Enhanced colour encoding of materials discrimination information for multiple view dual-energy X-ray imaging

• Main Author: Wang, Xun
• Published: Nottingham Trent University 2009
• Subjects: 539.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.510262

This thesis reports an investigation into dual-energy X-ray discrimination techniques. These techniques are designed to provide colour-coded materials discrimination information in a sequence of perspective images exhibiting sequential parallax. The methods developed are combined with a novel 3D imaging technique employing depth from motion or kinetic depth effect (KDE). This technique when applied to X-ray images is termed KDEX imaging and was developed previously by the university team for luggage screening applications at security checkpoints. A primary motivation for this research is that the dual-energy X-ray techniques, which are routinely incorporated into ‘standard’ 2D luggage scanners, provide relatively crude materials discrimination information. In this work it was critical that robust materials discrimination and colour encoding process was implemented as the sequential parallax exhibited by the KDEX imagery may introduce colour changes, due to the different X-ray beam paths associated with each perspective image. Any introduction of ‘colour noise’ into the resultant image sequences could affect the perception of depth and hinder the ongoing assessment of the potential utility of the dual-energy KDEX technique. Two dual-energy discrimination methods have been developed, termed K-II and W-E respectively. Employing the total amount of attenuation measured at each energy level and the weight fraction of layered structures, a combination of the K-II and the W-E techniques enables the computation and extraction of a target objects’ effective atomic number (Zeff) and its surface density (ρS) in the presence of masking layers. These material parameters (Zeff and ρS) together with laminographic layer thickness estimation enable mass density extraction. A series of experiments investigated the computation of Zeff and ρS as a function of system noise and repeatability. The estimation of thickness depends on the depth increment provided by the image capture geometry and the laminographic processing. Within the atomic number range of 6 to 30 and with up to 4 masking layers, the investigated techniques produce an accuracy of Zeff and ρS up to 97% and 95% respectively. The thickness estimation technique provided a relatively high accuracy for object thickness’ greater than 4cm. Although, the measurement accuracy for relatively thin layers is inherently limited by the minimum resolvable depth increment of the image collection geometry. The mass density extracted had an overall accuracy of greater than 90% for well-estimated thicknesses. Implementing a new four colours scheme highlights the presence of potential threat materials in the resultant KDEX imagery. This thesis forms part of a larger programme of activity in collaboration with and funded by the UK Home Office Scientific Development Branch, and the US Department of Homeland Security and the EPSRC.