LYSO Crystals Research Articles

A second-generation virtual-pinhole PET device for enhancing contrast recovery and improving lesion detectability of a whole-body PET/CT scanner.

We have developed a second-generation virtual-pinhole (VP) positron emission tomography (PET) device that can position a flat-panel PET detector around a patient's body using a robotic arm to enhance the contrast recovery coefficient (CRC) and detectability of lesions in any region-of-interest using a whole-body PET/computed tomography (CT) scanner. We constructed a flat-panel VP-PET device using 32 high-resolution detectors, each containing a 4 4 MPPC array and 16 16 LYSO crystals of 1.0 1.0 3.0mm3 each. The flat-panel detectors can be positioned around a patient's body anywhere in the imaging field-of-view (FOV) of a Siemens Biograph 40 PET/CT scanner by a robotic arm. New hardware, firmware and software have been developed to support the additional detector signals without compromising a scanner's native functions. We stepped a 22 Na point source across the axial FOV of the scanner to measure the sensitivity profile of the VP-PET device. We also recorded the coincidence events measured by the scanner detectors and by the VP-PET detectors when imaging phantoms of different sizes. To assess the improvement in the CRC of small lesions, we imaged an elliptical torso phantom measuring 316 228 162mm3 that contains spherical tumors with diameters ranging from 3.3 to 11.4mm with and without the VP-PET device. Images were reconstructed using a list mode Maximum-Likelihood Estimation-Maximization algorithm implemented on multiple graphics processing units (GPUs) to support the unconventional geometries enabled by a VP-PET system. The mean and standard deviation of the CRC were calculated for tumors of different sizes. Monte Carlo simulation was also conducted to image clusters of lesions in a torso phantom using a PET/CT scanner alone or the same scanner equipped with VP-PET devices. Receiver operating characteristic (ROC) curves were analyzed for three system configurations to evaluate the improvement in lesion detectability by the VP-PET device over the native PET/CT scanner. The repeatability in positioning the flat-panel detectors using a robotic arm is better than 0.15mm in all three directions. Experimental results show that the average CRC of 3.3, 4.3, and 6.0mm diameter tumors was 0.82%, 2.90%, and 5.25%, respectively, when measured by the native scanner. The corresponding CRC was 2.73%, 6.21% and 10.13% when imaged by the VP-PET insert device with the flat-panel detector under the torso phantom. These values may be further improved to 4.31%, 9.65% and 18.01% by a future dual-panel VP-PET insert device if DOI detectors are employed to triple its detector efficiency. Monte Carlo simulation results show that the tumor detectability can be improved by a VP-PET device that has a single flat-panel detector. The improvement is greater if the VP-PET device employs a dual-panel design. We have developed a prototype flat-panel VP-PET device and integrated it with a clinical PET/CT scanner. It significantly enhances the contrast of lesions, especially for those that are borderline detectable by the native scanner, within regions-of-interest specified by users. Simulation demonstrated the enhancement in lesion detectability with the VP-PET device. This technology may become a cost-effective solution for organ-specific imaging tasks.

An FPGA-Based Fast Linear Discharge Readout Scheme Enabling Simultaneous Time and Energy Measurements for TOF-PET Detectors

The readout electronics in time-of-flight positron emission tomography (TOF-PET) detector modules conventionally contain a separate timing circuit and a separate energy measurement circuit. For channel-by-channel readouts, the front-end electronics will be unacceptably large and must rely on application specific integrated circuit (ASIC) chips. In this paper, we present a novel field programmable gate array (FPGA)-based readout scheme that unifies the time and energy measurements with only one analog amplifier per channel in addition to the FPGA. By constructing a simple and effective fast linear discharge circuit, one time-to-digital converter (TDC) per channel can implement time and energy readouts simultaneously with a short measurement dead time. To evaluate the performance of the new approach, a 16-channel PET detector module consisting of LYSO crystals coupled with a silicon photomultiplier (SiPM) array was constructed connecting to the readout electronics. Using a reference detector with a 164.7-ps time resolution to perform 22Na coincidence measurements, the average energy, and time resolution among 16 channels were measured as 12.44% and 372.6 ps, respectively. The typical measurement dead time is approximately 300 ns. The test results show the simplicity and feasibility of the proposed readout scheme, which is particularly useful for laboratory instrumentation.

