number of the material, thereby distinguishing between low effective atomic number (referred to as Zeff


number) materials and high Zeff


ones.


A fl uoroscopic X-ray image of a bag and a fl uoroscopic X-ray system for checked baggage screening.


Another early system was the Electronic


Transmission X-ray, in which images result when X-rays pass through and are absorbed by objects. When viewed on a transmission monitor, these X-rays create "shadowgram" images, similar to the result of a medical X-ray. In single-energy X-ray techniques, an X-ray beam, generated by an X-ray tube with one constant voltage, is used and the image indicates the degree of absorption of the X-rays. Objects in images can be identified by their shape and contrast.


Commonly Used X-ray Systems Today The most commonly found X-ray systems


currently in use for carry-on and checked baggage screening are based on Dual-Energy transmission and Computed Tomography (CT). Dual-Energy transmission systems are based


on the use of low-energy and high-energy X-rays to image materials. X-ray data is obtained at both X-ray energies and the two independent images are computer-processed to compare low-energy to high-energy X-ray absorption. The


“…one of the key reasons that Dual-Energy X-ray transmission systems are so prevalent is that Zeff


enables effective detection of explosives, metals and other materials while maintaining a relatively high throughput…” Transmission X-ray,


commonly used


for people, baggage and cargo screening provides the ability to identify threat items. However with a single energy, there is no ability to provide material discrimination and so the electronic X-ray images provided, are monochromatic (black and white). While such images can allow identification of items such as weapons, it is more difficult to effectively identify more sophisticated threat items such as explosives, without any indication of material composition of the objects being screened. In addition, the use of Transmission X-ray for screening people, despite being subject to stringent safety regulations, has recently raised some public health concerns in Europe and the U.S. over exposure of passengers to low dose radiation. Finally, Backscatter X-ray is another early imaging technique that detects the radiation reflected from a target, usually forming a 2D image. The scattering pattern exhibited by a material is correlated with the atomic


–based material discrimination


displayed results, in addition to single-energy technique capabilities, characterise and identify the various materials by their Zeff


number, and


artificial colours are assigned to different Zeff numbered materials. One of the key reasons


that Dual-Energy X-ray transmission systems are so prevalent is that Zeff


–based material


discrimination enables effective detection of explosives, metals and other materials while maintaining a relatively high throughput at a reasonable cost and with high reliability. However it can be difficult both to separate objects from one another in such X-ray images and to determine the thickness of objects, as well as accurate material differentiation with multiple objects or clutter in the path of the beam. More modern Dual-Energy systems use multiple views or angles that provide two or more images of the object from different points of view which helps overcome some of the issues with single- view Dual-Energy X-ray systems.


This allows a backscatter image to highlight organic materials like concealed explosive devices. Typically two images (front and back) are taken of scanned individuals or objects. Early applications of this technology were for cargo and baggage screening. A more recent application is for screening passengers. While backscatter X-ray imaging can help display the size, shape and location of suspicious concealed objects, it provides a limited material differentiation ability (detection of anomalies and of objects with high nitrogen content) compared to other more recent X-ray based detection principles such as CT and X-ray Diffraction.


Computed Tomography (CT) is another X-ray technique that produces 2D images of cross-sectional “slices” through an object (e.g. a bag) at many angles and then combines these slices to obtain a 3D image. The source and detector (located on a gantry) both move and rotate around a scan circle in which the object is centred. Multiple sets of projection data are obtained at many angles which are then computer-processed and displayed as CT slices. Objects’ mass can be computed (measuring their volume and their average density) and objects of certain pre-defined masses can be automatically detected as explosives or other contraband materials. More advanced and recent CT technology uses a full volumetric or helical scanning approach (with multiple detector rows on the gantry), which can generate high- resolution 3D images of objects and bags while maintaining high throughput. Stationary gantry CT uses a number of fixed X-ray sources (or micro X-ray sources) with rapid electronic sequencing between these sources coupled with reconstruction techniques related to the geometry of sources and detectors. This technique mimics the mechanical rotation of a CT gantry but at higher rates with the advantage of less moving parts which can lead to better maintainability and higher availability. CT-based systems produce 3D X-ray images which can allow effective detection of hidden objects as well as explosive-like materials through accurate discrimination of such materials from most other innocuous objects (through the accurate determination of their mass and density). However, such CT-based Explosive Detection Systems (EDS) are typically fairly large, complex, and expensive. Most commonly used CT-based systems are for checked baggage screening.


A 3D CT image of a bag with highlighted detected threat


Enhancing X-ray Images of Current Systems There are several interesting technological and operational solutions being offered by various X-ray equipment manufacturers that can enhance current X-ray systems and can provide screeners with improved X-ray images and better image processing tools. Some of these solutions rely on sending such


A regular transmission X-ray image compared to a backscatter X-ray image.


December 2012 Aviationsecurityinternational


A Dual-Energy image of a bag compared to a backscatter image of the same bag


X-ray images to remote screening centres. For example On-Screen Resolution (OSR) conducted by TSA screeners in the U.S. is typically conducted in an OSR room that is remote from the location of the actual CT-based EDS. Sending such CT images to a remote location allows screeners to work in a better environment


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