Cerium-doped Lutetium-yttrium Oxyorthosilicate Research Articles

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.

Timing advances of commercial divalent-ion co-doped LYSO:Ce and SiPMs in sub-100 ps time-of-flight positron emission tomography

Objective. Together with novel photodetector technologies and emerging electronic front-end designs, scintillator material research is one of the key aspects to obtain ultra-fast timing in time-of-flight positron emission tomography (TOF-PET). In the late 1990s, Cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) has been established as the state-of-the-art PET scintillator due to its fast decay time, high light yield and high stopping power. It has been shown that co-doping with divalent ions, such as Ca 2+ and Mg 2+, is beneficial for its scintillation characteristics and timing performance. Therefore, this work aims to identify a fast scintillation material to combine it with novel photosensor technologies to push the state of the art in TOF-PET. Approach. This study evaluates commercially available LYSO:Ce,Ca and LYSO:Ce,Mg samples manufactured by Taiwan Applied Crystal Co., LTD regarding their rise and decay times as well as their coincidence time resolution (CTR) with both ultra-fast high-frequency (HF) readout and commercially available readout electronics, i.e. the TOFPET2 ASIC. Main results. The co-doped samples exhibit state-of-the-art rise times of on average 60 ps and effective decay times of on average 35 ns. Using the latest technological improvements made on NUV-MT SiPMs by Fondazione Bruno Kessler and Broadcom Inc., a 3 × 3 × 19 mm3 LYSO:Ce,Ca crystal achieves a CTR of 95 ps (FWHM) with ultra-fast HF readout and 157 ps (FWHM) with the system-applicable TOFPET2 ASIC. Evaluating the timing limits of the scintillation material, we even show a CTR of 56 ps (FWHM) for small 2 × 2 × 3 mm3 pixels. A complete overview of the timing performance obtained with different coatings (Teflon, BaSO4) and different crystal sizes coupled to standard Broadcom AFBR-S4N33C013 SiPMs will be presented and discussed. Significance. This work thoroughly evaluates commercially available co-doped LYSO:Ce crystals and, in combination with novel NUV-MT SiPMs, shows a TOF performance that significantly exceeds the current state of the art.

On the Prospects of BaF2 as a Fast Scintillator for Time-of-Flight Positron Emission Tomography Systems

Future time-of-flight positron emission tomography (TOF-PET) will be in need of ultrafast scintillation materials, with potential seen in cross-luminescence (CL). BaF2, for example, shows a sub-100-ps decay time with 300 photons produced per MeV. However, it poses challenges, such as medium radiation length, low photofraction, moderate light yield, and vacuum ultraviolet (VUV) emission around 200nm. Recent developments in UV-sensitive solid-state photodetectors (SiPMs) have the potential to establish CL for ultrafast TOF in PET. In this work, we aim to study BaF2, read out by these new VUV SiPMs, as a viable alternative to cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce). In order to investigate the prospects of BaF2 in TOF-PET, we compare the CTR of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\times $ </tex-math></inline-formula> 20 mm 3 undoped and yttrium-doped BaF2 crystals, readout by Hamamatsu S13370–3050CN SiPMs with a maximum photon detection efficiency (PDE) of 14% at 200nm and air coupling, to LYSO:Ce:Ca crystals of the same size. With high-frequency readout electronics, we reached 233ps with undoped BaF2 and 213ps with BaF2:Y, while the performance of LYSO:Ce:Ca was 125ps (measured with HPK S14160–3050HS, PDE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 60$ </tex-math></inline-formula> % and glue coupling). Conducting measurements at different depth of interaction positions shows a pronounced impact on the CTR. Furthermore, we investigated the performance for systems of BaF2 with TOFPET2c ASIC measurements and Geant4 simulations for effective sensitivity comparisons to LYSO:Ce:Ca.

