High position resolution gamma-ray imagers consisting of a monolithic MPPC array with submillimeter pixelized scintillator crystals

We report on the development of two versatile, high spatial resolution gamma-ray imagers for medical imaging. One is a compact gamma-ray camera, the other is a tweezers type coincidence imaging system. These applications consisting of a large-area monolithic Multi-Pixel Photon Counter (MPPC) and submillimeter pixelized scintillator matrices. The MPPC array has 4×4 channels with a three-side buttable, very compact package. Each channel has a photosensitive area of 3×3 mm² and 3600 Geiger mode avalanche photodiodes (APD). For a typical operational gain of 7.5×10⁵ at + 20 degrees, gain fluctuation over the entire MPPC device is only ±5.6%, and dark count rates (as measured at the 1 p.e. level) amount to ≤400 kcps per channel. We particularly selected Ce-doped (Lu,Y) ₂(SiO₄)O (Ce:LYSO) and a brand-new scintillator, Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) due to their high light yield and density. To improve the spatial resolution, these scintillators were fabricated to 22×22 or 15×15 matrices of 0.5×0.5 mm² pixels. These scintillator matrices were coupled to the MPPC array with an acrylic light guide with 1 mm thick, and signals were read out using the charge division resistor network, which compiles signals into four position-encoded analog outputs. The spatial resolution of 1.2 mm was achieved with the compact gamma-ray camera using collimated ⁵⁷Co source, and a radiography image of a bearing was successfully obtained. On the other hand, the spatial resolution of 1.1 mm was achieved with the coincidence imaging system using a ²²Na source. Furthermore the experimental measurements for a PET scanner was performed, and the spatial resolution of 0.91 mm was achieved. These results suggest that the gamma-ray imagers has excellent potential for their uses as a high spatial medical imaging, and also be promising for positron emission tomography (PET).