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 ${}^{\mathrm{99m}}$Tc (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 $\times$ 22 array of GAGG crystals (0.9 $\times 0.9\times$ 6 mm³ each) was optically coupled to the 8 x 8 multi-pixel photon-counter (MPPC) array (3 $\times$ 3 mm² pixel, 50 $\times$ $50\mathrm{\mu }\mathrm{m}$² 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.

硅光电倍增管（SiPM）已成为与PET中的闪烁体耦合的标准光电检测器，但是SiPM饱和度限制了所用检测器可实现的性能，特别是整个伽马成像（WGI）所需的高能量分辨率。WGI的概念是将散射检测器环插入PET环中，从而将PET和Compton摄像机结合在一起。不仅典型的SPECT放射性核素例如${}^{\mathrm{99m}}$Tc（140 keV），但也有不寻常的正电子发射器，例如⁸⁹ Zr（909 keV）和⁴⁴ ZrSc（1157 keV）可以成为成像目标。为了在康普顿成像中获得更好的空间分辨率，散射探测器需要针对较大范围的沉积能量具有更好的能量分辨率。必须使用诸如GAGG之类的明亮闪烁体，但是SiPM饱和会阻止充分使用此类明亮闪烁体。我们期望在GAGG和SiPM之间插入厚的光导可以将闪烁光子散布到周围的SiPM并消除饱和效应。另一方面，光导越厚，可以吸收的闪烁光子的数量就越大。因此，在本文中，我们研究了光导厚度与能量分辨率之间的关系。22岁$\times$ 22阵列的GAGG晶体（0.9 $\times 0。9\times$每个6 mm ³）光耦合到8 x 8多像素光子计数器（MPPC）阵列（3$\times$3 mm ²像素，50$\times$ $50\mathrm{\mu }\mathrm{米}$²个子像素）通过一个光导，其厚度从0（即没有光导）更改为8 mm。使用不同能量的点源（¹³³ Ba，²² Na和¹³⁷CS），我们比较了晶体识别性能，输出信号的线性度和能量分辨率。增加光导厚度会逐渐降低晶体识别性能，但会改善输出信号的线性度。随着光导厚度的增加，在81 keV时的能量分辨率不断降低。随着光导厚度增加到一定值，在356、511和662 keV处的能量分辨率提高了，此后它们恶化了。对于356、511和662 keV的能量分辨率，开始劣化的厚度分别为2.0 mm，3.0 mm和4.0 mm。我们发现，目标能量范围的最佳光导厚度为2.0 mm，对于该厚度，能量分辨率值为81 keV时为22.0％，356 keV时为7.6％，511 keV时为8.3％和662 keV时为8.2％。