In recent years, silicon photomultiplier (SiPM) is replacing the photomultiplier tube (PMT) in positron emission tomography (PET) systems due to its superior properties, such as fast single-photon timing response, small gap between adjacent photosensitive pixels in the array, and insensitivity to magnetic fields. One of the technical challenges when developing SiPM-based PET systems or other position-sensitive radiation detectors is the large number of output channels coming from the SiPM array. Therefore, various signal multiplexing methods have been proposed to reduce the number of output channels and the load on the subsequent data acquisition (DAQ) system. However, the large PN-junction capacitance and quenching resistance of the SiPM yield undesirable resistance–capacitance delay when multiple SiPMs are combined, which subsequently causes the accumulation of dark counts and signal fluctuation of SiPMs. Therefore, without proper SiPM signal handling and processing, the SiPMs may yield worse timing characteristics than the PMTs. This article reviews the evolution of signal readout and multiplexing methods for the SiPM. In this review, we focus primarily on analog electronics for SiPM signal multiplexing, which allows for the reduction of DAQ channels required for the SiPM-based position-sensitive detectors used in PET and other radiation detector systems. Although the applications of most technologies described in the article are not limited to PET systems, the review highlights efforts to improve the physical performance (e.g. spatial, energy, and timing resolutions) of PET detectors and systems.

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The multiple thresholds used in multi-voltage threshold (MVT) method provide more detailed information about the pulse amplitude of the input analog pulses than the simple time-over-threshold (TOT) method, thus allowing for better energy estimation and pulse reconstruction capabilities. However, as the number of thresholds increases, the number of comparators and digital signal readout channels required for MVT also increases. This requirement owing to the increased number of thresholds is the main disadvantage of MVT implementation using field-programmable gate array (FPGA)-based time-to-digital converters (TDCs) because the FPGA resources required for TDC implementation are substantial and FPGAs have a limited number of input/output ports. Therefore, we propose a new single-line MVT method to improve the integrity of the FPGA-only data acquisition system without analog-to-digital converters by reducing the FPGA input channels required for the MVT method. The proposed method, which applies three different levels of thresholding, reduces the digital output signal line in the MVT by employing a 3-input XOR logic gate. The XOR gate integrates the output signals from the comparators and generates 1-bit line digital pulse train. We evaluated the energy performance of the proposed single-line MVT method using three different energy estimators. The energy estimates were compared with the ground truth energy calculated using domino-ring-sampler 4 (DRS4)-sampled analog pulses. The proposed method showed virtually equivalent energy resolution to that of DRS4-based pulse digitization method and better energy linearity than the conventional TOT method. Among the energy estimation methods used in single-line MVT, the crossing-point triangular sum method showed the best energy linearity. The proposed single-line MVT method will be useful when data acquisition systems without ADCs are implemented using FPGA-based TDCs. This is because the proposed method alleviates the problem of limited input ports and the numerous resources required for TDCs in FPGAs.