LYSO Crystals Research Articles

First experimental verification of prompt gamma imaging with carbon ion irradiation

Prompt Gamma Imaging (PGI) is a promising technique for range verification in Particle Therapy. This technique was already tested in clinical environment with a knife-edge-collimator camera for proton treatments but remains relatively unexplored for Carbon Ion Radiation Therapy (CIRT). Previous FLUKA simulations suggested that PG profile shifts could be detected in CIRT with a precision of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} 4 mm (\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2 \\sigma$$\\end{document}) for a particle statistic equal to \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5 \\cdot 10^{7}$$\\end{document} C-ions using a 10 × 10 cm2 camera. An experimental campaign was carried out at CNAO (Pavia, Italy) to verify these results, using a knife-edge-collimator camera prototype based on a 5 × 5 cm2 pixelated LYSO crystal. PG profiles were measured irradiating a plastic phantom with a C-ion pencil beam at clinical energies and intensities, also moving the detector to extend the FOV to 13 × 5 cm2. The prototype detected Bragg-peak shifts with \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} 4 mm precision for a statistic of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 4 \\cdot 10^{8}$$\\end{document} C-ions (\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3 \\cdot 10^{8}$$\\end{document} for the extended FOV), slightly larger than expected. Nevertheless, the detector demonstrated significant potential for verifying the precision in dose delivery following a treatment fraction, which remains fundamental in the clinical environment. For the first time to our knowledge, range verification based on PGI was applied to a C-ion beam at clinical energy and intensities.

The Scintillation Counters of the High-Energy Particle Detector of the China Seismo-Electromagnetic (CSES-02) Satellite

The High-Energy Particle Detector (HEPD-02) is one of the scientific payloads of the China Seismo-Electromagnetic Satellite (CSES-02). The HEPD-02’s main purpose is to characterize the particle environment in the Earth’s vicinity, identifying sudden changes in the fluxes and correlating them with solar and terrestrial phenomena. Additionally, HEPD-02 also has capabilities in detecting Gamma-Ray Bursts. At the core of HEPD-02, a tower of scintillation counters made of plastic and LYSO crystals is able to recognize electrons in the range between 3 and 100 MeV, protons and nuclei between 30 and 200 MeV/n. Plastic scintillators covering the calorimeter on five sides allow to reject particles entering from the top and not completely absorbed within its volume. In this work, the design of the HEPD-02 is reviewed in comparison to its predecessor, HEPD-01, highlighting the innovations of the new design. The design of each scintillation counter type has been fully validated through a campaign of prototype realization, testing, and characterization. The production of the scintillation counters, including the PMT selection process, is also discussed. Finally, the performance of the counters is compared with simulations, showing an agreement of within 20% with the expected performance, thereby meeting expectations.

4D-tracking with digital SiPMs

Silicon Photomultipliers (SiPMs) are the state-of-the-art technology in single-photon detection with solid-state detectors. Single Photon Avalanche Diodes (SPADs), the key element of SiPMs, can now be manufactured in CMOS processes, facilitating the integration of a SPAD array into custom monolithic ASICs. This allows implementing features such as signal digitization, masking, full hit-map readout, noise suppression, and photon counting in a monolithic CMOS chip. The complexity of the off-chip readout chain is thereby reduced.These new features allow new applications for digital SiPMs, such as 4D-tracking of charged particles, where spatial resolutions of the order of 10µm and timestamping with time resolutions of a few tens of ps are required.A prototype of a digital SiPM was designed at DESY using the LFoundry 150nm CMOS technology. Various studies were carried out in the laboratory and at the DESY II test-beam facility to evaluate the sensor performance in Minimum Ionizing Particles (MIPs) detection. The direct detection of charged particles was investigated for bare prototypes and assemblies coupling dSiPMs and thin LYSO crystals. Spatial resolution ∼20µm and a full-system time resolution of ∼50ps are measured using bare dSiPMs in direct MIP detection. Efficiency >99.5%, low noise rate and time resolution <1ns can be reached with the thin radiator coupling.

The Ziré instrument onboard the NUSES space mission

The Ziré experiment is an instrument on board the NUSES (NeUtrino and Seismic Electromagnetic Signals) satellite dedicated to test new techniques for the study of low energy cosmic radiation, Sun–Earth environment, space weather and magnetosphere–ionosphere–lithosphere coupling (MILC). Ziré will consist of a fiber tracker, a stack of plastic scintillators, an array of LYSO crystals and an anticoincidence system. A dedicated Low Energy Module (LEM) will extend the sensitive energy range down to the MeV scale for charged particles. In this work, a general overview of the Ziré payload will be given, together with a focus on the design and the review of dedicated tests on the first prototype.