Preliminary results on a small animal SPECT system based on H13700 PSMPT coupled with CRY018 array

Abstract In this paper, we describe a new single photon emission computed tomography (SPECT) device made up of two detectors (High Resolution System, HRS) integrated into a rotation structure suitable for preclinical imaging applications. The overall objective is to develop an improved scintigraphic system based on our previous detector having high spatial resolution. For this purpose, we have assessed the performance of the new position-sensitive photomultiplier tube (PSPMT) Hamamatsu H13700 with 256 anodes, which replaces the Hamamatsu H9500 model. The adoption of the new PSPMT required the development of a dedicated compact electronics: a new resistive chain readout was optimized and integrated with an ADC system. Moreover, the new detector was engineered from both the mechanical and the electronic point of view. In fact, the entire system was miniaturized in order to reduce the overall dimensions. Alternatively, to the LYSO scintillator used in the previous detector, we have evaluated the CRY018 scintillator crystal, which provides a high light output, a medium density, a short decay time and excellent energy resolution, obtaining high efficiency comparable with the LYSO scintillator. We assessed the detector with the H13700 coupled with LYSO and CRY018 crystals of the same size. The H13700 model shows better quantum efficiency on crystal emission light than the H9500, and besides, the light collection near the edges has improved. Moreover, the use of CRY018 is preferred over the LYSO matrix for issues related to the reduced natural radioactivity of 176Lu. The experimental results were confirmed by an extensive evaluation carried out by GATE/GEANT4 Monte Carlo simulations. Finally, the performance results achieved, pave the way to the development of a Small Animal Imaging SPECT made up by two detectors based on the H13700 coupled with CRY018 crystal array.

Impact on timing and light extraction of a photonic crystal as measured on a half patterned LYSO crystal: implications for time of flight PET imaging

The improvement of the energy and of the timing resolution is always a challenge in scintillation-based detectors. A large fraction of the photons produced by scintillation remains trapped inside the crystal. Photonic crystals have been suggested as a solution to improve light extraction. Here we will present results obtained with a nanostructured TiO2 coating on a 50×50 mm2 LYSO crystal. The objective of the present paper is to characterize the performance of this coating in both light extraction and timing performance as both parameters are critical to spatial resolution of PET systems. To avoid tricky calibration problems, especially for timing, we have manufactured a monolithic crystal with one half patterned with a photonic crystal and with one half bare. We are rotating the crystal π/2 relative to the photo-detector between each measure. We have chosen a digital Si-PM Philips DPC 3200 as photo-detector due to its excellent timing precision and stability. The impact on light extraction of the photonic crystal is very strong as 30% of the light only escaped through the naked face vs 70% through the textured face for each position. The timing effect is much more subtle and quite at odd with previous results. By averaging the measurements on four positions, we are detecting a time lag effect of photon extraction with a probability of 98%. The average lag is only of 17 ps on the detection in the textured part of the crystal. This effect, although without practical consequences for PET imaging, is nevertheless perplexing as we were foreseeing a faster exit of photons on the textured face. We propose an explanation for the effect observed.

Modeling the Nonlinear Response of Silicon Photomultipliers

A statistical model of the nonlinear response of silicon photomultipliers is presented. It includes losses of both the photodetection efficiency and the gain during pixel recovery periods as well as the effect of correlated and uncorrelated noise. The model provides either the mean output charge of a SiPM for incident light pulses of arbitrary shape or the output current for continuous light. The dependence of the SiPM response on both the overvoltage and the pulse shape is also properly described. The model has been validated for two different silicon photomultipliers using scintillation light pulses from a LYSO crystal as well as continuous light from a LED. Good agreement is found with experimental data at moderate nonlinearity.

