying. And the bumbling bombers underscored flaws in airport security efforts.
Governments worldwide spend millions of dollars every year to stop those menaces. Transportation Security Administration TSA has implemented a vast array of security tactics intended to minimize the chances of future attacks, from limiting the size of shampoo bottles to aggressive pat-down procedures. One of the newest security technologies being deployed is called backscatter X-ray scanners. These machines scan peoples' bodies for potential weapons by creating a detailed outline that's been likened to a chalk drawing or an extremely detailed X-ray image.
In doing so, the scanners highlight materials that metal detectors and older X-ray machines don't see, such as plastic explosives, illicit drugs or nonmetallic weapons made of ceramic or other materials. The technology has drawn the ire of privacy groups, which say the images are a violation of citizens' personal rights.
Others question whether the technology puts human health at risk. Concerns aside, backscatter X-ray scanners combine some fascinating physics with cutting-edge technology.
Let's begin by reviewing a legacy technology -- traditional X-ray machines that doctors have been using for decades -- to understand how this new technology is different. 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. 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. When the wheel rotates, the radial slits traverse across the length of the fan-beam and results in the pencil beam that scans in the horizontal direction; this combination of the horizontal slit and the rotating aperture produces what is commonly referred to as a raster scan.
The large area detectors located beside the source integrate the X rays that are backscattered from the person. The resulting signal strength is recorded as a function of the position of the pencil beam.
This results in a single horizontal line image of the X rays backscattered from the person. Because the total vertical travel of the X-ray head is less than the desired scan height, the tube and collimator assembly also rotate about the horizontal axis. As the X-ray assembly approaches the lower limit of travel, it tilts down in order to image the feet of the subject, and as it reaches the top of its travel, it tilts up to cover the tallest individual.
The AIT system can scan in both the upward and the downward direction. The result is a two-dimensional X-ray backscattered image of the person being screened. The large area detector consists of two vertical arrays of fluorescent screens that are viewed by large photomultiplier tubes see Figure 3.
An image is generated based on the intensity of backscattered X rays as a function. Hudson, J. Glover, and R.
As stated earlier, the X-ray backscatter manufacturers use the same physics and differ mainly in their implementation. Several round apertures or holes see Figure 3. The large area detector on the source side integrates the X rays that are backscattered from the person, and the signal is coordinated with the beam location. As each hole traverses the cone-beam of X-rays from the source inside the cylinder, a scanning beam of X rays is generated.
In diagram a , only a line is swept out; in diagram b , the unit is raised so that an area is swept out. This generates a single horizontal line one-dimensional image of the X rays that are backscattered from the person located in the scanning environment. Differences in backscatter intensity provide contrast in the line image. The entire housing of the X-ray source, fan-beam collimator, and canister assembly is vertically translated during the scan to move the horizontally raster-scanned pencil beam of X rays along the vertical direction.
This results in a two-dimensional X-ray backscattered image of the person being screened. A second detector array, on the opposite side of the subject, produces an outline image of the subject. Click here to buy this book in print or download it as a free PDF, if available. Do you enjoy reading reports from the Academies online for free? Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released.
Get This Book. Visit NAP. Looking for other ways to read this? No thanks. Page 57 Share Cite. In interpreting and comparing the results from these reports, the committee identified two issues that may result in variations in effective dose to scanned individuals: The model of the AIT system evaluated, and The techniques used to measure kerma and half-value layer HVL.
Page 58 Share Cite. Page 59 Share Cite. Page 60 Share Cite. Measurements Person being scanned — Air kerma measurements were made using a 10X ionization chamber manufactured by Radcal Corporation. The wall thickness was 3.
This corresponds to the aluminum equivalent thickness of 1. The active volume was 1, cm 3. The central collector was a cylinder centrally located in the sensitive volume of the detector with a length of 7. Measurements were made using the full vertical and horizontal rasterscanning conditions at a standard location, representative of the exposure received by the individual being scanned.
The detector was centered midway between the entry and exit portals and 30 cm from the surface of the wall where the beam exits. The ion chamber was orientated so that the incident beam was parallel to the long axis of the cylindrical detector. Page 61 Share Cite. Page 62 Share Cite. The date of calibration was indicated, but details of calibration vendor and beam quality were not indicated. The Rapiscan was operated in the full scanning mode with vertical translation of the X-ray assembly, horizontal collimator, and rotating chopper wheel.
Lead shielding was wrapped around the ionization chamber to shield from ambient radiation and inscatter, but the beam was not collimated before the attenuation foils Figure 6. The first HVL for the anterior unit was found to be 1. No explanation was given for this unusually large difference in HVL for nominally identical sources.
