The performance of an 8 ×8 array of 6.0 ×6.0 mm ² (active area) SiPMs was evaluated for PET applications using crystal arrays with different pitch sizes (3.4, 1.5, 1.35, and 1.2 mm) and custom designed five-channel front-end readout electronics (four channels for position information and one channel for timing information). The total area of this SiPM array is 57.4 ×57.4 mm ² , and the pitch size is 7.2 mm. It was fabricated using enhanced blue sensitivity SiPMs (MicroFB-60035-SMT) with peak spectral sensitivity at 420 nm. The performance of the SiPM array was characterized by measuring flood histogram decoding quality, energy resolution, timing resolution and saturation at several bias voltages (from 25.0 to 30.0 V in 0.5 V intervals) and two different temperatures ( 5 ^° C and 20 ^° C). Results show that the best flood histogram was obtained at a bias voltage of 28.0 V and 5 ^° C and an array of polished LSO crystals with a pitch as small as 1.2 mm can be resolved. No saturation was observed up to a bias voltage of 29.5 V during the experiments, due to adequate light sharing between SiPMs. Energy resolution and timing resolution at 5 ^° C ranged from 12.7 ±0.8% to 14.6 ±1.4% and 1.58 ±0.13 ns to 2.50 ±0.44 ns, for crystal array pitch sizes of 3.4 and 1.2 mm, respectively. Superior flood histogram quality, energy resolution and timing resolution were obtained with larger crystal array pitch sizes and at lower temperature. Based on our findings, we conclude that this large-area SiPM array can serve as a suitable photodetector for high-resolution small-animal PET or dedicated human brain PET scanners.

The goal of this study was to exploit the excellent spatial resolution characteristics of a position-sensitive silicon photomultiplier (SiPM) and develop a high-resolution depth-of-interaction (DOI) encoding positron emission tomography (PET) detector module. The detector consists of a 30 × 30 array of 0.445 × 0.445 × 20 mm ³ polished LYSO crystals coupled to two 15.5 × 15.5 mm ² linearly-graded SiPM (LG-SiPM) arrays at both ends. The flood histograms show that all the crystals in the LYSO array can be resolved. The energy resolution, the coincidence timing resolution and the DOI resolution were 21.8 ± 5.8%, 1.23 ± 0.10 ns and 3.8 ± 1.2 mm, respectively, at a temperature of −10 °C and a bias voltage of 35.0 V. The performance did not degrade significantly for event rates of up to 130 000 counts s ⁻¹ . This detector represents an attractive option for small-bore PET scanner designs that simultaneously emphasize high spatial resolution and high detection efficiency, important, for example, in preclinical imaging of the rodent brain with neuroreceptor ligands.

The leading edge timing pick-off technique is the simplest timing extraction method for PET detectors. Due to the inherent time-walk of the leading edge technique, corrections should be made to improve timing resolution, especially for time-of-flight PET. Time-walk correction can be done by utilizing the relationship between the threshold crossing time and the event energy on an event-by-event basis. In this paper, a time-walk correction method is proposed and evaluated using timing information from two identical detectors both using leading edge discriminators. This differs from other techniques that use an external dedicated reference detector, such as a fast PMT-based detector using constant fraction techniques to pick-off timing information. In our proposed method, one detector was used as reference detector to correct the time-walk of the other detector. Time-walk in the reference detector was minimized by using events within a small energy window (508.5-513.5 keV). To validate this method, a coincidence detector pair was assembled using two SensL MicroFB SiPMs and two 2.5 mm × 2.5 mm × 20 mm polished lutetium-yttrium oxyorthosilicate crystals. Coincidence timing resolutions (CTRs) using different time pickoff techniques were obtained at a bias voltage of 27.5 V and a fixed temperature of 20 °C. The CTR without time-walk correction were 389.0 ± 12.0 ps (425-650 keV energy window) and 670.2 ± 16.2 ps (250-750 keV energy window). The timing resolution with time-walk correction improved to 367.3 ± 0.5 ps (425-650 keV) and 413.7 ± 0.9 ps (250-750 keV). For comparison, timing resolutions were 442.8 ± 12.8 ps (425-650 keV) and 476.0 ± 13.0 ps (250-750 keV) using constant fraction techniques, and 367.3 ± 0.4 ps (425-650 keV) and 413.4 ± 0.9 ps (250-750 keV) using a reference detector based on the constant fraction technique. These results show that the proposed leading edge-based time-walk correction method works well. Timing resolution obtained using this method was equivalent to that obtained using a reference detector and was better than that obtained using constant fraction discriminators.

