Delay grid multiplexing: simple time-based multiplexing and readout method for silicon photomultipliers

Abstract

In this paper, we propose a fully time-based multiplexing and readout method that uses the principle of the global positioning system. Time-based multiplexing allows simplifying the multiplexing circuits where the only innate traces that connect the signal pins of the silicon photomultiplier (SiPM) channels to the readout channels are used as the multiplexing circuit. Every SiPM channel is connected to the delay grid that consists of the traces on a printed circuit board, and the inherent transit times from each SiPM channel to the readout channels encode the position information uniquely. Thus, the position of each SiPM can be identified using the time difference of arrival (TDOA) measurements. The proposed multiplexing can also allow simplification of the readout circuit using the time-to-digital converter (TDC) implemented in a field-programmable gate array (FPGA), where the time-over-threshold (ToT) is used to extract the energy information after multiplexing. In order to verify the proposed multiplexing method, we built a positron emission tomography (PET) detector that consisted of an array of 4 × 4 LGSO crystals, each with a dimension of 3 × 3 × 20 mm, and one- to-one coupled SiPM channels. We first employed the waveform sampler as an initial study, and then replaced the waveform sampler with an FPGA-TDC to further simplify the readout circuits. The 16 crystals were clearly resolved using only the time information obtained from the four readout channels. The coincidence resolving times (CRTs) were 382 and 406 ps FWHM when using the waveform sampler and the FPGA-TDC, respectively. The proposed simple multiplexing and readout methods can be useful for time-of-flight (TOF) PET scanners.

Physics and technology of time-of-flight PET detectors

“…Compact and affordable digitizers with sampling rates >1 GHz exist that facilitate multichannel implementation of this approach (Cho et al 2011, Ronzhin et al 2013, Ashmanskas et al 2014, Park et al 2018. Time-multiplexing can be used to reduce costs, but leads to an increased DCR on the multiplexed channel (Grant and Levin 2015, Kim et al 2016, Bieniosek et al 2016a, Won et al 2016a. At the time of writing, waveform sampling of individual events is mostly used in research setups.…”

Analog and digital signal processing method using multi‐time‐over‐threshold and FPGA for PET

“…However, the resulting performance of TOT suffers from a distortion in the energy spectrum degrading energy resolution because of nonlinear input energy to output pulse width conversion. [7][8][9][10][11][12][13][14][15][16][17] MVT is also a digital sampling method, analogous to TOT, to generate samples of an event waveform of PET but employs multiple thresholds to extract energy and time information of a PET event. This method showed that implementing TOT with multiple triggering thresholds could overcome the problems inherent to nonlinear performance of TOT.…”

“…Therefore, compared with other digital sampling methods, such as DRS and TOT, MVT can provide high performance because it has shorter conversion time for storing amplitudes than DRS achieving high count rate and can obtain more samples per detector output pulse than TOT method improving nonlinearity of energy resolution. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] For design of PET systems using digital sampling methods, such as TOT, DRS, and MVT, a digital DAQ system usually has been employed. These systems consist of an analog-to-digital converter (ADC), time-to-digital converter (TDC), and field-programmable-gate-array (FPGA) for extracting energy and timing information for a detected event.…”

“…TOT is a digital sampling method that calculates time and energy information by measuring arrival time and pulse duration over the user‐defined threshold. However, the resulting performance of TOT suffers from a distortion in the energy spectrum degrading energy resolution because of nonlinear input energy to output pulse width conversion . MVT is also a digital sampling method, analogous to TOT, to generate samples of an event waveform of PET but employs multiple thresholds to extract energy and time information of a PET event.…”

“…This method showed that implementing TOT with multiple triggering thresholds could overcome the problems inherent to nonlinear performance of TOT. Therefore, compared with other digital sampling methods, such as DRS and TOT, MVT can provide high performance because it has shorter conversion time for storing amplitudes than DRS achieving high count rate and can obtain more samples per detector output pulse than TOT method improving nonlinearity of energy resolution …”