Gamma Spot Monitor at SLEGS beamline

The Gamma Spot Monitor(GSM) at SLEGS beamline is designed to provide a direct view of the gamma beam profile. The main part of the GSM is a LYSO crystal and an astronomical camera. A set of experimental tests on its performance were carried out. The results show that the magnification are 21.0% in X and 21.2% in Y direction, and the relative distortion is proved to be lower than 0.7%. The spatial resolution of the GSM evaluated by the Modulation Transfer Function(MTF) is better than 6.4 lp/mm. The contrast resolution is better than 14%. Beamline alignment is essential in beam modulation, and there are several ways for beamline alignment. In this paper, alignment capability of the GSM is demonstrated, also, the absorption coefficient distribution of a model composed of different materials is shown.

Complete recovery of rare earths from waste LYSO crystals by thermal shock treatment and reinforcement decomposition

Waste lutetium yttrium silicate (LYSO) crystals, consisting of nearly 70 % lutetium, is an important secondary resource. However, traditional processes recycling rare earths (REs) from LYSO faced issues of incomplete rare earth phase transformation, low REs recovery, and filtration difficulty of silicate residues. To solve these issues, a dramatically rapid cooling thermal shock treatment, contrary to the mild heating for LYSO crystal growth, was proposed to disrupt the LYSO crystal structure preliminarily. Then, the mechanism and process optimization of alkali roasting-water washing, HCl decomposition, and REs extraction were systematically investigated. Experimental results demonstrated that the phase of LYSO was effectively transformed from Lu2SiO5 to NaLuO2 after roasting at 800 °C with a NaOH/LYSO mass ratio of 1:1 for 2 h after thermal shock treatment. Subsequently, over 99.95 % of REs were leached from alkali slags with the L/S ratio of 8:1, 3 mol/L HCl at 80 ℃ for 20 min. To separate with silicate, REs in leachate were co-extracted with HEH(EHP)-ROH system at the O/A ratio of 2:1, equilibrium pH of 0.71, 25 ℃. Then, after water scrubbing, the loaded organic was stripped with 4 mol/L HCl, O/A=4:1, and a pure rare earth chloride product was obtained, which can be used as the precursor for generating REC2O4. This work achieves the reinforcement decomposition of LYSO crystals with no filtration difficulty and high REs recovery (RLu=99.70 %, RY=95.09 %), which can be a better alternative approach for REs recovery from LYSO.

Precise positioning of gamma ray interactions in multiplexed pixelated scintillators using artificial neural networks

Introduction. The positioning of γ ray interactions in positron emission tomography (PET) detectors is commonly made through the evaluation of the Anger logic flood histograms. machine learning techniques, leveraging features extracted from signal waveform, have demonstrated successful applications in addressing various challenges in PET instrumentation. Aim. This paper evaluates the use of artificial neural networks (NN) for γ ray interaction positioning in pixelated scintillators coupled to a multiplexed array of silicon photomultipliers (SiPM). Methods. An array of 16 Cerium doped Lutetium-based (LYSO) crystal pixels (cross-section 2 × 2 mm2) coupled to 16 SiPM (S13360-1350) were used for the experimental setup. Data from each of the 16 LYSO pixels was recorded, a total of 160000 events. The detectors were irradiated by 511 keV annihilation γ rays from a Sodium-22 (22Na) source. Another LYSO crystal was used for electronic collimation. Features extracted from the signal waveform were used to train the model. Two models were tested: i) single multiple-class neural network (mcNN), with 16 possible outputs followed by a softmax and ii) 16 binary classification neural networks (bNN), each one specialized in identifying events occurred in each position. Results. Both NN models showed a mean positioning accuracy above 85% on the evaluation dataset, although the mcNN is faster to train. Discussion The method’s accuracy is affected by the introduction of misclassified events that interacted in the neighbour’s crystals and were misclassified during the dataset acquisition. Electronic collimation reduces this effect, however results could be improved using a more complex acquisition setup, such as a light-sharing configuration. Conclusions The methods comparison showed that mcNN and bNN can surpass the Anger logic, showing the feasibility of using these models in positioning procedures of future multiplexed detector systems in a linear configuration.

