Dual-ended Readout Research Articles

Development of a dual-ended readout detector with segmented crystal bars made using a subsurface laser engraving technique

Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial resolution of positron emission tomography (PET) systems, especially for small field-of-view PET such as small animal PET and human brain PET. We have already developed a series of X’tal cube detectors for isotropic spatial resolution and we obtained the best isotropic resolution of 0.77 mm for detectors with six-sided readout. However, it is still challenging to apply the detector for PET systems due to the high cost of six-sided readout electronics and carrying out segmentation of a monolithic cubic scintillator in three dimensions using the subsurface laser engraving (SSLE) technique. In this work, we propose a more practical X’tal cube with a two-sided readout detector, which is made of crystal bars segmented in the height direction only by using the SSLE technique. We developed two types of prototype detectors with a 3 mm cubic segment and a 1.5 mm cubic segment by using 3 × 3 × 20 mm3 and 1.5 × 1.5 × 20 mm3 crystal bars segmented into 7 and 13 DOI segments, respectively, using the SSLE technique. First, the performance of the detector, composed of one crystal bar with different DOI segments and two thorough silicon via (TSV) multi-pixel photon counters (MPPCs) as readout at both ends of the crystal bar, were evaluated in order to demonstrate the capability of the segmented crystal bars as a DOI detector. Then, performance evaluation was carried out for a 4 × 4 crystal array of 3 × 3 × 20 mm3 with 7 DOI segments and an 8 × 8 crystal array of 1.5 × 1.5 × 20 mm3 with 13 DOI segments. Each readout included a 4 × 4 channel of the 3 × 3 mm2 active area of the TSV MPPCs. The three-dimensional position maps of the detectors were obtained by the Anger-type calculation. All the segments in the 4 × 4 array were identified very clearly when there was air between the crystal bars, as each crystal bar was coupled to one channel of the MPPCs; however, it was necessary to optimize optical conditions between crystal bars for the 8 × 8 array because of light sharing between crystal bars coupled to one channel of the MPPCs. The optimization was performed for the 8 × 8 array by inserting reflectors fully or partially between the crystal bars and the best crystal identification performance was obtained with the partial reflectors between the crystal bars. The mean energy resolutions at the 511 keV photo peak for the 4 × 4 array with air between the crystal bars and for the 8 × 8 array with partial reflectors between the crystal bars were 10.1% ± 0.3% and 10.8% ± 0.8%, respectively. Timing resolutions of 783 ± 36 ps and 1.14 ± 0.22 ns were obtained for the detectors composed of the 4 × 4 array and the 8 × 8 array with partial reflectors, respectively. These values correspond to single photon timing resolutions. Practical X’tal cubes with 3 mm and 1.5 mm DOI resolutions and two-sided readout were developed.

Development of depth encoding small animal PET detectors using dual-ended readout of pixelated scintillator arrays with SiPMs.

The performance of current small animal PET scanners is mainly limited by the detector performance and depth encoding detectors are required to develop PET scanner to simultaneously achieve high spatial resolution and high sensitivity. Among all depth encoding PET detector approaches, dual-ended readout detector has the advantage to achieve the highest depth of interaction (DOI) resolution and spatial resolution. Silicon photomultiplier (SiPM) is believed to be the photodetector of the future for PET detector due to its excellent properties as compared to the traditional photodetectors such as photomultiplier tube (PMT) and avalanche photodiode (APD). The purpose of this work is to develop high resolution depth encoding small animal PET detector using dual-ended readout of finely pixelated scintillator arrays with SiPMs. Four lutetium-yttrium oxyorthosilicate (LYSO) arrays with 11×11 crystals and 11.6×11.6×20mm3 outside dimension were made using ESR, Toray and BaSO4 reflectors. The LYSO arrays were read out with Hamamatsu 4×4 SiPM arrays from both ends. The SiPM array has a pixel size of 3×3mm2 , 0.2mm gap in between the pixels and a total active area of 12.6×12.6mm2 . The flood histograms, DOI resolution, energy resolution and timing resolution of the four detector modules were measured and compared. All crystals can be clearly resolved from the measured flood histograms of all four arrays. The BaSO4 arrays provide the best and the ESR array provides the worst flood histograms. The DOI resolution obtained from the DOI profiles of the individual crystals of the four array is from 2.1 to 2.35mm for events with E>350keV. The DOI ratio variation among crystals is bigger for the BaSO4 arrays as compared to both the ESR and Toray arrays. The BaSO4 arrays provide worse detector based DOI resolution. The photopeak amplitude of the Toray array had the maximum change with depth, it provides the worst energy resolution of 21.3%. The photopeak amplitude of the BaSO4 array with 80μm reflector almost doesn't change with depth, it provides the best energy resolution of 12.9%. A maximum timing shift of 1.37ns to 1.61ns among the corner and the center crystals in the four arrays was obtained due to the use of resistor network readout. A crystal based timing resolution of 0.68ns to 0.83ns and a detector based timing resolution of 1.26ns to 1.45ns were obtained for the four detector modules. Four high resolution depth encoding small animal PET detectors were developed using dual-ended readout of pixelated scintillator arrays with SiPMs. The performance results show that those detectors can be used to build a small animal PET scanner to simultaneously achieve uniform high spatial resolution and high sensitivity.

