Scalable electronic readout design for a 100 ps coincidence time resolution TOF-PET system

Abstract

We have developed a scalable detector readout design for a 100 ps coincidence time resolution (CTR) time of flight (TOF) positron emission tomography (PET) detector technology. The basic scintillation detectors studied in this paper are based on 2 × 4 arrays of 3 × 3 × 10 mm³'fast-LGSO:Ce' scintillation crystals side-coupled to 6 × 4 arrays of 3 × 3 mm²silicon photomultipliers (SiPMs). We employed a novel mixed-signal front-end electronic configuration and a low timing jitter Field Programming Gate Array-based time to digital converter for data acquisition. Using a²²Na point source, >10 000 coincidence events were experimentally acquired for several SiPM bias voltages, leading edge time-pickoff thresholds, and timing channels. CTR of 102.03 ± 1.9 ps full-width-at-half-maximum (FWHM) was achieved using single 3 × 3 × 10 mm³'fast-LGSO' crystal elements, wrapped in Teflon tape and side coupled to a linear array of 3 SiPMs. In addition, the measured average CTR was 113.4 ± 0.7 ps for the side-coupled 2 × 4 crystal array. The readout architecture presented in this work is designed to be scalable to large area module detectors with a goal to create the first TOF-PET system with 100 ps FWHM CTR.

Keywords: FPGA-based TDC; coincidence time resolution; front-end electronics; positron emission tomography; scintillation crystal; silicon photomultipliers; time-of-flight.

Figure 1.

Crystal to photosensor coupling: (a) Conventional end-readout configuration. (b) Side readout configuration used in this study.

Figure 2.

Schematic of concept for a detector module that achieves ~100 ps CTR. From left to right: Single side-coupled detector layer; Sub-module, and full detector module design we employ to build a full 100 ps CTR TOF-PET system.

Figure 3.

Prototype mixed-signal electronic readout and data acquisition scheme for our proposed TOF-PET detectors: (a) Schematic and (b) implementation of electronic readout and data acquisition. (c) Experimental coincidence setup using two 2 × 4 arrays of 3 × 3 × 10 mm³ fast-LGSO crystal elements coupled to a 6 × 4 array of 3 × 3 mm² SiPMs. This configuration of two rows of 10 mm crystals creates an effective crystal length of 20 mm needed for clinical PET systems.

Figure 4.

Coincidence experimental setup to enable testing the robustness of performance for different NINO channels.

Figure 5.

Single photon charge spectrum (SPCS, shown in blue) of amplified timing signal for our prototype TOF-PET detector. The amplified timing signal is shown in red; the x-axis is the time (ns) and y-axis is its amplitude (mV).

Figure 6.

(a)CTR for different SiPM bias and NINO threshold. (b) An example of detector coincidence energy spectra (left) and time spectrum (right) at positive NINO threshold of $V_{\text{th}}^{+}=1.45\mathrm{V}$, optimum SiPM bias 31.5 V, and FPGA energy threshold of E_th = 0.45 V.

Figure 7.

Calibrated TOT-based energy spectra of the detectors (data acquired in singles mode).

Figure 8.

Average CTR values at optimum SiPM biasing of 31.5 V and for different positive NINO thresholds achieved using 2 × 4 arrays of 3 × 3 × 10 mm³ chemically etched fast LGSO:Ce crystals arrays coated with 260 μm thick BaSO₄.

Figure 9.

CTR values across several NINO channels at 31.5 V optimum SiPM biasing. Note that ‘CH_M,N’ are the two NINO channels M and N used in different coincidence measurements presented.