100 μA-100 kV Photon-counting X-ray Computed Tomography System Using an LSO-MPPC Detector and a High-Speed Comparator...

A high-sensitive x-ray computed tomography (CT) system is useful for decreasing absorbed dose for patients, and we performed preliminary experiments for first-generation photon-counting CT using a high-sensitive single detector. X-ray photons are detected using an LSO [Lu₂(SiO₄)O] single crystal scintillator and a multipixel photon counter (MPPC). The photocurrent from the MPPC is amplified by a current-voltage amplifier and an integrator, and the event pulse is sent to a high-speed 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 count rate decreased with increase in lower level voltage of the comparator V_l, and the maximum count rate was 265 kcps at a V_l of 0.4 V. The exposure time for obtaining a tomogram was 10 minutes at a scan step of 0.5 mm and a rotation step of 1.0°. The image contrast of gadolinium medium slightly varied with change in V_l. We carried out low-dose-rate photon-counting CT at a tube current of 100 μA and a tube voltage of 100 kV. The energy-dispersive effect of the CT image was confirmed by selecting V_l. The absorbed dose for objects can be reduced using the linear detector consisting of plural LSO-MPPC detectors.