Feasibility of PET detector readout by high-density Silicon Photomultipliers with epitaxial quenching resistors

Silicon photomultipliers (SiPMs) have high gain, low bias voltage, excellent timing properties and are insensitive to magnetic fields, which have become the best readout choice for advanced positron emission tomography (PET) systems, such as PET/MRI instruments and time-of-flight (TOF) PET imaging. Novel Device Laboratory (NDL) has been developing an unique SiPM technology that uses the bulk resistors in the epitaxial layer as quenching resistors (EQR SiPM); it features small micro cells with high fill factor, fast response to even a single photon, and simple fabrication technology. This paper reports the results to verify that EQR SiPM is promising in the applications of advanced PET imaging. For the EQR SiPM with active area of 3 × 3 mm2 and 90000 individual cells, the dark count rate of 6.3 MHz at room temperature, a peak PDE of 34% for 420 nm photons and an intrinsic single photon timing resolution (SPTR) of 81 ps at 9 V over-voltage were obtained. By employing a pair of EQR SiPMs 1-to-1 coupled with 2.84 × 2.84× 6 mm3 LYSO crystals, the energy resolution (ER) of ∼10.1% and the coincidence timing resolution (CTR) of ∼195 ps (FWHM) were demonstrated; by employing a pair of EQR SiPMs 1-to-1 coupled with 2.84 × 2.84× 10 mm3 LYSO crystals, the ER of ∼9.8% and the CTR of ∼226 ps (FWHM) were demonstrated.

Design and Detector Performance of the PET Component of the TRIMAGE PET/MR/EEG Scanner

The TRIMAGE project aims at developing a brain-dedicated PET/MR/EEG system able to perform simultaneous PET, MR, and EEG acquisitions for application in schizophrenia. Both PET and MR components have been designed in this project. The PET component consists of a full ring with 18 sectors each comprising three square detector modules. The modules are based on dual-layer staggered matrices of LYSO crystals read out by silicon photomultipliers. This paper describes in full detail the final version of the PET detectors and the related electronics. It also reports on the preliminary performance of a pair of sectors in terms of pixel resolvability index (RI), energy resolution, singles count rate capability, and coincidence time resolution (CTR). The procedures used for the optimization and calibration of PET detector are described. Results demonstrate the pixel/layer identification performance with an RI of about 0.2 while the energy resolution resulted in 20% and 22% FWHM for the bottom and top layer, respectively. The maximum singles count rate of a PET detector is about 700 kcps and the CTR of two sectors is 515 ps.

Four-layered DOI-PET detector with quadrisected top layer crystals

Previously, we had developed a four-layered depth-of-interaction (DOI) PET detector based on the light sharing method. Reflectors , which were inserted in every two lines of crystal segments and shifted differently for each layer, projected 3D crystal positions onto a 2D position histogram without any overlapping after applying the Anger-type calculation. The best crystal separation we ever obtained was for the four-layered 32 × 32 array of LYSO crystals sized at 1.45 × 1.45 × 5 mm 3 . However, assembling small crystals tended to cost a lot, and fine tuning of the front-end circuit was required to get fine crystal identification. In this paper, therefore, we proposed a more practical four-layered DOI detector. Its key concept was that the crystals in the top layer, which have the highest detection efficiency, are the biggest contributors to the PET spatial resolution. We applied two new ideas: (1) using quarter size crystals only for the first (top) layer and (2) inserting a thin light guide between the first and the second layers of the crystal array. In the developed prototype detector, we used 24 × 24 LYSO crystals of quarter size (1.4 × 1.4 × 5.0 mm 3 ) in the first layer and the other layers were 12 × 12 arrays of crystals of 2.8 × 2.8 × 5.0 mm 3 . For better crystal identification of small crystals in the first layer, we optimized the optical condition between crystals by using an optical adhesive and air. Also, the thin light guide of 0.5 mm thickness was inserted between the first and the second layers for improvement of crystal identification of the first layer. With the appropriate insertion of the light guide, all crystals of the first layer were identified as well as the crystals in the other layers. Our developed four-layered DOI detector showed good potential for high spatial resolution without a large increase in the number of crystals.

