Optimization of energy windows to calculate quantitative X-ray images using an energy-resolving photon-counting detector: A simulation study

Purpose: Energy-resolving photon-counting detectors (ERPCDs) are expected to enable novel functional diagnosis using quantitative X-ray images such as effective atomic number (Z_eff) images. In this simulation study, we sought to optimize the settings related to tube voltage and energy windows. Methods: We assumed a virtual phantom composed of polymethylmethacrylate (PMMA, Z_eff = 6.5) and aluminum (Z_eff = 13.0), and simulated the conventional energy integrating detector (EID) image and Z_eff images obtained by the ERPCD. The investigational phantom is composed of elements with ρt ranging from 0.1 to 80 g/cm². In order to perform optimization using a quantitative index, we defined a system performance function (SPF) that takes into account the contributions of the contrast to noise ratio (CNR) of the EID image and the uncertainty (δZ_eff) of the Z_eff image. The tube voltage was varied to be 60, 90, and 120 kV, and the variable that determines the separation energy between the middle and high energy windows was changed from 35 to 115 keV with 5 keV interval. In addition, preclinical images of the digital phantoms based on computed tomography (CT) images were created for demonstration. Results: We were able to determine the imaging conditions that yielded the better image quality for tube voltages of 60, 90, and 120 kV. Among these, the 120 kV condition (20-30-50-120 keV) showed the smallest SPF value, and was therefore adopted as the optimal condition. Furthermore, high-quality EID and Z_eff images of preclinical phantoms were obtained under the optimal condition. Conclusions: We determined the optimal condition suitable for deriving both qualitative and quantitative images using ERPCDs.