Optimization for spectral modulator in fast-KV-switching dual-energy CBCT imaging

Fast kV-switching (FKS) is one of the dual-energy (DE) techniques that has successfully been employed in the commercial computed tomography (CT) systems. However, the FKS in cone-beam CT (CBCT) spectral imaging still faces challenges such as scatter contamination and relatively moderate energy separation, which results in suboptimal quantitative performance. To improve the performance of FKS CBCT spectral imaging, we investigated the feasibility of adopting spectral modulator (SM) in FKS CBCT spectral imaging. In this work, we utilized the Cram´er-Rao lower bound (CRLB) method to establish parameter optimization principle for FKS dual-energy CBCT with a spectral modulator, and quantitatively analyzed the impact of the different spectral modulators including materials, thickness and spatial frequencies on the FKS spectral imaging performance. Both the numerical simulation and physics experiments were conducted to validate the improvement of the spectral modulator for FKS CBCT spectral imaging. In the parameters optimization analysis, we found that each metallic material has an optimal thickness for each basis material image under the 80/140 kVp pair scan. And the performance of FKS spectral imaging improves as the spectral modulator become denser but with greater challenges to power capacity of the system. The copper (Cu) with thickness of 0.2mm and molybdenum (Mo) with thickness of 0.07mm were selected as the parameter of the spectral modulator in physics experiments based the CRLB analysis. In physics experiments, the multi-energy phantom was scanned with 80/120kVp pairs. The CNR of water density image for multi-energy phantom in fan-beam scan are boosted by an average improvement of 20.5% and 22.5% f or Mo modulator and Cu modulator, respectively. The CNR of iodine density image for multi-energy phantom in fan-beam scan are boosted by an average improvement of 25.3% and 30.8% for Mo modulator and Cu modulator, respectively. The parameters optimization principle for FKS CBCT spectral imaging with a spectral modulator was established. The feasibility of the spectral modulator to improve FKS CBCT spectral imaging performance was validated.