Validation of the design of a high-sensitivity and high-resolution PET system for a preclinical PET/EPR hybrid scanner

We report the development of a high-sensitivity and high-resolution PET subsystem for a next-generation preclinical PET/EPR hybrid scanner for investigating and improving hypoxia imaging with PET. The PET subsystem consists of 14 detector modules (DM) installed within a cylindrical supporting frame whose outer and inner diameters are 115 mm and 60 mm, respectively. Each DM contains eight detector units (DU) in a row and each DU is made of a 12 × 12 array of 1 × 1 × 10mm3 LYSO crystals (with a 1.05 mm pitch) coupled to a 4 × 4 silicon photomultiplier (SiPM) array that has a 3.2 mm pitch (Hamamatsu multi-pixel photon counter (MPPC) array 14161-3050HS-04). The PET subsystem has a 104 mm axial field-of-view (AFOV) that is sufficient for full-body mouse imaging, therefore enabling temporal and spatial correlation studies of tumor hypoxia between PET and EPR. It employs 1mm-width crystals to support sub-millimeter image resolution that is desired for mouse imaging. Although a DM contains 1,152 LYSO crystals, by use of a newly devised signal readout method only six outputs are produced. Recently a partial prototype of this subsystem consisting of four DMs is built. In this paper, we present performance measurement results obtained for the developed DMs and initial imaging results obtained by the prototype. The developed DMs show uniformly superior performance in identifying the hit crystal and detector unit, in energy resolution, and in coincidence time resolution. The images obtained for a 22Na point source and a 18F-filled U-shaped tube source show an image resolution of about 1.1 mm and 1.2 mm FWHM in the transverse and axial directions respectively, and demonstrate successful imaging over the entire 104 mm AFOV of the prototype. This estimated image resolution however includes the contribution by the source size.

Determination of lutetium density in LYSO crystals: methodology and PET detector applications

Objective. Lutetium yttrium oxyorthosilicate (LYSO) scintillation crystals are used in positron emission tomography (PET) due to their high gamma attenuation, fair energy resolution, and fast scintillation decay time. The enduring presence of the 176Lu isotope, characterized by a half-life of 37.9 billion years, imparts a consistent radiation background (BG) profile that depends on the geometry and composition attributes of the LYSO crystals. Approach. In this work, we proposed a methodology for estimating the composition of LYSO crystals in cases where the exact Lutetium composition remains unknown. The connection between BG spectrum intensity and intrinsic radioactivity enables precise estimation of Lutetium density in LYSO crystal samples. This methodology was initially applied to a well-characterized LYSO crystal sample, yielding results closely aligned with the known composition. The composition estimation approach was extended to several samples of undisclosed LYSO crystals, encompassing single crystal and crystal array configurations. Furthermore, we model the background spectrum observed in the LYSO-based detector and validate the observed spectra via simulations. Main results. The estimated Lutetium composition exhibited adequate consistency across different samples of the same LYSO material, with variations of less than 1%. The result of the proposed approach coupled with the simulation successfully models the background radiation spectra in various LYSO-based detector geometries. Significance. The implications of this work extend to the predictive assessment of system behaviors and the autonomous configuration parameters governing LYSO-based detectors.

A multi-resolution TOF-DOI detector for human brain dedicated PET scanner

Objective. We propose a single-ended readout, multi-resolution detector design that can achieve high spatial, depth-of-interaction (DOI), and time-of-flight (TOF) resolutions, as well as high sensitivity for human brain-dedicated positron emission tomography (PET) scanners. Approach. The detector comprised two layers of LYSO crystal arrays and a lightguide in between. The top (gamma ray entrance) layer consisted of a 16 × 16 array of 1.53 × 1.53 × 6 mm3 LYSO crystals for providing high spatial resolution. The bottom layer consisted of an 8 × 8 array of 3.0 × 3.0 × 15 mm3 LYSO crystals that were one-to-one coupled to an 8 × 8 multipixel photon counter (MPPC) array for providing high TOF resolution. The 2 mm thick lightguide introduces inter-crystal light sharing that causes variations of the light distribution patterns for high DOI resolution. The detector was read out by a PETsys TOFPET2 application-specific integrated circuit. Main result. The top and bottom layers were distinguished by a convolutional neural network with 97% accuracy. All crystals in the top and bottom layers were resolved. The inter-crystal scatter (ICS) events in the bottom layer were identified, and the measured average DOI resolution of the bottom layer was 4.1 mm. The coincidence time resolution (CTR) for the top–top, top-bottom, and bottom–bottom coincidences was 476 ps, 405 ps, and 298 ps, respectively. When ICS events were excluded from the bottom layer, the CTR of the bottom–bottom coincidence was 277 ps. Significance. The top layer of the proposed two-layer detector achieved a high spatial resolution and the bottom layer achieved a high TOF resolution. Together with its high DOI resolution and detection efficiency, the proposed detector is well suited for next-generation high-performance brain-dedicated PET scanners.

