X-ray screening can be used to aid in the detection of a variety of threats, including items such as explosives and narcotics. Conventional x-ray imaging provides limited means of distinguishing between different materials. Gratings-based phase contrast x-ray imaging provides images reflecting three distinct materials signatures: absorption, similar to conventional x-ray imaging; phase contrast, proportional to electron density variations; and dark field, which can be related to material texture. These additional signatures may enhance the ability to distinguish materials in x-ray images, leading to detection improvements. Here we discuss considerations for system design and improved detection: approach to measurements at relevant energies for security screening, signature studies of explosives and opioids as well as common benign materials, and development of a prototype screening system.
Gratings-based phase contrast x-ray imaging offers enhanced material information in an x-ray imaging measurement, a key consideration for improving performance in explosives detection. Application of phase contrast imaging to explosives detection requires addressing several key technical issues: identifying a patterning element (grating) that offers an appropriate tradeoff between sensitivity and robust operation at high energies, developing techniques that allow for quantitative interpretation of new signatures under a broad range of attenuation conditions, and designing a system that allows for rapid measurement while providing sufficient signal-to-noise. We present results illustrating the value of phase contrast x-ray signatures for explosives detection, and demonstrate the ability to obtain quantitative metrics in the presence of intervening materials. Finally, we demonstrate preliminary results from a gratings-based phase contrast system in a scanning configuration.
The Pacific Northwest National Laboratory (PNNL) has recently developed an active 3D microwave/millimeter-wave shoe scanner. This system is designed to detect threats concealed within the soles of common footwear. The system was designed in response to the security incident involving Richard Reid, known as the “Shoe Bomber”. The system operates over the 10-40 GHz frequency range. Waves in this band readily pass through common shoe materials, such as leather, rubber, plastics, foams, and synthetic and natural cloth materials. The shoe scanner system consists of a linear array positioned underneath a low loss dielectric window that the person is directed to stand upon. The linear array is positioned so the antenna propagation is vertical, and the array axis is horizontal across the width of the shoes. A linear mechanical scan translates the arrays along the length of the shoes. A frequency-modulated continuous wave (FM-CW) transceiver is used to collect the signal scattered from the scene. The data collected from the system is fully 3D covering two spatial and one frequency dimensions. The system presents several challenges for efficient image reconstruction, including the dielectric window, multi-row linear arrays, and focusing close to the antenna elements. The dielectric window presents a significant challenge for image reconstruction since the waves will travel through an inhomogeneous layered media. In this paper, an efficient back-projection reconstruction algorithm is presented that overcomes these challenges. Experimental imaging results are shown that demonstrate high-resolution imaging performance for this new scanner.
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