Siwei Xie

This study aims to develop a compact, low-cost, and high-performance benchtop small-animal PET/MRI scanner that achieves functional and anatomical image fusion. The system is designed to address challenges in cost reduction, spatial resolution, sensitivity, image quality, and quantitative accuracy. The PET/MRI system was developed with a parallel configuration, integrating a custom-designed PET scanner and a 0.5-T permanent magnet MRI system. Quantitative assessments included spatial resolution, sensitivity, image quality, and quantitative accuracy, as well as signal-to-noise ratio (SNR), geometric distortion (GD), and image uniformity for MRI. The spatial resolution at the axial center is 1.31 mm (axial), 1.26 mm (radial), and 1.22 mm (tangential), with a center sensitivity of 8.05% under a wide energy window. Image quality tests using an IQ phantom demonstrated a uniformity of 10.08 standard deviation, recovery coefficients (RC) ranging from 0.23 to 0.96, and spill-over ratios (SOR) of 0.08 and 0.18 in air and water regions, respectively. The MRI system achieved an SNR of 14.16 in phantom tests, a geometric distortion of less than 1%, and image uniformity of 90.13%. Fusion imaging of PET and MRI demonstrated high registration accuracy in both phantom and mouse studies, with complementary functional and anatomical information. The proposed PET/MRI system achieves high spatial resolution, sensitivity, image quality, and quantitative accuracy while maintaining a simple, low-cost design. The parallel configuration facilitates precise PET/MRI image fusion and allows for efficient multi-animal imaging. The results highlight the potential of this system for preclinical research and its feasibility for future in-vehicle imaging applications. Further optimization of the MRI system and data transmission methods will enhance its performance in high-activity studies and broaden its application scope, with potential applications in preclinical research and invehicle imaging.

The spatial resolution and image quality of positron emission tomography (PET) scanners are significantly affected by the uncertainties in the depth-of-interaction (DOI) measurement in PET detectors. The existing methods for implementing the DOI measurements mainly include dual-layer and dual-end crystal detectors. However, dual-layer crystal detectors have poor DOI resolution and limited performance improvement, whereas dual-end crystal detectors have a complex structure and high cost. To address these shortcomings, this paper proposes a cost-effective detector with a light-sharing window comprising a simple structure and good DOI resolution. The proposed detector leverages the ratio of photons passing through the internal light-sharing window at different interaction depths to determine the DOI. The proposed design is validated using an LYSO scintillation crystal detector, with a single crystal of size 2 mm × 2 mm × 20 mm, and a feedforward neural network. The results show that the DOI resolution achieve a full-width at half-maximum of 3.0 mm and a mean absolute error of 2.1 mm, which significantly improves the PET image quality. Moreover, the results also indicate a direct correlation between the energy and time resolution of the detector and the DOI. Furthermore, the results show that the proposed detector calibrates energy and time measurements through DOI, thus improving its performance and providing better PET image quality. The methods and results presented in this study can help in further improving the performance of PET detectors and PET scanners.

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.

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.

Dual-layer-offset or multi-layer-offset design of a PET detector can improve spatial resolution while maintaining high sensitivity. In this study, three dual-layer-offset LYSO detectors with three different reflectors (ESR, Toray, and BaSO₄) were developed. The top layer consisted of a 17 × 17 array of crystals 1 × 1 × 6.5 mm³ in size and the bottom layer consisted of an 18 × 18 array of crystals 1 × 1 × 9.5 mm³ in size. Neither light guides nor optical glue were used between the two layers of crystals. A custom-designed electronics system, composed of a 6 × 6 SiPM array, two FPC cables, and a custom-designed data processing module, was used to read out signals. An optimized interaction-decoding algorithm using the center of gravity to determine the position and threshold of analog signals for timing methods was applied to generate decoding flood histograms. The detector performances, in terms of peak to valley ratio of the flood histograms and energy resolutions, were calculated and compared. The dual-layer-offset PET detector constructed with BaSO₄ reflectors performed much better than the other two reflectors in both crystal identification and energy resolution. The average peak-to-valley ratio and the energy resolution were approximately 7 and 11%, respectively. In addition, the crystals in the bottom layer showed better performance at crystal identification than those in the top layer. This study can act as a reference providing guidance in choosing scintillator reflectors for multi-layer dedicated DOI detectors designed for small-animal PET imaging.

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.

Portable radiation detectors are widely used in environmental radiation detection and medical imaging due to their portability feature, high detection efficiency, and large field of view. Lutetium-yttrium oxyorthosilicate (LYSO) is a widely used scintillator in gamma radiation detection. However, the structure and the arrangement of scintillators limit the sensitivity and detection accuracy of these radiation detectors. In this study, a novel portable sensor based on a monolithic LYSO ring was developed for the detection of environmental radiation through simulation, followed by construction and assessments. Monte Carlo simulations were utilized to prove the detection of gamma rays at 511 keV by the developed sensor. The simulations data, including energy resolutions, decoding errors, and sensitivity, showed good potential for the detection of gamma rays by the as-obtained sensor. The experimental results using the VA method revealed decoding errors in the energy window width of 50 keV less than 2°. The average error was estimated at 0.67°, a sufficient value for the detection of gamma radiation. In sum, the proposed radiation sensor appears promising for the construction of high-performance radiation detectors and systems.

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.