Characterization of external optical crosstalk reduction for SiPM-based scintillation detectors with an optical bandpass filter

External optical crosstalk remains a significant source of correlated noise in scintillation detectors optically coupled to silicon photomultiplier (SiPM) arrays and can influence achievable performance by limiting the maximum overvoltage at which a detector can be operated. This study evaluated the potential benefits of incorporating an optical bandpass filter, which is absorptive to optical crosstalk and transmissive for scintillation photons, between the scintillator and SiPM array. Several SiPM and bandpass configurations were characterized, including single-element and SiPM arrays with bare, filter-coupled, crystal-coupled, and crystal-filter-coupled interfaces. Coupling the optical filter between a 4 × 4 array of Broadcom AFBR-S4N44P164 M (NUV-MT) SiPMs and a 12 × 12 × 20 mm3 LYSO crystal decreased the total number of detected optical photon noise by up to 92%. The impact of the filter on crosstalk probability reduction was also characterized by the single-channel measurements with the same SiPM, suppressing external crosstalk up to 64%. With the filter coupled, SiPMs were operable at very high overvoltage. The results of these studies demonstrate that optical filtering is a promising technique to significantly reduce correlated noise in scintillation detectors employing SiPMs.

Design study of a novel geometrical arrangement for an in-beam small animal positron emission tomography scanner

Objective. We designed a geometrical solution for a small animal in-beam positron emission tomography (PET) scanner to be used in the project SIRMIO (Small animal proton irradiator for research in molecular image-guided radiation-oncology). The system is based on 56 scintillator blocks of pixelated LYSO crystals. The crystals are arranged providing a pyramidal-step shape to optimize the geometrical coverage in a spherical configuration. Approach. Different arrangements have been simulated and compared in terms of spatial resolution and sensitivity. The chosen setup enables us to reach a good trade-off between a solid angle coverage and sufficient available space for the integration of additional components of the first design prototype of the SIRMIO platform. The possibility of moving the mouse holder inside the PET scanner furthermore allows for achieving the optimum placement of the irradiation area for all the possible tumor positions in the body of the mouse. The work also includes a study of the scintillator material where LYSO and GAGG are compared with a focus on the random coincidence noise due to the natural radioactivity of Lutetium in LYSO, justifying the choice of LYSO for the development of the final system. Main results. The best imaging performance can be achieved with a sub-millimeter spatial resolution and sensitivity of 10% in the center of the scanner, as verified in thorough simulations of point sources. The simulation of realistic irradiation scenarios of proton beams in PMMA targets with/without air gaps indicates the ability of the proposed PET system to detect range shifts down to 0.2 mm. Significance. The presented results support the choice of the identified optimal design for a novel spherical in-beam PET scanner which is currently under commissioning for application to small animal proton and light ion irradiation, and which might find also application, e.g. for biological image-guidance in x-ray irradiation.

The effects of various penalty parameter values in Q.Clear algorithm for rectal cancer detection on 18F-FDG images using a BGO-based PET/CT scanner: a phantom and clinical study