A compact scintillator based position sensitive detector system for gamma ray tracking applications

Abstract A Cerium doped Lutetium Yttrium Orthosilicate (LYSO) crystal based precision gamma detection system has been developed using a position sensitive photomultiplier tube, with minimal pulse processing requirements. The assembly’s detailed characterization has been reported in this work. This kind of a setup with a precise spatial resolution can be very useful for scanning high purity germanium detectors that use pulse shape analysis algorithms for signal processing in gamma ray spectroscopy experiments. The highly pure LYSO crystal used here is custom-made and the overall setup including the multi anode readout photomultiplier tube is compact. The system has been tested for energy, timing and position information regarding gamma photons interacting within the crystal using a coincidence mechanism. Position sensitivity has been found to be in the anticipated range of a few millimeters. This kind of a detector can be used in various fields, vis-a-vis, medical imaging, radioactive waste management, nuclear proliferation control, and various applications that deal with precisely identifying and locating radioactivity.

TOF studies for dedicated PET with open geometries

Recently, two novel PET devices have been developed with open geometries, namely: breast and prostate-dedicated scanners. The breast-dedicated system comprises two detector rings of twelve modules with a field of view of 170 mm × 170 mm × 94 mm. Each module consists of a continuous trapezoidal LYSO crystal and a PSPMT. The system has the capability to vary the opening of the rings up to 60 mm in order to allow the insertion of a needle to perform a biopsy procedure. The prostate system has an open geometry consisting on two parallel plates separated 28 cm. One panel includes 18 detectors organized in a 6 × 3 matrix while the second one comprises 6 detectors organized in a 3 × 2 matrix. All detectors are formed by continuous LYSO crystals of 50 mm × 50 mm ×15 mm, and a SiPM array of 12 × 12 individual photo-detectors. The system geometry is asymmetric maximizing the sensitivity of the system at the prostate location, located at about 2/3 in the abdomen-anus distance. The reconstructed images for PET scanners with open geometries present severe artifacts due to this peculiarity. These artifacts can be minimized using Time Of Flight information (TOF). In this work we present a TOF resolution study for open geometries. With this aim, the dedicated breast and prostate systems have been simulated using GATE (8.1 version) with different TOF resolutions in order to determine the image quality improvements that can be achieved with the existing TOF-dedicated electronics currently present in the market. The images have been reconstructed using the LMOS algorithm including TOF modeling in the calculation of the voxel-Line Of Response emission probabilities.

The EDM polarimeter development at COSY-Jülich

The JEDI (Julich Electric Dipole moment Investigations) collaboration performs a set of experiments at the COSY storage ring in Julich, to search for the Electric Dipole Moments (EDMs) of charged particles (JEDI Collaboration [2011]). A measurement of proton and deuteron EDMs is a sensitive probe of yet unknown CP violation. The method of charged particle EDM search will exploit stored polarized beams in order to observe a miniscule rotation of the polarization axis as a function of time due to the interaction of a finite EDM with large electric fields. Key challenge is the provision of a sensitive and efficient method to determine the tiny change of the beam polarization. Elastic scattering of the beam particles on carbon nuclei will provide the polarimetry reaction. To perform these measurements, an EDM polarimeter needs to be developed. The polarimetry concept realized within the JEDI collaboration is based on a heavy crystal (LYSO) hadron calorimeter. LYSO as a fast, dense and radiation hard, novel scintillating material was chosen to fulfill these specifications. The polarimeter is designed in a compact and modular fashion consisting of modules made from LYSO crystals coupled to silicon photomultipliers (SiPM).

CaloCube and “Tracker In Calorimeter” projects for the direct measurement of high energy charged astro-particles and gamma rays.

