As backscatter x-ray full-body scanner systems lose ground in favor of the "nonionizing" radiation from terahertz and millimeter-wave scanning and imaging systems, safety questions continue while scanner exposure levels are being defined for defense and security applications.
Full-body scanners for the detection of guns and other metal objects as well as narcotics, explosives, and hazardous chemicals are seeing increasing deployment at airports, security checkpoints, and cargo facilities. But first, some definitions are in order. All scanners can be either active (showering imaging subjects with radiation to improve detection) or passive (using existing reflected radiation to detect concealed objects). Backscatter x-ray systems illuminate the subject with low-dose x-rays at 0.01 to 10 nm wavelengths; millimeter-wave imaging systems utilize radiation with 1 to 10 mm wavelengths—longer than infrared (IR) or x-rays, but shorter than radio waves or microwaves; and terahertz imagers are called "sub-millimeter" systems and operate at 0.1 to 1.0 mm wavelengths.
The first full-body scanners were deployed at Amsterdam's Schiphol airport in late 2009. By late 2011, Europe had banned full-body x-ray scanners at its airports due to radiation concerns, even though the machines deliver x-ray doses equivalent to 2 min of high-altitude airline flight time or only 1/2500 of the American College of Radiology (Reston, VA) recommended annual x-ray dose.1 Still, academics such as David Brenner of Columbia University (New York, NY) and David Agard from the University of California-San Francisco note that even small doses of x-ray radiation directly on the skin can affect high-risk populations including young children, individuals with certain gene mutations that prevent DNA-damage repair due to x-ray exposure, or persons at risk for skin cancer.2
In September 2012, aviation policy specialist Bart Elias wrote in a Congressional Research Service (www.crs.gov) report to the US Congress that 700 x-ray backscatter and millimeter-wave technology scanners had been deployed by the Transportation Security Administration (TSA) at US airports, with an estimated 1800 to be in place by the end of 2014 at a cost of about $175,000 per unit.3
Elias adds that in addition to backscatter x-ray safety concerns, passengers being screened were also concerned about privacy issues, citing the "revealing" nature of the images acquired by these scanners. As a result, the TSA has required that privacy algorithms be applied to the images produced by commercial systems, or in the case of millimeter-wave systems, that images be eliminated entirely for viewing and replaced by Automated Target Recognition (ATR) software that only indicates the visual presence of a threat (see Fig. 1).
Just recently (January 2013), a Bloomberg article reported that the TSA plans to remove x-ray backscatter-based Rapiscan units from OSI Systems (Hawthorne, CA) because the company failed to write software to make passenger images less revealing. The TSA plans to replace the systems with millimeter-wave scanners from L-3 Communications Holdings (New York, NY) that do include adequate privacy software.
"Nonionizing" radiation
With privacy concerns being addressed through software, safety issues are the major factor in the reduction of backscatter x-ray body scanner use worldwide. Security officials are increasingly in favor of nonionizing radiation sources used in millimeter-wave and terahertz scanners. Radiation is not even a factor if the systems are "passive" in nature—meaning no radiation whatsoever is used to illuminate the subject; rather, the instrumentation detects signals passively emitted by a passenger being screened.
"Even nonionizing radiation is dangerous at extreme power levels; our ThruVision TS4 [indoor use] and TS5 [outdoor use] terahertz scanners do not use any emissions to reflect off the person," says Stefan Hale, head of ThruVision products for Digital Barriers (London, England). "Our systems are passive—just seeing what is emitted [much as the eye sees] without illuminating the object. Technically, our systems are passive differential radiometers—not entirely dissimilar to those used in space telescopes—that detect concealed objects by analyzing the specific absence of a heat signature given off naturally by the human body."
ThruVision terahertz scanners are passive heterodyne systems that employ a mixer as the main element to receive the blackbody radiation given off by a subject. A local oscillator (LO) signal is used to pump the mixer and down-convert the received 250 GHz (radio frequency or RF) signal to an intermediate frequency (IF) of a few gigahertz. At Digital Barriers, the mixer is subharmonic, wherein the local oscillator signal is approximately half the frequency of the received signal.
The mixer uses an antiparallel pair of Schottky mixer diodes mounted on a quartz filter circuit and the pumping of the double diodes by the half-LO frequency creates a current component within the diodes at the RF frequency. The nonlinearity of this current means that an IF frequency is produced at a low-gigahertz frequency that contains all the amplitude information that was in the terahertz RF frequency. This signal is now low enough in frequency to be amplified efficiently and then detected using a Schottky diode detector. The amplitude of the signal is then fed into the software using an A-to-D converter. Optical metal mirrors and a mechanical scanning mirror are used to scan eight receiving channels across the subject and the image is built up within the software.
