Improved visualization of plaque in cardiac CT angiography by optimal energy weighting with photon counting detector: A simulation study

For the detecting plaque buildup and calcification in the coronary artery, the cardiac computed tomography (CT) angiography is widely used in clinic. However, the soft and calcified plaque is hard to distinguish from contrast enhanced blood vessel with conventional CT. The photon counting detector has an advantage over the conventional detector by detecting energy information of each photon. The application of optimal energy weighting factor for each energy window based on energy information of each photon can be improve the identification of plaque. The purpose of this study is to investigate, with simulation, the ability to distinguish plaque from contrast enhanced vessel using optimal energy weighting factor. The photon counting imaging system consists of a micro focus x-ray source and CdTe sensor was simulated using the Geant4 Application for Tomographic Emission (GATE) V6. The source to phantom rotation distance and phantom rotation to detector distance were set as 600 and 400 mm, respectively. The energy spectra were calculated at 80 kVp with 1 mm aluminum filter using Spekcalc program and used as source. A phantom consisted of iodine and hydroxyapatite (HA) which simulates contrasts enhanced blood vessel and plaque, respectively. The projection based energy weighting factors at each energy window were calculated and applied. The identification of HA with respect to iodine was evaluated using signal-difference-to noise ratio (SDNR). The SDNR improvements, as compared to image from charge integrating detector were 1.07 and 1.42 for photon counting and energy weighted image at 50 mg/ml of HA, respectively. The SDNR were improved from 1.43 to 21.88 with optimal energy weighting for each concentration. The results showed that the plaque could be distinguished from contrast enhanced blood vessel with optimal energy weighting. However, the result can be depends on the concentration of background contrast media, therefore additional simulation required.