GAGG–MPPC detector with optimized light guide thickness for combined Compton-PET applications

Silicon photomultipliers (SiPMs) have become a standard photodetector to be coupled with scintillators in PET, but the SiPM saturation is limiting the performance achievable with the detectors employed, in particular the high energy resolution which is necessary for whole gamma imaging (WGI). The concept of WGI combines PET and a Compton camera by inserting a scatterer detector ring into a PET ring. Not only typical SPECT radionuclides such as ^99mTc (140 keV), but also unusual positron emitters such as ⁸⁹Zr (909 keV) and ⁴⁴Sc (1157 keV) can be imaging targets. For better spatial resolution in Compton imaging, the scatterer detector requires better energy resolution for a wide range of deposited energies. The use of bright scintillators such as GAGG is essential, but the SiPM saturation may prevent full use being made for such bright scintillators. We expected that inserting a thick light guide between GAGG and SiPM could spread scintillation photons to surrounding SiPMs and eliminate the saturation effect. On the other hand, the thicker the light guide becomes, the greater the number of scintillation photons that may be absorbed. Therefore, in this paper, we investigated the relationship between the light guide thickness and the energy resolution. A 22 × 22 array of GAGG crystals (0.9 ×0.9× 6 mm³ each) was optically coupled to the 8 x 8 multi-pixel photon-counter (MPPC) array (3 × 3 mm² pixel, 50 ×50μm² sub-pixel) via a light guide, for which thickness was changed from 0 (i.e., without the light guide) to 8 mm. Using point sources with different energies (¹³³Ba, ²²Na and ¹³⁷Cs), we compared crystal identification performance, linearity of the output signal and energy resolution. Increasing the light guide thickness gradually degraded crystal identification performance but improved linearity of the output signals. Energy resolution at 81 keV constantly deteriorated with increasing light guide thickness. Energy resolutions at 356, 511 and 662 keV were improved with increasing light guide thickness to a certain value after which they deteriorated; the thicknesses at which deterioration started were 2.0 mm, 3.0 mm and 4.0 mm, respectively, for the energy resolutions at 356, 511 and 662 keV. We found that the optimum light guide thickness for the target energy range was 2.0 mm, and for this thickness, energy resolution values were 22.0% at 81 keV, 7.6% at 356 keV, 8.3% at 511 keV and 8.2% at 662 keV.