Simulation study of a brain PET scanner using TOF-DOI detectors equipped with first interaction position detection

Objective. The aim of this study is to evaluate the performance characteristics of a brain positron emission tomography (PET) scanner composed of four-layer independent read-out time-of-flight depth-of-interaction (TOF-DOI) detectors capable of first interaction position (FIP) detection, using Geant4 application for tomographic emission(GATE). This includes the spatial resolution, sensitivity, count rate capability, and reconstructed image quality. Approach. The proposed TOF-DOI PET detector comprises four layers of a 50 × 50 cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) scintillator array with 1 mm pitch size, coupled to a 16 × 16 multi-pixel photon counter array with 3.0 mm × 3.0 mm photosensitive segments. Along the direction distant from the center field-of-view (FOV), the scintillator thickness of the four layers is 2.5, 3, 4, and 6 mm. The four layers were simulated with a 150 ps coincidence time resolution and the independent readout make the FIP detection capable. The spatial resolution and imaging performance were compared among the true-FIP, winner-takes-all (WTA) and front-layer FIP (FL-FIP) methods (FL-FIP selects the interaction position located on the front-most interaction layer in all the interaction layers). The National Electrical Manufacturers Association NU 2-2018 procedure was referred and modified to evaluate the performance of proposed scanner. Main results. In detector evaluation, the intrinsic spatial resolutions were 0.52 and 0.76 mm full width at half-maximum (FWHM) at 0° and 30° incident γ-rays in the first layer pair, respectively. The reconstructed spatial resolution by the filter backprojection (FBP) achieved sub-millimeter FWHM on average over the whole FOV. The maximum true count rate was 207.6 kcps at 15 kBq ml−1 and the noise equivalent count rate (NECR_2R) was 54.7 kcps at 6.0 kBq ml−1. Total sensitivity was 45.2 cps kBq−1 and 48.4 cps kBq−1 at the center and 10 cm off-center FOV, respectively. The TOF and DOI reconstructions significantly improved the image quality in the phantom studies. Moreover, the FL-FIP outperformed the conventional WTA method in terms of the spatial resolution and image quality. Significance. The proposed brain PET scanner could achieve sub-millimeter spatial resolution and high image quality with TOF and DOI reconstruction, which is meaningful to the clinical oncology research. Meanwhile, the comparison among the three positioning methods indicated that the FL-FIP decreased the image degradation caused by Compton scatter more than WTA.

Comparative characterization study of LYSO:Ce crystals for timing applications

Cerium-doped Lutetium-Yttrium Oxyorthosilicate (LYSO:Ce) is one of the most widely used Cerium-doped Lutetium based scintillation crystals. Initially developed for medical detectors it rapidly became attractive for High Energy Particle Physics (HEP) applications, especially in the frame of high luminosity particle colliders. In this paper, a comprehensive and systematic study of LYSO:Ce ([Lu(1-x)Y x ]2SiO5:Ce) crystals is presented. It involves for the first time a large number of crystal samples (180) of the same size from a dozen of producers. The study consists of a comparative characterization of LYSO:Ce crystal products available on the market by mechanical, optical and scintillation measurements and aims specifically, to investigate key parameters of timing applications for HEP.

Simulation study of potential time-of-flight capabilities for a multilayer DOI-PET detector with an independent readout structure

A multilayer depth-of-interaction positron emission tomography (DOI-PET) detector with an independent readout structure has a potential advantage as a time-of-flight (TOF)-PET detector. The thin scintillator block of each detector layer can afford an improved coincidence time resolution (CTR) of ∼100 ps because the photon transfer time spread within the scintillator inherently decreases. To evaluate the potential TOF capabilities of a multilayer DOI-PET detector, which consists of thin layers of a cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) scintillator coupled to a multi-pixel photon counter (MPPC) array, we examined the detector’s CTR performance via Monte Carlo simulations. We used several types of scintillator structures: a monolithic plate, laser-processing array with 3.2 mm pitch, fine laser-processing array with 1.6 mm pitch, and pixelated array with 3.2 mm pitch, with 2, 4, 6, and 8 mm thickness values of a 25.6 mm × 25.6 mm scintillator cross-section. The MPPC array was composed of 3.0 mm × 3.0 mm photosensitive segments arranged in an 8 × 8 array. Here, we note that the CTR performance also significantly depends on the timing detection method, which generates a timing trigger signal for coincidence detection. Thus, we evaluated the CTRs for each scintillator structure by adopting four timing detection methods: using the total sum signal of 64 MPPC chips (T_sum), the maximum signal in the 64 MPPC chips (Max), the sum signal of a partial number of MPPC chips located at and in the vicinity of the γ-ray interaction position (P_sum), and the average of the timestamps generated at several MPPC chips (Ave). When using the T_sum for timing detection, the CTR full width at half-maximum values were ∼100 ps regardless of the scintillator structure. However, when using the Max signal approach, the CTRs of the monolithic plates, laser-processing arrays, and fine-pitch laser-processing arrays were drastically degraded with increasing thickness. On the other hand, the CTRs of the pixelated arrays exhibited almost no degradation. To improve the CTRs of the monolithic plate and the (fine-pitch) laser-processing array that exhibit a large light spread in the scintillator block, we applied the P_sum and Ave methods. The resulting CTRs significantly improved upon using P_sum; however, in the Ave approach the improvement effect disappeared when the thickness was <6 mm in case of our simulation.

