Development of a MPPC-based DOI-PET module with submillimeter 3-D resolution

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) cubic crystals measuring 3 × 3 × 3 mm³ in size, separated by a layer of air. When the light signals output from both ends are read with the MPPCs, the position of each crystal is clearly distinguished with a spatial uncertainty of 0.48 ± 0.03 mm. For 3-D measurements, we then fabricated three different type arrays: [A] 4 × 4 × 4 matrix of 3 × 3 × 3 mm³ pixels, [B] 5 × 5 × 5 matrix of 2 × 2 × 2 mm³ pixels, and [C] 10 × 10 × 10 matrix of 1 × 1 × 1 mm³ pixels, with each pixel divided by a BaSO₄ reflector in the 2-D direction and by a layer of air in the DOI direction. We demonstrated that the 3-D position of each Ce:GAGG pixel was clearly distinguished when illuminated by 662 keV gamma rays uniformly. Average energy resolutions of 9.8 ± 0.8 %,9.8 ± 0.9 %, and 13.2 ± 1.7 % were obtained for types A, B and C, respectively. These results suggest that our proposed method is simple and offers promise in achieving 1 mm 3-D spatial resolution for future medical imaging, partic