Development of a Dual-Sided Readout DOI-PET Module Using Large-Area Monolithic MPPC-Arrays

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 approximately 10 μm-thick. When the light signals output from both ends are read with the 3×3 mm² MPPCs, the position of each crystal is clearly distinguished. The same measurements were also made using Ce-doped (Lu,Y)₂(SiO₄)O (Ce:LYSO), achieving a similarly good separation. We then fabricated thin Ce:GAGG 2-D crystal arrays consisting of two types: [A] 4 × 4 matrix of 3×3×3 mm³ pixels, and [B] 10 × 10 matrix of 0.8×0.8×5 mm³ pixels, with each pixel divided by a BaSO₄ reflector 0.2 mm-thick. Then four arrays are laid on top of each other facing the DOI direction through a layer of air 10 μm-thick. We demonstrated that the 3-D position of each Ce:GAGG pixel is clearly distinguished in both the 2-D and DOI directions for type A and B when illuminated by 662 keV gamma rays. Average energy resolutions of 9.8 ± 0.8% and 11.8 ± 1.3% were obtained for types A and B, respectively. These results suggest that our proposed method is simple and offers promise in achieving both excellent spatial and energy resolutions for future medical imaging, parti- ularly in positron emission tomography (PET).