250 kcps photon-counting X-ray CT system using a YAP (Ce)-MPPC detector and a high-speed inverse comparator and its application to iodine imaging

Abstract

A photon-counting X-ray computed tomography (CT) system was developed to perform fundamental study on high-speed photon-energy dispersion utilizing scintillation counting. X-ray photons are detected using a YAP (Ce) [cerium-doped yttrium aluminum perovskite] single crystal scintillator with a decay time of 30 ns and an MPPC (multipixel photon counter) module. The photocurrent from the MPPC was amplified by a current-voltage amplifier and an integrator, and the event pulse is sent to a high-speed inverse comparator. Logical pulses are then produced by the comparator and are counted by a counter card. Tomography is accomplished by repeated linear scans and rotations of an object, and projection curves of the object are obtained by the linear scan. The image contrast of iodine media decreased with increase in lower-level voltage (V_l) of the comparator. The count rate decreased with increase in V_l, and the maximum count rate was 250 kcps at a V_l of 0.15 V, a tube current of 1.0 mA, and a tube voltage of 70 kV. The exposure time for obtaining a tomogram was 10 min at a scan step of 0.5 mm and a rotation step of 1.0°.