Engineering Aviation Security Environments—Reduction of False Alarms in Computed Tomography-Based Screening of Checked Baggage (2013) / Chapter Skim
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2 Overview of Deployed Explosive Detection System Technologies
2 Overview of Deployed Explosive Detection System Technologies
Pages 17-28
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From page 17... ...
All EDSs must be certified by the TSL, as discussed in the subsection below entitled "Testing at the Transportation Security Laboratory." OVERVIEW OF A COMPUTED TOMOGRAPHY SCANNER A typical CT scanner (Figure 2-1) consists of a support frame and five key subsystems: (1)
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From page 18... ...
Most scanners use a process called filtered back-projection (FBP) to reconstruct the crosssectional images; 2 the algorithms used in image reconstruction were developed for medical imaging and have not been optimized for use in security applications, which is one potential source of error that can lead to false alarms.
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From page 19... ...
If the projections were acquired using helical scanning, then the movement of the bag during data acquisition is removed using interpolation.
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From page 20... ...
As part of its analysis, the ATR algorithm may also compensate for imperfect correction in the CT reconstruction step and extracts features such as density, atomic number, and feature size. Once this information has been extracted, it is compared to the density and properties of known explosives.
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From page 21... ...
Narrowing the detection windows in order to eliminate the misidentification of non-threat materials carries with it the risk of decreasing the detection rate and missing a true threat. Expanding the detection window to ensure the capture of all threat materials will result in capturing non-threat materials and increasing the false alarm rate.
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From page 22... ...
L-3 Communications also indicated that it had developed TSL-certified software to reduce false alarms, but that, as of the committee's meeting in 2009, this software had not been purchased. FUNDAMENTAL LIMITATIONS OF COMPUTED TOMOGRAPHY-BASED EXPLOSIVE DETECTION SYSTEMS BASED ON THE PHYSICS OF THE TECHNOLOGY As stated above, CT-based explosive detection systems are not systems that detect explosives, but rather systems that can identify materials that have specific properties.
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From page 23... ...
Image artifacts caused by imperfections in both the CT hardware and the software reconstruction lower confidence in the estimated characteristics of an object within a bag, forcing the threat-defining windows to be widened, which results in a concurrent increase in false alarms. Thus, improvements in the image reconstruction and correction process would enable the more accurate measurement of objects and could lead to a lower false alarm rate.
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From page 24... ...
TESTING AT THE TRANSPORTATION SECURITY LABORATORY Certification testing of EDSs and their subsequent performance testing in an airport setting are one way to gain a better understanding of EDS performance and of the causes of false alarms. To be 9 Elan Scheinman, Reveal Imaging Technologies, Inc., presentation to the committee, April 29, 2009, Washington, D.C.
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From page 25... ...
10 Additionally, because the bags and objects scanned in the airport are more varied than those in the TSLcertified test set, the probability of detection established for an EDS at the TSL may not be maintained an airport setting. Beyond these limitations, additional shortcomings in the effectiveness of an EDS system may develop over time, including, for example, the lack of a feedback system by which false alarms are analyzed and fed back into the ATR software development (discussed in greater detail in Chapter 6)
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From page 26... ...
When the ATR algorithm determines that an object or objects within a scanned bag meet the established threat criteria, a human screener must resolve the alarm. Information is presented on a display to a human screener at a baggage-viewing station.
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From page 27... ...
When a bag is sent to the baggage-inspection room, screeners may open it to visually inspect the objects indentified as potential threats and -- depending on the object indentified -- may also employ explosive trace detection to attempt to clear the bag. If the airport's integration of the baggage-inspection room with the rest of the baggage-screening system is robust enough to permit it, this inspection may be guided and informed by other data related to the bags being inspected including CT slices and the outputs of the ATR algorithm-although it is possible that the threat indentified by the ATR algorithm will not be found by the transportation security officer (TSO)
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From page 28... ...
Modifying an EDS's operation to emphasize image quality over operational requirements (such as throughput) by such means as slowing the scan speed, improving reconstruction algorithms, or changing parameter settings within ATR algorithms based on threat level could also achieve the same aim -- that is, it could improve the estimate of materials properties or object segmentation.
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