Experimental optimization of single‐exposure dual‐energy angiography with photon‐counting x‐ray detectors

Background Photon‐counting x‐ray detectors may enable single‐exposure dual‐energy (DE) x‐ray angiography. Purpose The purpose of this paper is to experimentally optimize the energy thresholds and tube voltage for single‐exposure DE x‐ray angiography. Methods We optimized single‐exposure DE x‐ray angiography using the iodine signal‐difference‐to‐noise ratio (SDNR) per root patient air kerma (κ) as a figure of merit. We measured the iodine SDNR by imaging an iodine stepwedge immersed in a water tank with a depth of 30 cm in the direction of x‐ray propagation. The stepwedge was imaged using tube voltages ranging from 90 to 150 kV and a cadmium telluride (CdTe) x‐ray detector with two energy bins and analog charge summing for charge sharing suppression. The energy threshold that separates the two energy bins was varied from approximately 35 keV to approximately 75% of the maximum energy of the x‐ray beam. Curve fitting was used to determine the threshold that maximized SDNR/κ$\mathrm{SDNR}/\sqrt {\kappa }$. The effect of scatter was determined from measurements of the scatter‐to‐primary ratios (SPRs) of the low‐energy and high‐energy images and a semi‐empirical model of the relationship between SDNR and SPR. Using the optimal parameters, we imaged a phantom with vessel‐simulating structures and background clutter. Results The optimal energy thresholds increased monotonically from ∼50 to ∼85 keV over the range of tube voltages considered. For tube voltages greater than 90 kV, the optimal energy thresholds consistently allocated approximately two thirds of all detected primary photons to the low energy bin; this ratio was preserved without scatter. Consistent with prior modeling studies, SDNR/κ$\mathrm{SDNR}/\sqrt {\kappa }$ increased monotonically with tube voltage from 90 to 150 kV; SDNR/κ$\mathrm{SDNR}/\sqrt {\kappa }$ at 150 kV was approximately 38% higher than that at 90 kV for an iodine area density of ∼50 mg/cm2. Scatter reduced SDNR by approximately 25% for SPRs of ∼1 and 0.4 in low‐energy and high‐energy images, respectively. Conclusions Achieving optimal image quality in single‐exposure DE angiography with photon‐counting x‐ray detectors will require high tube voltages (i.e., >130 kV) and, for thick patients, energy thresholds that allocate approximately two thirds of all primary photons to the low‐energy image. Future work will compare the image quality of singe‐exposure photon‐counting and kV‐switching approaches to DE x‐ray angiography.