The terahertz signal that Digital Barriers detects with its ThruVision scanners is simply blackbody radiation, emitted from the body like any other warm object, but with tiny power levels on the order of 10-12 W. ThruVision uses state-of-the-art mixer design techniques to make low noise-temperature mixers together with very low-noise-figure IF amplifiers in order to produce the current images of objects concealed on a person's body.
And what about privacy concerns? Hale responds, "The shorter wavelengths of terahertz imagers have lower penetration through clothing. At a wavelength of 1.2 mm, our systems have lower penetration through clothing than the current crop of active millimeter-wave imagers but still have enough penetration to see threat items such as guns and suicide bombs through clothing."
Hale adds that ThruVision TS4 systems operate at low resolution (less than 200 × 100 pixels), so anatomical detail is not shown, but guns and suicide bombs are still clearly visible (see Fig. 2). Hale argues that current active millimeter-wave systems operate at higher wavelengths, higher resolutions, and as reflection images that increase the anatomical detail that can be seen—extra detail that is not necessary to identify threats hidden under clothing.
Defining safe exposure limits
Published articles and even books comparing millimeter-wave to terahertz imaging technology are readily available online, and each technology has its own merits and drawbacks.4,5 It is difficult to estimate how many millimeter-wave and terahertz scanners are commercially deployed in full-body scanner applications, considering that most companies do not discuss specific customers or programs. And while passive systems are inherently safe whether based on millimeter-wave or terahertz technology, active imaging systems are always a radiation concern.
A study from Los Alamos National Laboratory (LANL; Los Alamos, NM) and Harvard Medical School (Boston, MA) researchers discusses "DNA breathing dynamics in the presence of a terahertz field" and concludes in its abstract that "a specific terahertz radiation exposure may significantly affect the natural dynamics of DNA, and thereby influence intricate molecular processes involved in gene expression and DNA replication."6 However, LANL researcher Boian Alexandrov emphasizes, "Our project is still at the level of exploratory basic research, and we have drawn no connections to full-body scanners."
We all know that the business of quantifying radiation exposure limits for any photonics instrument is a tricky one, especially considering how various parts of the human body interact with the scanning beam. For example, retinal injury research has impacted laser safety standards. A comprehensive summary on the "Effects of Terahertz Radiation on Biological Systems" undertaken by Universität Würzburg researchers finds that only a small amount of safety data exists for terahertz radiation from 0.1 to 1.0 THz, but virtually nothing exists for frequencies higher than 4.0 THz.7
In a more targeted study from the same researchers, terahertz radiation at 0.380 THz and 2.520 THz at intensity levels ranging from 0.03 to 0.9 mW/cm2 and 2 to 8 hr exposure times does not lead to DNA damage in skin cells in vitro.8 While this may sound like good news for terahertz full-body scanner manufacturers, it is difficult to source specifications for any commercially available full-body scanner system and safety standards for active terahertz systems simply don't exist.
Even though future terahertz scanners may be deemed safe by the TSA or whatever security agency directs their installation, the public often decides the outcome of new radiation technologies as evidenced by the demise of seemingly safe backscatter x-ray scanners. For now, scanner manufacturers should continue advancing passive imaging technology and let the naturally occurring radiation do the talking.
REFERENCES
1. K. Burke and J. Straw, "Europe bans full-body X-ray scanners over radiation concerns, but no such luck for U.S. travelers," New York Daily News (Nov. 16, 2011).
2. R. Knox, "Scientists Question Safety of New Airport Scanners," NPR News (May 17, 2010).
3. B. Elias, "Airport Body Scanners: The Role of Advanced Imaging Technology in Airline Passenger Screening," Congressional Research Service report R42750; http://www.fas.org/sgp/crs/homesec/R42750.pdf.
4. Committee on Assessment of Security Technologies for Transportation, National Research Council, Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons, National Academies Press (Jan. 29, 2007); http://bit.ly/Xx1ukt.
5. Abstract, R. Appleby, "Passive millimetre-wave imaging and how it differs from terahertz imaging," Phil. Trans. R. Soc. Lond.A 15, 362, 1815, 379–393 (Feb. 15, 2004); doi:10.1098/rsta.2003.1323.
6. B.S. Alexandrov et al., Phys. Lett. A, 374, 1214–1217 (2010).
7. H. Hintzsche and H. Stopper, Critical Rev. in Environ. Sci. and Technol., 42, 2408–2434 (2012).
8. H. Hintzsche et al., Radiation Research, 179, 38–45 (2013).