Dual-Energy Micro-CT Using GAGG:Ce and LYSO:Ce Scintillators

Dual-energy computed tomography (CT) has a better material determination capability than traditional CT. In this article, we develop a dual-layer dual-energy micrometer CT (DEMCT) imaging system with micrometer spatial resolution. The detector of the DEMCT system uses cerium-doped gadolinium aluminum gallium garnet (GAGG:Ce) and cerium-doped lutetium yttrium oxyorthosilicate (LYSO:Ce) scintillators to absorb X-rays of different energies, which is a new application of GAGG:Ce scintillator. As GAGG:Ce and LYSO:Ce have high light yields and their emission wavelengths match well with the quantum efficiency of common charge-coupled device (CCD), thicknesses of the scintillators are allowed to be thin, which is of great significance for high-resolution imaging due to the limitation of the depth of field of objectives. We propose a mechanical adjustment method for registering high-energy and low-energy images, which saves the postprocessing time and avoids oscillating or nonconvergent iteration results of mathematical registration methods. Meanwhile, in order to increase the dual-energy spectra separation of materials detection in the DEMCT, a fast optimal kilovolt peak selection method is proposed. Results show that the DEMCT has the spatial resolution of 2 μm in low-energy imaging and 4 μm in high-energy imaging. In addition, materials of a silica mixture sample are clearly separated by the pixel selection of the energy map based on the DEMCT data. The proposed DEMCT system has the potential to separate materials that cannot be correctly determined on traditional CT.

A Smart Multi-Plane Detector Design for Ultrafast Electron Beam X-ray Computed Tomography.

In this paper, a smart detector design for novel multi-plane ultrafast electron beam X-ray computed tomography is presented. The concept is based on multi-plane electron beam scanning on a transparent X-ray target and elongated cuboid-shape scintillation detectors for radiation detection over an extended axial scanning range. The optical part of the scintillation detector acts as both an X-ray sensitive scintillator with a fast time response and a light guide. With that, we reduce detector complexity, number of detector elements, overall power consumption, and detector costs. We investigated the performance of this new multi-plane detector design with an evaluation detector setup that is made of cerium doped lutetium yttrium oxyorthosilicate (LYSO:Ce) as scintillation material and an avalanche photodiode (APD) array. Thereby, we assessed two design variants: A monolithic LYSO bar detector and a sandwich detector made of multiple LYSO crystals and glass light-guides. Both types reveal excellent linear detector responses, long-term stabilities, and comparable signal qualities.

Directional Control and Enhancement of Light Output of Scintillators by Using Microlens Arrays.

Scintillators play an important role in the field of nuclear radiation detection, such as nuclear medical imaging, dark matter detection, nuclear physics experiments, and national security. However, the light extraction efficiency of a scintillator with a high refractive index is severely restricted because of the total internal reflection. In this paper, microlens arrays have been applied onto the surface of a cerium-doped lutetium-yttrium oxyorthosilicate scintillator to improve the light extraction efficiency and to control the directivity of the light output. Compared to that of a reference sample, a 3.26-fold enhancement with an emission angle of 45° has been obtained by using microlens arrays with optimal parameters. It was also found that the enhancement ratio can be affected by the refractive index of the microlens, the spacing of individual microlens. The control mechanism of microlens arrays is revealed by a combination of simulations and experiments. X-ray imaging characteristics exhibit an improved gray scale amplitude without any loss of the spatial resolution. The present results suggest that the application of microlens arrays to scintillators is beneficial to the field of nuclear radiation detection.