Thallium bromide (TlBr) and thallium chloride (TlCl) are semiconductor materials with high transparency to visible light, a high index of refraction, and high detection efficiency for gamma rays and annihilation photons. This manuscript reports on measurements of the light intensity and timing response of Čerenkov light emitted in one 3 mm ×3 mm ×5 mm slab of each of these materials operated in coincidence with a lutetium fine silicate (LFS) crystal with dimensions of 3 mm ×3 mm ×20 mm. A ²² Na radioactive source was used. The measured average number of detected photons per event was 1.5 photons for TlBr and 2.8 photons for TlCl when these materials were coupled to a silicon photomultiplier. The simulation predicts these results with an overestimation of 12%. The best coincidence time resolution (CTR) for events in TlBr and TlCl was 329 ± 9 and 316 ± 9 ps, respectively, when events with four photons and >7 photons were selected. The simulation showed the CTR degraded from 120 to 405 ps in TlCl, and from 160 to 700 ps in TlBr when the first or second Čerenkov photon was selected. Results of this work show that TlCl has a stronger Čerenkov light emission compared to TlBr and a greater potential to obtain the best timing measurements. Results also stress the importance of improving detection efficiency and transport of light to capture the first Čerenkov photon in timing measurements.

PET scanners using SiPMs as photodetectors could have tens of thousands of SiPMs. To simplify the readout electronics, analog signal multiplexing readouts are always preferred to be used as early as possible. In this paper, two simple analog signal multiplexing readouts, a capacitive charge-division readout, and a resistive charge-division readout were evaluated and compared using dual-ended readout detectors based on 10 × 10 arrays of SensL MicroFJ-30035 SiPMs coupled to both ends of a 20 × 20 LYSO array with a pitch size of 1.5 mm and a length of 20 mm. The performance of the detectors were evaluated at different bias voltages (from 27.0 V to 30.5 V with an interval of 0.5 V) and a temperature of 22.8 °C. The flood histograms show that all the crystals in the LYSO array were clearly identified, whilst better flood histogram was obtained using the resistive charge-division readout. At a bias voltage of 29.5V, the flood histogram quality, energy resolution, DOI resolution, and timing resolution of the detector obtained using the capacitive charge-division readout were 3.28 ± 0.85, 18.9% ± 6.2%, 1.93 ± 0.20 mm, 1.25 ± 0.11 ns respectively, and those obtained using the resistive charge-division readout were 3.57 ± 0.81, 16.9% ± 6.5%, 1.96 ± 0.23 mm and 1.23 ± 0.07 ns, respectively. Overall, the detector with the resistive charge-division readout provided better performance.

The performance of dual-ended readout depth-encoding positron emission tomography (PET) detectors based on bismuth germanate (BGO) coupled to silicon photomultipliers (SiPM) arrays was measured for the first time and compared to lutetium-yttrium oxyorthosilicate (LYSO)-based detectors using the same readout. The BGO and LYSO crystal arrays all had a crystal pitch of 2.2 mm and were coupled to 8 × 8 SiPM arrays with a matching pitch of 2.2 mm, using a one-to-one coupling configuration. Three types of crystals with Toray reflector were used: polished LYSO, polished BGO, and unpolished BGO, and for two different crystal thicknesses of 20 mm and 30 mm. All the crystal elements in the BGO arrays were clearly resolved in the flood histogram. Better flood histograms were obtained using the LYSO arrays for a selected crystal thickness, and better flood histograms were obtained using the 20 mm thick crystal arrays for a selected crystal type. The average crystal level energy resolution and timing resolution for 20 mm polished LYSO, polished BGO and unpolished BGO crystals at their optimal SiPM bias voltage were 18.6 ± 1.3% and 1.19 ± 0.20 ns, 17.8 ± 0.8% and 4.43 ± 0.47 ns, and 18.0 ± 1.0% and 4.68 ± 1.0 ns, respectively. Depth-of-interaction (DOI) resolution of the 20 mm polished LYSO array was 2.31 ± 0.17 mm and for the 20 mm unpolished BGO array was 3.53 ± 0.25 mm. However, polished BGO arrays with Toray reflector did not provide DOI information. Our key conclusion is that dual-ended readout depth-encoding 20 mm thick unpolished BGO detectors are good candidates for low-activity PET systems with small field-of-view and low timing performance requirements, such as preclinical or compact organ-dedicated PET systems, with the advantage over LYSO of having no background radiation and significantly lower cost.