Measurements of high energy cosmic rays in the “knee” region (about 1015 eV) are currently available only with ground detectors: new observations of cosmic particles up to these energies with direct measurements are one of the main goals of the next generation space experiments. To achieve those aims, a large acceptance, good energy resolution and particle identification are needed. CaloCube is the design of a space borne calorimeter which is capable to accept particles coming from any direction, increasing the acceptance with respect to traditional telescopes. A good performance for both hadronic and electromagnetic showers is achieved with a 3-D sampling capability: the basic picture of CaloCube is a cubic homogeneous calorimeter which consists of cubic scintillating crystals. MC simulations, concerning different materials and geometrical configurations, and several beam tests with different versions of the CaloCube prototype have been employed to optimize both the detector design and the data analysis method. Taking advantage of the CaloCube project, the space experiment HERD (“High Energy Cosmic Radiation Detection”) will include a large acceptance cubic calorimeter with cubic LYSO crystals. It will be installed on-board of the Chinese space station around 2025. Beside the charged particle observations, high energy gamma-rays provide direct information about the galactic cosmic ray sources. A new project named “Tracker In Calorimeter” (TIC) was approved by INFN in 2017 with the main purpose of the optimization of the calorimeter design for the reconstruction of the gamma-ray direction, without the requirement of additional not homogeneous pre-shower detector. A TIC prototype was recently assembled and tested at the PS-CERN and SPS-CERN accelerators.

Novel method to measure the intrinsic spatial resolution in PET detectors based on monolithic crystals

The main aim of this work is to provide a method to retrieve the intrinsic spatial resolution of a gamma-ray detector block based on monolithic crystals within an assembled scanner. This method consists on a discrimination of the data using a software collimation process. The results are compared with an alternative method of separating two detector blocks far enough to produce a “virtual” source collimation due to the geometric constraints on the allowed coincidence event angles.A theoretical model has been deduced to fit the measured light distribution profiles, allowing estimating the detector intrinsic spatial resolution. The detector intrinsic spatial resolution is expected to follow a Gaussian distribution and the positron-emitter source shape, given the small size of a 22Na source with 0.25 mm in diameter, can be assumed to follow a Lorentzian profile. However, the collimation of the data modifies the source shape that is no longer a pure Lorentzian distribution. Therefore, the model is based on the convolution of a Gaussian shaped distribution (contribution of the detector) and a modified Lorentzian distribution (contribution of the collimated source profile) that takes into account the collimation effect.Three LYSO crystals geometries have been studied in the present work, namely a 10 mm thick trapezoidal monolithic block, and two rectangular monolithic blocks with thicknesses of 15 mm and 20 mm, respectively. All the blocks have size dimensions of 50 mm × 50 mm. The experimental results yielded an intrinsic detector spatial resolution of 0.64 ± 0.02 mm, 0.82 ± 0.02 and 1.07 ± 0.03 mm, for the 10 mm, 15 mm and 20 mm thick blocks, respectively, when the source was placed at the center of the detector. The detector intrinsic spatial resolution was moreover evaluated across one of the axis of each crystal. These values worsen to an average value of 0.68 ± 0.04 mm, 0.90 ± 0.14 and 1.29 ± 0.19 mm, respectively, when the whole crystal size is considered, as expected. These tests show an accurate method to determine the intrinsic spatial resolution of monolithic-based detector blocks, once assembled in the PET system.

Anisotropy in pulsed laser performance of Nd:LYSO crystal

We report on a Cr:YAG Q-switched laser using monoclinic, disordered Nd:LYSO crystal as gain medium. Anisotropy in pulsed laser performances is investigated with samples in different cut directions. With Y-cut Nd:LYSO, pulse energy of 150.8 μJ and peak power of 14.7 kW are achieved at 1079 nm. However, in the case of X-cut Nd:LYSO, multi-wavelength emission at 1076, 1077.5 and 1079 nm are observed, the total pulse energy and peak power are 120.9 μJ and 7.8 kW respectively.