Fabrication and luminescent properties of fine-grained cerium-doped lutetium-yttrium oxyorthosilicate ceramics

When compared to Lu2SiO5:Ce (LSO:Ce), cerium-doped lutetium-yttrium oxyorthosilicate solid solutions not only possess a similar density, but they also exhibit good scintillation properties (high light yield, short decay time and good energy resolution). Fine-grained Lu1.8Y0.2SiO5:Ce (LYSO:Ce) ceramics were fabricated via Spark Plasma Sintering (SPS) at a temperature of 1050 °C and an applied pressure of 100 MPa from a nano-sized LYSO:Ce powder. The average grain size of the sintered LYSO:Ce ceramics is 210 nm. To eliminate the oxygen vacancies that result from the sintering process, the sintered LYSO:Ce ceramics were subsequently annealed at 950 °C for 40 h in air. The in-line transmittance of the LYSO:Ce ceramics is 1.7% at 420 nm. The light yield of the annealed LYSO:Ce ceramics is 23,600 photons/MeV. It is suggested that SPS combined with annealing in air is a promising route to fabricate fine-grained LYSO:Ce ceramics with good scintillation performance.

SiPM-based dual-ended-readout DOI-TOF PET module based on mean-time method

Positron emission tomography (PET) with high resolution and high sensitivity is desirable for detecting cancers and neurological diseases. In this work, a depth-of-interaction (DOI)-time-of-flight (TOF) PET has been attempted to achieve both high spatial and high timing resolution. Dual-ended readout is a simple technique that can provide excellent timing and DOI resolutions and consistent signal arrival times, regardless of the DOI position along the scintillation crystal, as a mean-time method is used. A dual-ended readout DOI-TOF PET module consisting of a 6 × 6 array of 2 × 2 × 20 mm3 saw-cut cerium-doped lutetium-yttrium oxyorthosilicate (LYSO) crystals is constructed. Both ends of the LYSO crystal array are optically coupled to a multi-pixel photon counter (MPPC) with 4 × 4 channels. The sixteen MPPC outputs are reduced to four position signals using a charge division circuit (CDC) board, and the timing signal is extracted from the common cathode of the MPPC. The four position signals from the MPPC are digitized by a DRS4-based high-speed waveform digitizer with a sampling rate of 5 GSa/s. A 22Na source is placed in front of a reference detector and at the side of the dual-ended readout DOI-TOF PET module in five steps of 2 mm, 6 mm, 10 mm, 14 mm, and 18 mm to measure the DOI, coincidence timing resolutions (CTRs), and mean-times. The full-width-half-maximums (FWHMs) of DOI resolutions and CTRs varied from 3.0 mm to 3.8 mm, with an average of 3.5 mm, and from 333 ps to 367 ps, with an average of 349 ps, respectively. The average of the slopes of the mean-time versus DOI position, for the 36 crystals, was −0.60 ± 1.68 ps/nm, which was consistent with the null value. The dual-ended readout DOI-TOF PET module based on the mean-time method produced both good DOI and CTRs, and consistent signal arrival times. The found solution would be the most advantageous in the small aperture PET systems, such as those for brain and breast imaging.

NUV-Sensitive Silicon Photomultiplier Technologies Developed at Fondazione Bruno Kessler.

