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Currently, energy-selective X-ray imaging is performed using monochromatic photons, and enhanced iodine (I) Kedge imaging (Mori et al., 1996; Hyodo et al., 1998) has been performed using extremely clean monochromatic parallel beams formed by silicon (Si) single crystals. The energies of monochromatic photons are approximately 35 keV beyond I-K-edge energy of 33.2 keV; these photons are absorbed effectively by I atoms. On the contrary, monochromatic imaging can also be performed using photon-counting energy-dispersive imaging, and several cadmium-telluride (CdTe) detector arrays (Feuerlein et al., 2008; Wang et al., 2011; Ogawa et al., 2012) have been developed to perform energy-dispersive (ED) K-edge imaging including computed tomography (CT) using I and gadolinium (Gd) contrast media. To perform fundamental studies on photon-counting X-ray imaging, we have developed several ED-CT scanners using CdTe and Si-PIN detectors. First, the photon energy range was determined using a multichannel analyzer (MCA) (Matsukiyo et al., 2011), and the photon count rate was measured using a counter. Second, the threshold energy was determined by a comparator, and the photon-count energy subtraction (Sato et al., 2012) was performed by a personalcomputer (PC) program using two sets of projection data, which were not obtained simultaneously. In the cases where a fairly available CdTe detector for measuring X-ray spectra is used, the energy resolution and the maximum count rate without pileups of event pulses were approximately 1% at 122 keV and 5 kilocounts per second (kcps), respectively. Therefore, we have developed a frequency-voltage converter (FVC) (Shimamura et al., 2014) to improve the image granulation of ED tomograms owing to low count rates of the CdTe detector. Subsequently, an energyAbstract Dual-energy photon counting was performed using an energy-selecting device (ESD) and a cadmium telluride detector. The ESD is used to determine a low-energy range for computed tomography (CT) and consists of two comparators and a microcomputer (MC). The two threshold energies are determined using low and high-energy comparators, respectively. The MC in the ESD produces a single logical pulse when only a logical pulse from the low-energy comparator is input to the MC. To determine the high-energy range, logical pulses from the highenergy comparator are input to the MC outside the ESD. Logical pulses from the two MCs are input to frequencyvoltage converters (FVCs) to convert count rates into voltages. The output voltages from the two FVCs are sent to a personal computer through an analog-digital converter to reconstruct tomograms. Dual-energy CT was accomplished at a tube voltage of 70 kV and a maximum count rate of 4.5 kilocounts per second, and twodifferent-energy tomograms were obtained simultaneously using iodine media.

Dual-energy X-ray computed tomography scanner using an energy-selecting device and a cadmium telluride detector