230. A novel geometry for gamma-prompt imaging in proton therapy

Purpose Proton therapy is a radiotherapy technique particularly engaging since it allows for a well-defined penetration and dose delivery. The estimation of the beam penetration is essentially based on the preliminary treatment planning. Real-time and in vivo estimation of the range of the proton beam will be very useful [1] : in this context, imaging from gamma-prompt plays a key role for therapy assessment [2] . Methods In this work we propose a novel geometry for gamma-prompt (see Figure). The setup is based on two detectors placed, along a rail, perpendicularly to the proton beam. Each detector is composed by a knife-edge collimator and a small Field-of-View (50 × 50 mm2) Gamma-camera based on a LYSO crystal and a position-sensitive photodetector. This detection geometry has been studied through Monte Carlo simulations developed with GATE and GEANT4. Results Many tests have been made in order to obtain the optimal detection configuration. With the best one, the following results have been achieved: proper counting-rate (about 50 kHz) and spatial resolution (about 4 mm). Conclusions The proposed geometry allows a good trade-off between counting rate and achievable spatial resolution in the region of interest undergoing to proton therapy. Further experimental measurements of clinical protocols are planned.

Optimization of a depth of interaction encoding PET block detector for a PET/MRI insert

Preclinical positron emission tomography, combined with magnetic resonance imaging (PET/MRI), is increasingly used as a tool to simultaneously characterize functional processes in vivo. Many emerging preclinical applications, however, are limited by PET detection sensitivity, especially when generating short imaging frames for quantitative studies. One such application is dynamic multifunctional imaging, which probes multiple aspects of a biological process, using relationships between the datasets to quantify interactions. These studies have limited accuracy due to the relatively low sensitivity of modern preclinical PET/MRI systems.The goal of this project is to develop a preclinical PET/MRI insert with detection sensitivity above 15% (250–750 keV) to improve quantitation in dynamic PET imaging. To achieve this sensitivity, we have developed a detector module incorporating a 2 cm thick crystal block, which will be arranged into a system with 8 cm axial FOV, targeting mice and rats. To maintain homogenous spatial resolution, the detector will incorporate dual-ended depth-of-interaction (DOI) encoding with silicon photomultiplier (SiPM) based photodetector arrays. The specific aim of this work is to identify a detector configuration with adequate performance for the proposed system. We have optimized the SiPM array geometry and tested two crystal array materials with pitch ranging from 0.8 to 1.2 mm and various surface treatments and reflectors. From these configurations, we have identified the best balance between crystal separation, energy resolution, and DOI resolution.The final detector module uses two rectangular SiPM arrays with 5 × 6 and 5 × 4 elements. The photodetector arrays are coupled to a 19 × 19 array of 1 mm pitch LYSO crystals with polished surfaces and a diffuse reflector. The prototype design has 14.3% ± 2.9% energy resolution, 3.57 ± 0.88 mm DOI resolution, and resolves all elements in the crystal array, giving it sufficient performance to serve as the basis for the proposed high sensitivity PET/MRI insert.

A depth-of-interaction encoding PET detector module with dual-ended readout using large-area silicon photomultiplier arrays

The performance of a depth-of-interaction (DOI) encoding PET detector module with dual-ended readout of LYSO scintillator arrays using large-area SiPM arrays was evaluated. Each SiPM array, with a surface area of 50.2 × 50.2 mm2, consists of 12 × 12 C-series SiPMs from SensL (SensL, Inc). The LYSO array, with a total size of 46 × 46 mm2 and a pitch size of 1.0 mm, consists of a 46 × 46 array of 0.945 × 0.945 × 20 mm3 polished LYSO crystals, separated by Toray reflector. Custom front-end electronics were designed to reduce the 288 SiPM signals of one detector module to nine signals, eight for position information and 1 for timing information. Schottky diodes were used to block noise from SiPMs that did not detect a significant number of scintillation photons following a gamma interaction. Measurements of noise, signal, signal-to-noise ratio, energy resolution and flood histogram quality were obtained at different bias voltages (26.0 to 31.0 V in 0.5 V intervals) and at two temperatures (5 °C and 20 °C). Clear acrylic plates, 2.0 mm thick, were used as light guides to spread the scintillation photons. Timing resolution, depth of interaction resolution, and the effect of event rate on detector performance were measured at the bias voltage determined to be optimal for the flood histograms. Performance obtained with and without the noise-blocking Shottky diodes was also compared. The results showed that all crystals in the LYSO array can be clearly resolved, and performance improved when using diodes to block noise, and at the lower temperature. The average energy resolution, flood histogram quality, timing resolution and DOI resolution were 23.8% ± 2.0%, 1.54 ± 0.17, 1.78 ± 0.09 ns and 2.81 ± 0.13 mm respectively, obtained at a bias voltage of 30.0 V and a temperature of 5 °C using the diode readout method. The event rate experiments showed that the flood histogram and energy resolution of the detector were not significantly degraded for an event rate of up to 150 000 counts s−1.

