Zero-dark-counting X-ray photon detection using a YAP(Ce)–MPPC detector and its application to computed tomography using gadolinium contrast media

To measure X-ray spectra and to perform photon-counting computed tomography (PC-CT) with high count rates, we developed a zero-dark-counting spectrometer using a short-decay-time scintillator. A method exploiting a YAP(Ce) [cerium-doped yttrium aluminum perovskite] single crystal scintillator with a decay time of 30ns and an MPPC (multipixel photon counter) has been developed to count X-ray photons. The photocurrent from the MPPC was amplified by a high-speed current–voltage amplifier, and the event pulse was sent to a multichannel analyzer (MCA) to measure X-ray spectra. The MPPC was driven under pre-Geiger mode at a bias voltage of the MPPC of 70.7V and a temperature of 23°C. The PC-CT was accomplished by repeated linear scans and rotations of an object, and projection curves of the object were obtained by the linear scan at a tube current of 1.0mA. The exposure time for obtaining a tomogram was 10min at a scan step of 0.5mm and a rotation step of 1.0°. At a tube voltage of 100kV, the maximum count rate was 200kcps. In the PC-CT using gadolinium media, we observed image-contrast variations with changes in lower-level discrimination voltage of the event pulse using a comparator.