Different applications require different customizations of silicon photomultiplier (SiPM) technology. We present a review on the latest SiPM technologies developed at Fondazione Bruno Kessler (FBK, Trento), characterized by a peak detection efficiency in the near-UV and customized according to the needs of different applications. Original near-UV sensitive, high-density SiPMs (NUV-HD), optimized for Positron Emission Tomography (PET) application, feature peak photon detection efficiency (PDE) of 63% at 420 nm with a 35 um cell size and a dark count rate (DCR) of 100 kHz/mm2. Correlated noise probability is around 25% at a PDE of 50% at 420 nm. It provides a coincidence resolving time (CRT) of 100 ps FWHM (full width at half maximum) in the detection of 511 keV photons, when used for the readout of LYSO(Ce) scintillator (Cerium-doped lutetium-yttrium oxyorthosilicate) and down to 75 ps FWHM with LSO(Ce:Ca) scintillator (Cerium and Calcium-doped lutetium oxyorthosilicate). Starting from this technology, we developed three variants, optimized according to different sets of specifications. NUV-HD–LowCT features a 60% reduction of direct crosstalk probability, for applications such as Cherenkov telescope array (CTA). NUV-HD–Cryo was optimized for cryogenic operation and for large photosensitive areas. The reference application, in this case, is the readout of liquid, noble-gases scintillators, such as liquid Argon. Measurements at 77 K showed a remarkably low value of the DCR of a few mHz/mm2. Finally, vacuum-UV (VUV)-HD features an increased sensitivity to VUV light, aiming at direct detection of photons below 200 nm. PDE in excess of 20% at 175 nm was measured in liquid Xenon. In the paper, we discuss the specifications on the SiPM related to different types of applications, the SiPM design challenges and process optimizations, and the results from the experimental characterization of the different, NUV-sensitive technologies developed at FBK.

Brief measurement of X-ray spectra using a lutetium-yttrium-oxyorthosilicate crystal and a micro-photomultiplier tube

Brief measurement of X-ray spectra using a detector, consisting of an LYSO [Cerium doped Lutetium Yttrium Oxyorthosilicate, (Lu,Y)2SiO5:Ce] crystal and a μPMT (micro-photomultiplier tube) is described. X-ray photons are detected using the LYSO crystal and a 0.5-mm-diameter 3.0-mm-thick lead pinhole, and scintillation photons produced in the crystal are detected using the μPMT. The negative output voltages from the μPMT are input to a high-speed inverse-voltage-voltage amplifier, and the event pulses from the amplifier are sent to a multichannel analyzer to perform pulse-height analysis. Dark counts from the μPMT are not counted at all by increasing the time constant of the amplifier. Without the pinhole, the energy resolutions was approximately 40% at 59.5 keV. The photon energy was determined using two-point calibration using iodine-Kα fluorescence and tungsten Kα-photons from an X-ray-tube target. At a tube voltage of 100 kV, the maximum tube current without event-pulse pileups was 0.24 mA. Using the LYSO-μPMT detector, both the maximum and maximum-count energies increased with increasing tube voltage, and the maximum count rate was 72 kilocounts per second at a tube voltage of 100 kV and a current of 0.24 mA. Dual-energy computed tomography utilizing K- and L-edges was accomplished using gadolinium contrast media.

Performance of a PET Insert for High-Resolution Small-Animal PET/MRI at 7 Tesla.

We characterize a compact MR-compatible PET insert for simultaneous preclinical PET/MRI. Although specifically designed with the strict size constraint to fit inside the 114-mm inner diameter of the BGA-12S gradient coil used in the BioSpec 70/20 and 94/20 series of small-animal MRI systems, the insert can easily be installed in any appropriate MRI scanner or used as a stand-alone PET system. Methods: The insert consists of a ring of 16 detector-blocks each made from depth-of-interaction-capable dual-layer-offset arrays of cerium-doped lutetium-yttrium oxyorthosilicate crystals read out by silicon photomultiplier arrays. Scintillator crystal arrays are made from 22 × 10 and 21 × 9 crystals in the bottom and top layers, respectively, with respective layer thicknesses of 6 and 4 mm, arranged with a 1.27-mm pitch, resulting in a useable field of view 28 mm long and about 55 mm wide. Results: Spatial resolution ranged from 1.17 to 1.86 mm full width at half maximum in the radial direction from a radial offset of 0-15 mm. With a 300- to 800-keV energy window, peak sensitivity was 2.2% and noise-equivalent count rate from a mouse-sized phantom at 3.7 MBq was 11.1 kcps and peaked at 20.8 kcps at 14.5 MBq. Phantom imaging showed that features as small as 0.7 mm could be resolved. 18F-FDG PET/MR images of mouse and rat brains showed no signs of intermodality interference and could excellently resolve substructures within the brain. Conclusion: Because of excellent spatial resolvability and lack of intermodality interference, this PET insert will serve as a useful tool for preclinical PET/MR.

