Detection of improvised explosive devices at long-range using coded aperture imaging of backscattered X-rays with dynamic reconstruction

Abstract

Standoff detection of improvised explosive devices (IEDs) is a continuing problem for the U.S. military. Current X-ray detection systems cannot detect explosives at distances above a few meters and with a source-detector system moving in relation to the target. The aim of this study is to determine the feasibility of a large-area, Coded-Aperture Imaging (CAI) system using X-Ray backscatter as the source of radiation. A moving source-detector system required development of a new reconstruction technique, dynamic reconstruction (DR), which continually back-projects detected events on an event-by-event basis. This research imaged multiple low-Z (polyethylene and water-filled), area targets with backscattered X-rays using standard medical imaging equipment, coded aperture masks with ideal bi-level autocorrelation properties, and dynamic reconstruction (DR). Lower fill factor apertures were the primary metric investigated because contrast was shown to be inversely related to the mask's percentage of open area. This study experimentally determined the optimal mask fill factor, gamma camera imaging protocols, and experimental geometry by examining the resulting effects on image quality. Reconstructed images were analyzed for Contrast-to-noise ratio (CNR), Signal-to-noise Ratio (SNR), resolution, sharpness, the uniformity of the background (artifacts). In addition to changing the fill factor, additional methods of improving the contrast included changing the experimental geometry, reducing the X-ray tube filtration, and widening the X-ray source's cone beam (FOV).

(cont.) 14 studies were performed that found 25% fill factor mask reconstructions had the highest average CNR (14.7), compared to 50% and 12.5% fill factor (CNRs 8.50 and 6.9, respectively) with a system resolution of 25 mm at the target. Thus, this study's techniques confirmed that large-area, low fill factor coded apertures could successfully be used, in conjunction with dynamic reconstruction, to image complex, extended scenes at 5 meters with capabilities of up to 50 meters or more.