Positron emission tomography (PET) is widely used in the diagnosis of tumors, cardiovascular system diseases, and neurological diseases. Scintillation crystals are an important part of PET scanners; they can convert γ photons into fluorescent photons to obtain their energy, time, and position information. Currently, an important research goal in PET is to find scintillation crystals with better performance. In this work, the principle of scintillation crystals is introduced, and the properties and requirements of scintillation crystals in different PET scanners are analyzed. At present, Lu_2(1−x)Y_2xSiO₅ (LYSO) is the scintillation crystal with the best comprehensive properties. LaBr₃ performs even better regarding the timing characteristics and light output; however, LaBr₃ has not been used in any PET scanner because of its deliquescence. Detectors made of Gd₃(Ga, Al)₅O₁₂ (GAGG) exhibit a high depth of interaction (DOI) resolution and have considerable application potential. The application fields of PET are constantly expanding, and its future development aims to achieve high spatial resolution and high sensitivity, which require scintillation crystals with better performance.

The performance of radiation detectors used in positron-emission tomography (PET) is determined by the intrinsic properties of the scintillators, the geometry and surface treatment of the scintillator crystals and the electrical and optical characteristics of the photosensors. Experimental studies were performed to assess the timing resolution and energy resolution of detectors constructed with samples of different scintillator materials (LaBr₃, CeBr₃, LFS, LSO, LYSO: Ce, Ca and GAGG) that were fabricated into different shapes with various surface treatments. The saturation correction of SiPMs was applied for tested detectors based on a Tracepro simulation. Overall, we tested 28 pairs of different forms of scintillators to determine the one with the best CTR and light output. Two common high-performance silicon photomultipliers (SiPMs) provided by SensL (J-series, 6 mm) or AdvanSiD (NUV, 6 mm) were used for photodetectors. The PET detector constructed with 6 mm CeBr₃ cubes achieved the best CTR with a FWHM of 74 ps. The 4 mm co-doped LYSO: Ce, Ca pyramid crystals achieved 88.1 ps FWHM CTR. The 2 mm, 4 mm and 6 mm 0.2% Ce, 0.1% Ca co-doped LYSO cubes achieved 95.6 ps, 106 ps and 129 ps FWHM CTR, respectively. The scintillator crystals with unpolished surfaces had better timing than those with polished surfaces. The timing resolution was also improved by using certain geometric factors, such as a pyramid shape, to improve light transportation in the scintillator crystals.

This paper presented a non-uniform multiphase (NUMP) time-to-digital converter (TDC) implemented in a field-programmable gate array (FPGA) with real-time automatic temperature compensation. NUMP-TDC is a novel, low-cost, high-performance TDC that has achieved an excellent performance in Altera Cyclone V FPGA. The root mean square (RMS) for the intrinsic timing resolution was 2.3 ps. However, the propagation delays in the delay chain of some FPGAs (for example, the Altera Cyclone 10 LP) vary significantly as the temperature changes. Thus, the timing performances of NUMP-TDCs implemented in those FPGAs are significantly impacted by temperature fluctuations. In this study, a simple method was developed to monitor variations in propagation delays using two registers deployed at both ends of the delay chain and compensate for changes in propagation delay using a look-up table (LUT). When the variations exceeded a certain threshold, the LUT for the delay correction was updated, and a bin-by-bin correction was launched. Using this correction approach, a resolution of 8.8 ps RMS over a wide temperature range (5 °C to 80 °C) had been achieved in a NUMP-TDC implemented in a Cyclone 10 LP FPGA.

We investigate the effects of the signal walk in timing measurements and the methods to compensate for the error when leading-edge discriminator (LED) triggering was used. Three pairs of crystals (LaBr3 cubes, LYSO cubes, and LYSO rectangular prisms) were tested for coincidence timing resolution (CTR) to evaluate four compensation methods: Linear 1D, Logarithm 1D, Polynomial 2D, and Artificial Neural Network (ANN) 2D. The experimental results show that: (1) the signal walk causes a logarithmic relationship between energy and measured time. (2) The signal walk dramatically affects the timing measurement at a wide energy window. The measured FWHM CTR decreased by 86.2%, from 670 ps to 92.7 ps, when the lower cut of the energy window was used. (3) The logarithm 1D compensation can improve the timing performance. The FWHM CTRs decreased by 69.9% (from 326.8 ps to 101.3 ps) with an energy window of 300 keV -550 keV and by 12.5% (from 97.2 ps to 85.1 ps) with an energy window of 410 keV -550 keV for LaBr3 cube crystals. The results show that the logarithm 1D, polynomial 2D, and ANN 2D compensations work well, achieving at least a 10% improvement in timing performance.

The spatial resolutions of preclinical PET imagers are largely determined by the size of the crystals. This study explores methods to construct PET detectors using crystals with ultra-small cross-section for preclinical PET imagers with ultra-high resolution. Three 16×16 segmented LYSO: Ce crystal arrays were built with different reflectors and assembling techniques using 0.25×0.25×6.25 mm3 pixels. The crystal arrays were readout by 3 mm SiPMs with a crystal-to-SiPM pixel area ratio of approximately 1:94, and the signals were recorded with custom-designed read-out electronics. Two coupling configurations were conducted. The arrays were evaluated in terms of flood histogram, energy resolution, and timing resolution. The first array, constructed with discrete LYSO crystals filled with BaSO4 reflectors, had non-uniformly distributed decoding spots in the flood histogram. The second array, constructed with ESR reflectors using the slab-sandwich-slice (SSS) production method, had a distorted flood histogram. The third array, constructed with the combination of ESR and BaSO4 using the SSS production method, achieved the best flood histogram in terms of crystal spot uniformity and peak-to-valley ratio (2.80±0.53). The third array also demonstrated good energy resolution (14.89%±2.30%) and timing resolution (926.5 ps). These findings suggest that the SSS production method using the combined reflectors of ESR and BaSO4 is a potential method to construct detectors for ultra-high resolution PET imagers.

As a commonly used solution, the multi-ended readout can measure the depth-of-interaction (DOI) for positron emission tomography (PET) detectors. In the present study, the effects of the multi-ended readout design were investigated using the leading-edge discriminator (LED) triggers on the timing performance of time-of-flight (TOF) PET detectors. At the very first, the photon transmission model of the four detectors, namely, single-ended readout, dual-ended readout, side dual-ended readout, and triple-ended readout, was established in Tracepro. The optical simulation revealed that the light output of the multi-ended readout was higher. Meanwhile, the readout circuit could be triggered earlier. Especially, in the triple-ended readout, the light output at 0.5 ns was observed to be nearly twice that of the single-ended readout after the first scintillating photon was generated. Subsequently, a reference detector was applied to test the multi-ended readout detectors that were constructed from a 6 × 6 × 25 mm³ LYSO crystal. Each module is composed of a crystal coupled with multiple SiPMs. Accordingly, its timing performance was improved by approximately 10% after the compensation of fourth-order polynomial fitting. Finally, the compensated full-width-at-half-maximum (FWHM) coincidence timing resolutions (CTR) of the dual-ended readout, side dual-ended readout, and triple-ended readout were 216.9 ps, 231.0 ps, and 203.6 ps, respectively.