PKU-PET-II: A novel SiPM-based PET imaging system for small animals

The objective of this study was to introduce, describe, and validate the performance of a novel preclinical silicon photomultiplier (SiPM)-based PET system (PKU-PET-II). Briefly, the detector assembly consisted of cerium-doped lutetium–yttrium oxyorthosilicate (LYSO) crystals, with dimensions of 2×2×15 mm3, that offered a 60 mm transaxial field of view (FOV) and 32 mm axial FOV, respectively. The compact front-end electronics readout and digital controller implemented architecture in the FPGA were noteworthy improvements in PKU-PET-II over its predecessor (PKU-PET-I). Based on the National Electrical Manufacturers Association (NEMA) NU 04-2008 standards, the design of the PKU-PET-II system was validated by a phantom experiment. The results presented spatial resolution (evaluated as full width at half maximum) with a system range from 1.68±0.07 to 2.31±0.03 mm at the FOV center and from 1.43±0.02 to 2.10±0.10 mm at the 1/4th axial FOV, respectively. The system’s absolute sensitivity at the center position was 1.35% with the coincidence window of 6 ns and energy window of 300–700 keV. In addition, the NEMA image quality phantom and an animal study results validated the system imaging performance in preclinical imaging application. In conclusion, this SiPM-based, small-animal PET system (PKU-PET-II) provided higher-resolution, adequate sensitivity, and excellent image quality and has potential as a useful tool for real-time imaging of disease progression and development in vivo.

Evaluation of linearly-graded SiPMs for high resolution small-animal PET

The performance of a position-sensitive silicon photomultiplier (PS-SiPM), a linearly graded silicon photomultiplier (LG-SiPM), was evaluated for use as a high resolution small-animal PET detector. This SiPM utilizes a combined resistive /capacitive current divider and a double quenching resistor in every SiPM microcell for position estimation. In this paper, using a 6 × 6 array of 0.45 × 0.45 × 6 mm3 polished cerium-doped lutetium-yttrium oxyorthosilicate (LYSO) crystals, coupled to the center of the SiPM, we evaluated the detector performance, with a focus on applications in very high resolution small-animal PET. Measurements of signal-to-noise ratio, energy resolution and flood histogram quality were performed at different over-voltages (from ∼1.0 V to ∼7.5 V) and different temperatures (–6.5 °C, 0 °C, 10 °C and 20 °C) to find the optimal working conditions. The timing resolution was measured at an over-voltage of 3.0 V and at different temperatures. The results showed that all the crystals in the LYSO array could be clearly resolved, even at room temperature (20 °C). The best flood histogram quality was obtained at an over-voltage of ∼3.0 V and a temperature of –6.5 °C. Under these conditions, the average energy resolution, signal-to-noise ratio, and coincidence timing resolution were 22.3 ± 2.7%, 269.0 ± 24.2 and 828 ± 43 ps, respectively.

A Dual-Ended Readout Detector Using a Meantime Method for SiPM TOF-DOI PET

We have investigated a dual-ended readout detector based on two silicon photo-multipliers (SiPMs) using a 30 mm long cerium-doped lutetium-yttrium oxyorthosilicate ( ${\rm Lu}_{0.5}{{\rm Y}_{1.4}}{\rm Si}_{0.5}:{\rm Ce}$ , LYSO) crystal to study the feasibility of a time-of-flight (TOF) and depth-of-interaction (DOI) positron emission tomography (PET). To improve the timing resolution in the dual-ended readout detector, a novel meantime measurement was employed. The meantime was obtained by averaging two signal arrival times at two SiPMs, each of which was attached to both ends of the LYSO crystal. The meantime method minimizes the arrival time difference along the crystal. Both timing and DOI resolutions using the meantime method were better than those of a single-ended readout detector.