BackgroundThe Q.Clear algorithm is a fully convergent iterative image reconstruction technique. We hypothesize that different PET/CT scanners with distinct crystal properties will require different optimal settings for the Q.Clear algorithm. Many studies have investigated the improvement of the Q.Clear reconstruction algorithm on PET/CT scanner with LYSO crystals and SiPM detectors. We propose an optimum penalization factor (β) for the detection of rectal cancer and its metastases using a BGO-based detector PET/CT system which obtained via accurate and comprehensive phantom and clinical studies.Methods18F-FDG PET-CT scans were acquired from NEMA phantom with lesion-to-background ratio (LBR) of 2:1, 4:1, 8:1, and 15 patients with rectal cancer. Clinical lesions were classified into two size groups. OSEM and Q.Clear (β value of 100–500) reconstruction was applied. In Q.Clear, background variability (BV), contrast recovery (CR), signal-to-noise ratio (SNR), SUVmax, and signal-to-background ratio (SBR) were evaluated and compared to OSEM.ResultsOSEM had 11.5–18.6% higher BV than Q.Clear using β value of 500. Conversely, RC from OSEM to Q.Clear using β value of 500 decreased by 3.3–7.7% for a sphere with a diameter of 10 mm and 2.5–5.1% for a sphere with a diameter of 37 mm. Furthermore, the increment of contrast using a β value of 500 was 5.2–8.1% in the smallest spheres compared to OSEM. When the β value was increased from 100 to 500, the SNR increased by 49.1% and 30.8% in the smallest and largest spheres at LBR 2:1, respectively. At LBR of 8:1, the relative difference of SNR between β value of 100 and 500 was 43.7% and 44.0% in the smallest and largest spheres, respectively. In the clinical study, as β increased from 100 to 500, the SUVmax decreased by 47.7% in small and 31.1% in large lesions. OSEM demonstrated the least SUVmax, SBR, and contrast. The decrement of SBR and contrast using OSEM were 13.6% and 12.9% in small and 4.2% and 3.4%, respectively, in large lesions.ConclusionsImplementing Q.Clear enhances quantitative accuracies through a fully convergent voxel-based image approach, employing a penalization factor. In the BGO-based scanner, the optimal β value for small lesions ranges from 200 for LBR 2:1 to 300 for LBR 8:1. For large lesions, the optimal β value is between 400 for LBR 2:1 and 500 for LBR 8:1. We recommended β value of 300 for small lesions and β value of 500 for large lesions in clinical study.

Comparative studies of the sensitivities of sparse and full geometries of Total-Body PET scanners built from crystals and plastic scintillators

BackgroundAlongside the benefits of Total-Body imaging modalities, such as higher sensitivity, single-bed position, low dose imaging, etc., their final construction cost prevents worldwide utilization. The main aim of this study is to present a simulation-based comparison of the sensitivities of existing and currently developed tomographs to introduce a cost-efficient solution for constructing a Total-Body PET scanner based on plastic scintillators.MethodsFor the case of this study, eight tomographs based on the uEXPLORER configuration with different scintillator materials (BGO, LYSO), axial field-of-view (97.4 cm and 194.8 cm), and detector configurations (full and sparse) were simulated. In addition, 8 J-PET scanners with different configurations, such as various axial field-of-view (200 cm and 250 cm), different cross sections of plastic scintillator, and multiple numbers of plastic scintillator layers (2, 3, and 4), based on J-PET technology have been simulated by GATE software. Furthermore, Siemens’ Biograph Vision has been simulated to compare the results with standard PET scans. Two types of simulations have been performed. The first one with a centrally located source with a diameter of 1 mm and a length of 250 cm, and the second one with the same source inside a water-filled cylindrical phantom with a diameter of 20 cm and a length of 183 cm.ResultsWith regards to sensitivity, among all the proposed scanners, the ones constructed with BGO crystals give the best performance (sim 350 cps/kBq at the center). The utilization of sparse geometry or LYSO crystals significantly lowers the achievable sensitivity of such systems. The J-PET design gives a similar sensitivity to the sparse LYSO crystal-based detectors while having full detector coverage over the body. Moreover, it provides uniform sensitivity over the body with additional gain on its sides and provides the possibility for high-quality brain imaging.ConclusionTaking into account not only the sensitivity but also the price of Total-Body PET tomographs, which till now was one of the main obstacles in their widespread clinical availability, the J-PET tomography system based on plastic scintillators could be a cost-efficient alternative for Total-Body PET scanners.

Beam Test Results of the RADiCAL—A Radiation Hard Innovative EM Calorimeter

High performance calorimetry conducted at future hadron colliders, such as the FCC-hh, poses a significant challenge for applying current detector technologies due to unprecedented beam luminosities and radiation fields. Solutions include developing scintillators that are capable of separating events at the sub-fifty picosecond level while also maintaining performance after extreme and constant neutron and ionizing radiation exposure. The RADiCAL is an approach that incorporates radiation tolerant materials in a sampling ‘shashlik’ style calorimeter configuration, using quartz capillaries filled with organic liquid or polymer-based wavelength shifters embedded in layers of tungsten plates and LYSO crystals. This novel design intends to address the Priority Research Directions (PRD) for calorimetry listed in the DOE Basic Research Needs (BRN) workshop for HEP Instrumentation. Here we report preliminary results from an experimental run at the Fermilab Test Beam Facility in June 2022. These tests demonstrate that the RADiCAL concept is capable of < 50 ps timing resolution.