Geiger-Mueller and scintillator probes were used to survey the radiation outside the inspection volume. In order to simulate radiation scattered by a person being scanned, a phantom consisting of four 5-gallon containers of water supported by stacked bins was placed at the position of the person being scanned. Locations of maximum count rate were recorded and measurements were made at those locations with the cc ion chamber and with the AIT system in normal full-scanning operation. Background measurements for the same total time were made and subtracted from the readings of dose during scanning.
A maximum of 10 nGy per screening was observed in locations above the inspection volume. The maximum in a location that could be occupied by the equipment operator was 8. Calculations Person being scanned. Dose rate to bystanders was calculated to range from up to 1, nSv per hour, assuming screenings per hour. Page 63 Share Cite. This corresponds to an aluminum equivalent thickness of 1. Page 64 Share Cite. Measurements were made at a standard location representative of the exposure received by an individual, cm above the floor midway between the entry and exit portals and 30 cm from the surface of the wall where the beam exits to the center of the ion chamber Figure 6.
The ion chamber was orientated so that the incident beam was perpendicular to the long axis of the cylindrical detector. Measurements of air kerma were also taken at the ANSI-specified Reference Location representing the point of maximum exposure, but no closer than 30 cm to the surface where the beam exits, at a height of cm above the floor, midway between the entry and exit portals and 30 cm from where the beam exits the front panel.
This beam had a HVL of 1. The measured air kerma at the standard location was 47 nGy while at the maximum location cm above the floor; it was 92 nGy when averaged over the area of the ion chamber. The Rapiscan was operated in an engineering mode with vertical translation of the X-ray assembly disengaged.
The X-ray source remained at a fixed height. The horizontal collimator and rotating chopper were in place and operated normally such that the flying spot made a repeating horizontal trace. A collimator was placed outside of the X-ray assembly upstream of the aluminum absorbers.
This restricted the horizontal dimensions of the raster scanned beam incident upon the aluminum absorber. Another circular collimator was placed between the aluminum absorbers and ionization chamber to intercept scattered radiation from the aluminum and to create conditions for narrow beam geometry Figure 6. No significant difference was found between the first HVL for the anterior and posterior units.
The HVL was found to be 1. Air kerma was measured at representative locations outside the inspection area with a RANDO phantom being scanned. Measurements were made using the cc ion chamber by averaging the charge collected over 20 scans occurring in a 5-minute interval. At many locations, the kerma was found to be less than 0. At locations expected to have the highest kerma, based on results of a dose rate survey using a Technical Associates Neon-P8 probe, the kerma was found to range from 0.
Page 65 Share Cite. The result was A more realistic reference effective dose, typical of potentially occupied spaces, was 9.
Page 66 Share Cite. The aluminum-equivalent filtration of 1. The calculated effective dose for a properly positioned adult was The effective dose for an adult ranged from The effective dose to a standard 5-year-old child 19 kg, Ambient dose equivalent to bystanders was calculated using conversion factor 0. Procedures and Interlocks The NIST report did not indicate that potential failure mechanisms or verified safety interlocks were considered during the AIT system evaluation.
Page 67 Share Cite. Measurements were made on three AIT systems 1. Measurements were made at a standard location representative of the exposure received by the individual being screened.
This was similar to the NIST study reviewed above, but the height is 91 cm above the floor, midway between the entry and exit portals, and 30 cm from the surface of the wall where the beam exits.
The ionization chamber was calibrated using the M50 beam, 17 which has a HVL of 1. The average air kerma at the reference point for nine AIT systems was 46 nGy with a standard deviation of 3 nGy and a range of 40 to 52 Gy. Measurements at the factory were made with a 6 cc thimble ion chamber suitable for mammography beams with the scanning mechanisms completely disassembled. This created a stationary beam defined only by the horizontal slit attached to the X-ray tube housing.
Measurements were made with beam collimation before and after the aluminum absorber Figure 6. Measurements at the airport were made with the cc ion chamber described above.
For the measurements at the airport, the HVL was 0. An attempt was made to determine the dose to bystanders using both the cc chamber and a Fluke P survey meter. Measurements were made at the airport, with an anthropomorphic torso phantom in the AIT system. Locations expected to have maximum scattered radiation were measured by integrating the reading for 10 screenings, but no exposure above background was detected.
Page 68 Share Cite. The result was 11 nSv per screening. The input data for beam quality was 50 kV, 20 degree anode angle, and 1 mm of aluminum total filtration to give the measured 0. The results are summarized in Table 6.
No calculations were made for the bystander. Procedures and Interlocks The AAPM report includes potential failure mechanisms that could result in overexposure to the person being screened or the bystanders or screeners, including. Page 69 Share Cite. Page 70 Share Cite. Measurements Person being scanned. OSL dosimeters were placed on the surface of the plastic containers facing the anterior AIT system unit at nine different locations representing specific locations on the body.
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