Silicon photomultiplier modules for MRI-compatible PET

Silicon photomultiplier (SiPM) modules were developed for use in positron emission tomography-magnetic resonance imaging (PET-MRI), which is a hybrid medical imaging technology. A PET-MRI is very efficient in the early diagnosis of representative senile diseases, including cancer, Alzheimer's disease, and Parkinson's disease. SiPMs comprise the core image sensor for MR-compatible PET applications since they have a low operational voltage, high gain, good timing resolution, ruggedness, insensitivity to magnetic fields, compactness, and low cost. In PET systems, SiPM microcells can be optimized by making a trade-off between photon detection efficiency (PDE) and dynamic range. The SiPM modules used in this study were fabricated at the National NanoFab Center (NNFC) of South Korea by using a customized CMOS processes. The SiPM modules were evaluated by first packaging them with a cost-effective PCB package instead of with a conventional ceramic package. Measurements on 1,400 SiPMs indicated a uniform breakdown voltage of 20.54 V with a standard deviation of 0.07 V. Moreover, the SiPM modules present a high and uniform energy resolution of 13.6% with a standard deviation of 0.5% at 511 keV with 3 × 3 × 20 mm3 cerium-doped lutetium-yttrium oxyorthosilicate (Lu2(1−x)Y2xSiO5:Ce, LYSO) crystal coupling. These results indicated that the proposed devices offer adequate performance to form the foundation of an image sensor technology for MRI-compatible PET.

Structural, mechanical and light yield characterisation of heat treated LYSO:Ce single crystals for medical imaging applications

Five single crystals of cerium-doped lutetium yttrium oxyorthosilicate (LYSO:Ce) grown by the Czochralski method were submitted to structural characterisation by X-ray (XRD) and neutron (ND) diffraction, scanning (SEM) and transmission (TEM) electron microscopy and energy dispersive microanalysis (EDS). The Ultimate Tensile Strength (UTS), the Young Modulus (YM) and the Light Yield (LY) of the samples were also measured in order to correlate the mechanical and the optical behaviour of the crystals with the characteristics of their microstructure. Two of the samples analysed were also heat treated at 300°C for 10h to evidence possible variations induced by the temperature in the optical and mechanical response of the crystals. Results showed that the mean compositional variations evidenced by the structural analyses do not affect the mechanical and optical behaviour of the samples. On the contrary, the thermal treatment could induce the formation of coherent spherical particles (size 10 to 15nm), not uniformly distributed inside the sample, that strongly reduce the UTS and YM values, but it does not affect the optical response of the crystal. This latter result was attributed to the low value of the heating temperature (300°C) that is not sufficiently high to induce annealing of the oxygen vacancies traps that are responsible of the deterioration of the scintillation properties of the LYSO:Ce crystals.This study was carried out in the framework of the Crystal Clear Collaboration (CCC).

GATE simulation of a LYSO-based SPECT imager: Validation and detector optimization

This paper presents a small animal SPECT system that is based on cerium doped lutetium–yttrium oxyorthosilicate (LYSO) scintillation crystal, position sensitive photomultiplier tubes (PSPMTs) and parallel hole collimator. Spatial resolution test and animal experiment were performed to demonstrate the imaging performance of the detector. Preliminary results indicated a spatial resolution of 2.5mm at FWHM that cannot meet our design requirement. Therefore, we simulated this gamma camera using GATE (GEANT 4 Application for Tomographic Emission) aiming to make detector spatial resolution less than 2mm. First, the GATE simulation process was validated through comparison between simulated and experimental data. This also indicates the accuracy and effectiveness of GATE simulation for LYSO-based gamma camera. Then the different detector sampling methods (crystal size with 1.5, and 1mm) and collimator design (collimator height with 30, 34.8, 38, and 43mm) were studied to figure out an optimized parameter set. Detector sensitivity changes were also focused on with different parameters set that generated different spatial resolution results. Tradeoff curves of spatial resolution and sensitivity were plotted to determine the optimal collimator height with different sampling methods. Simulation results show that scintillation crystal size of 1mm and collimator height of 38mm, which can generate a spatial resolution of ~1.8mm and sensitivity of ~0.065cps/kBq, can be an ideal configuration for our SPECT imager design.