Impact of spectral separation on robustness to scatter errors in quantitative dual-energy radiography

Scatter reduces the quantitative accuracy of dual-energy (DE) material estimates. However, its impact likely depends on the condition number of the material decomposition problem, which in turn depends on energy channel separation of the DE system. We investigate this tradeoff for various realistic DE radiography configurations. The study involved high-fidelity Monte Carlo (MC) simulations of multi-material abdomen phantoms presenting a range of vertebral bone mineral (CaHA) concentrations. Polyenergetic DE radiographs were obtained in a standard PA contact-scan setting, with a source-to-detector distance of 180.3cm, beam collimation of 18 x 43cm (measured on the detector), and a Pb/Al anti-scatter grid (13:1 ratio, 92 lines/cm). To investigate the impact of spectral separation, four DE configurations at matched doses (~0.1mGy each) were investigated: (i) a single-exposure (120kV) protocol with a multi-layer detector consisting of three 0.2mm CsI layers, with the top providing low-energy (LE) data and bottom providing high-energy (HE) data; (ii)-(iv) three kV-switching protocols with (ii) 100kV LE and 140 kV (+0.08mm Ag) HE, resulting in effective energies matching the multi-layer spectral channels, (iii) 60kV LE and 120kV HE, and iv) 60kV LE and HE of 140kV (+0.08mm Ag). Moving-average denoising followed by projection-domain decomposition were performed to obtain areal distributions of water and CaHA pathlengths. Scatter correction with a tunable residual error was emulated by subtracting a fraction (ranging 0 to 200%) of the known simulated scatter from the total measured projections. Accuracy of the vertebral areal Bone Mineral Density (aBMD) estimates were compared for the different DE protocols and scatter estimation errors. Despite the presence of anti-scatter grid, the scatter-to-primary ratio (SPR) was >90% for both LE and HE data in all DE protocols. DE settings with relatively large LE-HE spectral overlap (multi-layer and 100/140 kV) appeared to require more accurate scatter correction than settings with increased spectral separation – e.g., to achieve <0.2g/cm² aBMD bias (compared to aBMD of ~1.3g/cm² in healthy bone and ~0.5g/cm² in osteoporotic bone), LE and HE scatter errors needed to be <10% for the multi-layer and 100/140 kV protocols, but a 30% error in LE scatter and 15% in HE scatter could be tolerated for the 60/120 and 60/140kV protocols. Interestingly, accurate aBMD could be obtained for certain ratios of LE-to-HE scatter errors even if the magnitude of the residual uncorrected scatter was high. However, the aBMD bias increased rapidly to >1g/cm² outside of a narrow band of such protocol-specific error ratios. Within the DE configurations tested in this study, quantitative aBMD estimates obtained using protocols with larger LE-HE spectral separation were generally more robust to scatter correction errors.