Monte Carlo simulation of the system performance of a long axial field-of-view PET based on monolithic LYSO detectors

BackgroundIn light of the milestones achieved in PET design so far, further sensitivity improvements aim to optimise factors such as the dose, throughput, and detection of small lesions. While several longer axial field-of-view (aFOV) PET systems based on pixelated detectors have been installed, continuous monolithic scintillation detectors recently gained increased attention due to their depth of interaction capability and superior intrinsic resolution. As a result, the aim of this work is to present and evaluate the performance of two long aFOV, monolithic LYSO-based PET scanner designs.MethodsGeant4 Application for Tomographic Emission (GATE) v9.1 was used to perform the simulations. Scanner designs A and B have an aFOV of 36.2 cm (7 rings) and 72.6 cm (14 rings), respectively, with 40 detector modules per ring each and a bore diameter of 70 cm. Each module is a 50 × 50 × 16 mm3 monolithic LYSO crystal. Sensitivity, noise equivalent count rate (NECR), scatter fraction, spatial resolution, and image quality tests were performed based on NEMA NU-2018 standards.ResultsThe sensitivity of design A was calculated to be 29.2 kcps/MBq at the centre and 27 kcps/MBq at 10 cm radial offset; similarly, the sensitivity of design B was found to be 106.8 kcps/MBq and 98.3 kcps/MBq at 10 cm radial offset. NECR peaks were reached at activity concentrations beyond the range of activities used for clinical studies. In terms of spatial resolution, the values for the point sources were below 2 mm for the radial, tangential, and axial full width half maximum. The contrast recovery coefficient ranged from 53% for design B and 4:1 contrast ratio to 90% for design A and 8:1 ratio, with a reasonably low background variability.ConclusionsLonger aFOV PET designs using monolithic LYSO have superior spatial resolution compared to current pixelated total-body PET (TB-PET) scanners. These systems combine high sensitivity with improved contrast recovery.

Validation of a Monte Carlo model of the uEXPLORER total-body PET scanner using GATE code

PurposeTo develop and validate a GATE model of the newly manufactured total-body PET scanner (uEXPLORER), in order to evaluate the capability of GATE to simulate PET scanners with long AFOV, and to provide a reliable model to be used in further optimization studies. MethodsThe detector geometry of our model consists of 564,480 LYSO crystals grouped into blocks and modules in accordance with the actual scanner's specifications. Signal processing chain was modeled directly in GATE through the digitizer modules. For model validation, we performed spatial resolution, sensitivity, count rate and scatter fraction tests following the National Electrical Manufactures Association (NEMA) NU 2–2018 protocol, in addition to other devised tests representative of total-body acquisitions. The simulation results were then compared to published measurements. ResultsOverall, our simulation model demonstrated good agreement with the experimental results, within 6% for sensitivity, 7% for scatter fraction and 17% for noise equivalent count rate. For the spatial resolution test, the model matched the experimental results to within 0.1–1.3 mm. ConclusionsWe have shown that our developed model predicts the uEXPLORER PET scanner's performances with good accuracy. Consequently, we conclude that GATE is reasonably suitable for simulating long AFOV PET scanners.

Ce:LYSO, from scintillator to solid-state lighting as a blue luminescent concentrator

Cerium-doped lutetium-yttrium oxyorthosilicate (Ce:LYSO) is a well-known single crystal scintillator used in medical imaging and security scanners. Recent development of high power UV LED, matching its absorption band, questions the possibility to use Ce:LYSO in a new way: as LED-pumped solid-state light source. Since Ce:LYSO is available in large size crystals, we investigate its potential as a luminescent concentrator. This paper reports an extensive study of the performance in close relation to the spectroscopic properties of this crystal. It gives the reasons why the Ce:LYSO crystal tested in this study is less efficient than Ce:YAG for luminescent concentration: limited quantum efficiency and high losses coming from self-absorption and from excited-state absorption are playing key roles. However, we demonstrate that a Ce:LYSO luminescent concentrator is an innovative source for solid-state lighting. Pumped by a peak power of 3400 W in quasi-continuous wave regime (40 µs, 10 Hz), a rectangular (1 × 22 × 105 mm3) Ce:LYSO crystal delivers a broadband spectrum (60 nm FWHM) centered at 430 nm. At full output aperture (20 × 1 mm2), it emits a peak power of 116 W. On a squared output surface (1 × 1 mm2) it emits 16 W corresponding to a brightness of 509 W cm–2 sr–1. This combination of spectrum power and brightness is higher than blue LEDs and opens perspectives for Ce:LYSO in the field of illumination namely for imaging.

