Detection with polychromatic x-ray pencil beam illumination: information-theoretic bounds.

Non-destructive testing (NDT) by x-ray imaging is commonly used for finding manufacturing defects, cargo inspection, or security screening. These tasks can be regarded as examples of a detection problem where a target is either present or not. Task-specific information (TSI) [J. Opt. Soc. Am. A24, B25 (2007)JOAOD60740-323210.1364/JOSAA.24.000B25; Appl. Opt.47, 4457 (2008)APOPAI0003-693510.1364/AO.47.004457] bounds, an information-theoretic based metric, are presented for a threat detection task. A system using polychromatic x-ray pencil beam object illumination and energy-resolving detectors for both absorption and diffraction measurements is employed for this task. Water and diesel are two liquids chosen as non-threat and threat materials, respectively, for this study. Three different threat class configurations are examined: a homogeneous object with fixed thickness, a homogeneous object with stochastic thickness, and a dual-material object (i.e., representing a target and clutter) with stochastic thickness, where the threat material has a fixed thickness. We find for the threat class composed of a dual-material object that a minimum threat thickness of 4.5 cm is needed to achieve a desired TSI≥0.7 using a joint absorption and diffraction measurement.