Performance evaluation of four-layer DOI detectors using multi-pixel photon counter arrays

Multi-pixel photon counters (MPPC) have many advantages for PET detectors, such as high internal gain, high photon detection efficiency, lower power supply and insensitivity to magnetic fields. However, the saturation effect is unavoidable phenomenon for PET detectors using MPPCs which need to collect large number of scintillation photons to achieve high energy and timing resolutions. In a lot of cases, pulse height of PET detectors using MPPCs are saturated in high energy region. On the other hand, since our DOI encoding method previously presented is sharing scintillation photons to multiple pixels, the saturation effect can be suppressed. We constructed prototype four-layer detectors using two MPPC arrays and evaluated their detector performances. We constructed the prototype detector consisting of four layers of a 6 × 6 array of Lu_2(1-x)Y_2x SiO₅ (LYSO) (Lu: 98%, Y: 2%) crystals (Proteus Inc., U. S. A.), and a MPPC array of 4 × 4 pixels (S11064 series, Hamamatsu Photonics K.K.). The size of each crystal element was 1.46 mm × 1.46 mm × 4.5 mm and all surfaces of crystal elements were chemically etched. For comparison, the two types of MPPC, S11064-050P (3600 cells) and S11064-025P (14400 cells), were used in the experiment. As a result, the all of the crystals of the detector using 050P were clearly identified while the crystal identification performance of the detector using 025P was degraded due to lower photon detection efficiency. The energy resolution of 11.0-13.6% (050P) and 12.7-15.2% (025P) were achieved. No saturations were observed for both detectors under the 511keV energy region.