Monolithic scintillator PET detector by using light sharing between adjacent detector modules

A monolithic crystal-based positron emission tomography (PET) detector has a low cost and high sensitivity and allows determining 3D positions by scintillation light distribution. However, the edge effect inherent to scintillators deteriorates the position resolution of the detector toward the crystal borders due to the escape and reflection of the scintillation light. An increase in crystal thickness will improve the detection efficiency, but the edge effect becomes severe. To improve the position resolution and detection efficiency of scintillators, the light-sharing technique is developed. Two identical trapezoidal monolithic cerium-doped lutetium yttrium orthosilicate (LYSO) crystals with a bottom size of 25.80 × 25.80 mm2 and a thickness of 20 mm are optically coupled at one lateral face by an optical medium. The scintillation light near the optically coupled interface is jointly collected by two adjacent 8 × 8 SiPM arrays from the bottom faces. The SiPM signals are individually read out and processed by using the multichannel readout application-specific integrated circuits (ASICs) of TOFPET2 to provide a light distribution. The transverse positions and depth of interaction (DOI) are calculated from the measured light distribution. As expected, the optical glue with a refractive index of n = 1.68 shows better light sharing between two LYSO crystals than the coupling media of silicon grease (n = 1.40) and air. For the 20 mm thick monolithic LYSO-based PET detector, the transverse position resolution near the crystal edge is improved by nearly 30% by using the light-sharing method compared with the black-painted treatment. However, the DOI resolution near the edge of the crystal is not significantly improved, which may be attributed to the incomplete collection of scintillation light. The DOI resolution of the detector would be improved by using electronics with lower noise and SiPM arrays with smaller pixel.

A CNN-based four-layer DOI encoding detector using LYSO and BGO scintillators for small animal PET imaging

Objective. We propose a novel four-layer depth-of-interaction (DOI) encoding phoswich detector using lutetium–yttrium oxyothosilicate (LYSO) and bismuth germanate (BGO) scintillator crystal arrays for high sensitivity and high spatial resolution small animal PET imaging. Approach. The detector was comprised of a stack of four alternating LYSO and BGO scintillator crystal arrays coupled to an 8 × 8 multi-pixel photon counter (MPPC) array and read out by a PETsys TOFPET2 application specific integrated circuit. The four layers from the top (gamma ray entrance) to the bottom (facing the MPPC) consisted of a 24 × 24 array of 0.99 × 0.99 × 6 mm3 LYSO crystals, a 24 × 24 array of 0.99 × 0.99 × 6 mm3 BGO crystals, a 16 × 16 array of 1.53 × 1.53 × 6 mm3 LYSO crystals and a 16 × 16 array of 1.53 × 1.53 × 6 mm3 BGO crystals. Main results. Events that occurred in the LYSO and BGO layers were first separated by measuring the pulse energy (integrated charge) and duration (time over threshold (ToT)) from the scintillation pulses. Convolutional neural networks (CNNs) were then used to distinguish between the top and lower LYSO layers and between the upper and bottom BGO layers. Measurements with the prototype detector showed that our proposed method successfully identified events from all four layers. The CNN models achieved a classification accuracy of 91% for distinguishing the two LYSO layers and 81% for distinguishing the two BGO layers. The measured average energy resolution was 13.1% ± 1.7% for the top LYSO layer, 34.0% ± 6.3% for the upper BGO layer, 12.3% ± 1.3% for the lower LYSO layer, and 33.9% ± 6.9% for the bottom BGO layer. The timing resolution between each individual layer (from the top to the bottom) and a single crystal reference detector was 350 ps, 2.8 ns, 328 ps, and 2.1 ns respectively. Significance. In conclusion, the proposed four-layer DOI encoding detector achieved high performance and is an attractive choice for next-generation high sensitivity and high spatial resolution small animal positron emission tomography systems.