Ultra-precise surface processing of LYSO scintillator crystals for Positron Emission Tomography

The Lutetium-Yttrium Oxyorthosilicate (LYSO) is one of the most widely used scintillation crystal in the high-performance Positron Emission Tomography (PET) systems. The quality of the surface finish of the LYSO has an important impact on the light output, the decoding performance, the energy resolution and timing resolution of the PET detectors and systems. In this paper, we present an ultra-precise method for processing the surface of LYSO crystals. The hardness and elastic modulus of the crystals were initially measured using Nano indentation technology. The scintillators were fixed onto the plate in sparse, serried and continuous arrangements and polished using an alumina (Al2O3) and cerium oxide (CeO2) polishing solution with particles of varying size. We used a magnetorheological-polishing technique to polish the LYSO crystals. The polishing solution here included hydroxyl iron powder and hard abrasives. The hardness and elastic modulus of the crystals in question was, respectively, 11.18 ± 0.50 and 155.78 ± 4 gigapascals (GPa). A 3D optical surface profiler (3D-OPS) and an atomic force microscope (AFM) were used to evaluate the quality of the polished surfaces. The average roughness of Ra 0.55 nm measured by 3D-OPS was achieved using a precise plate grinding and polishing technique. The magnetorheological-polishing method also obtained an excellent roughness of Ra 0.75 nm (3D-OPS). Our report of the use of these processing technologies can serve as a foundation for further in-depth research regarding the optimal techniques for scintillator surface processing.

Evaluation of LFS continuous scintillation crystals for PET

Abstract The suitability of monolithic Lutetium Fine Silicate (LFS) continuous scintillation crystals coupled to 64-channel SiPM-TSV matrices for small animal PET has been evaluated. Three crystals of size 25.8 × 25 . 8 × 15 mm 3 were tested: LYSO white coating, LFS white coating (LFS-W) and LFS specular coating (LFS-S). Electronically collimated measurements were carried out to compare the performance of LYSO and LFS crystals. Mean energy resolutions of 14%, 17% and 19% were measured for LYSO, LFS-S and LFS-W respectively. Similar intrinsic spatial resolutions were found for all three crystals in the central region. Different versions of the light model used to estimate the interaction position of the photons in the detector have been studied. Further measurements with two detector heads of each crystal type in coincidence have also been carried out with a moveable 22Na source. A ML-MLEM image reconstruction code is used to reconstruct the data. Final position resolutions for all three configurations show similar values. Taking into account that LYSO and LFS present similar performance levels (and considering that LFS has better timing resolution than LYSO) we conclude that LFS is a valid substitute for LYSO.

Characterization or Monolithic Scintillation Detectors Etched with Laser Induced Optical Barriers.

Several groups are actively investigating the performance of monolithic (continuous) scintillation detectors using a variety of crystal thicknesses, photo-sensor configurations, and surface treatments. This work explores the performance of thick LYSO crystals that would be applicable to a whole-body PET system. The crystals were etched with laser induced optical barriers (LIOBs) to alter the behavior of the light spread within the crystal in order to improve the performance of the detector. We studied the behavior of the LIOBs in response to optical light using small cubes of LYSO with a variety of laser etching parameters to characterize the impact of the optical barriers. We demonstrated that the opacity of the etchings can be altered by varying the parameters of the laser etching, which influences the depth-dependent light response and spatial resolution in the thick crystal. We successfully etched several crystals, as large as 50×50×25-mm3 thick, with a fine grid of LIOBs, and achieved an average spatial resolution close to 3 mm (FWHM) with 511-keV gammas.