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Determining the x-ray properties of materials with dual-energy computed tomography and 2D calibration surfaces

Dual-energy computed tomography (DECT) is widely used to identify explosives and characterize substances relevant to transportation security screening. The x-ray attenuating behavior of a material can generally be understood in terms of its electron density (ρe) and effective atomic number (Ze). These properties can be estimated from DECT data if the studied material is accompanied by several reference materials of known density and composition. Traditionally, the reference material ρe and Ze values have been used to generate calibration curves with dependencies on the high-energy computed tomography (CT) values and the ratio of low- to high-energy CT values, respectively. However, the accuracy of these 1- D curves, which treat ρe and Ze as functions of a single variable, breaks down when attempting to characterize materials with high effective atomic number. A more robust calibration technique was developed that instead generates 2-D surfaces from the reference material data. These surfaces treat ρe and Ze as functions of both the high- and low-energy CT values, which better reflects the expected behavior for these properties. Several test materials were packed into bottles and scanned in a commercial explosives detection system along with different sets of reference materials. The results have demonstrated that the developed calibration surfaces generally yield more accurate estimations for test material properties than the traditional approach, particularly for high-Z materials. This work was conducted with the U.S. Department of Homeland Security (DHS) Science and Technology Directorate (S&T) under contract 70RSAT19FR0000016. Any opinions contained herein are those of the author and do not necessarily reflect those of DHS S&T.