Open-field mouse brain PET: design optimisation and detector characterisation

‘Open-field’ PET, in which an animal is free to move within an enclosed space during imaging, is a very promising advance for neuroscientific research. It provides a key advantage over conventional imaging under anesthesia by enabling functional changes in the brain to be correlated with an animal’s behavioural response to environmental or pharmacologic stimuli. Previously we have demonstrated the feasibility of open-field imaging of rats using motion compensation techniques applied to a commercially available PET scanner. However, this approach of ‘retro-fitting’ motion compensation techniques to an existing system is limited by the inherent geometric and performance constraints of the system. The goal of this project is to develop a purpose-built PET scanner with geometry, motion tracking and imaging performance tailored and optimised for open-field imaging of the mouse brain. The design concept is a rail-based sliding tomograph which moves according to the animal’s motion. Our specific aim in this work was to evaluate candidate scanner designs and characterise the performance of a depth-of-interaction detector module for the open-field system. We performed Monte Carlo simulations to estimate and compare the sensitivity and spatial resolution performance of four scanner geometries: a ring, parallel plate, and two box variants. Each system was based on a detector block consisting of a 23 × 23 array of 0.785 × 0.785 × 20 mm3 LSO crystals (overall dim. 19.6 × 19.6 × 20 mm). We found that a DoI resolution capability of 3 mm was necessary to achieve approximately uniform sub-millimetre spatial resolution throughout the FoV for all scanners except the parallel-plate geometry. With this DoI performance, the sensitivity advantage afforded by the box geometry with overlapping panels (16% peak absolute sensitivity, a 36% improvement over the ring design) suggests this unconventional design is best suited for imaging the mouse brain. We also built and characterised the block detector modelled in the simulations, including a dual-ended readout based on 6 × 6 arrays of through-silicon-via silicon photomultipliers (active area 84%) for DoI estimation. Identification of individual crystals in the flood map was excellent, energy resolution varied from 12.4% ± 0.6% near the centre to 24.4% ± 3.4% at the ends of the crystal, and the average DoI resolution was 2.8 mm ± 0.35 mm near the central depth (10 mm) and 3.5 mm ± 1.0 mm near the ends. Timing resolution was 1.4 ± 0.14 ns. Therefore, the DoI detector module meets the target specifications for the application and will be used as the basis for a prototype open-field mouse PET scanner.

Performance of a high-resolution depth encoding PET detector using barium sulfate reflector

Small animal positron emission tomography (PET) is a well-established imaging modality in preclinical biomedical research. The performance of current small animal PET scanners is mainly limited by the detector performance and depth-encoding detectors are required to simultaneously achieve high spatial resolution and high sensitivity. In this work, the performance of a high-resolution dual-ended readout lutetium-yttrium oxyorthosilicate (LYSO) array using barium sulfate powder (BaSO4) as the inter-crystal reflector was measured for the first time and compared to that of a LYSO array using the most commonly used enhanced specular reflector (ESR). Both LYSO arrays have 18 × 18 crystals and the crystal size is about 0.62 × 0.62 × 20 mm3. The LYSO arrays are readout by two position-sensitive photomultiplier tubes (PSPMTs) from both ends. The flood histograms, energy resolution, depth of interaction (DOI) resolution and timing resolution were measured. The flood histograms of the LYSO array with BaSO4 reflector is much better than that of the LYSO array with ESR reflector. For the BaSO4 array, all crystals can be clearly resolved. For the ESR array, all crystals in one direction can be clearly resolved, but the edge 2–3 columns of the crystals in the other direction cannot be resolved. The average energy resolution of the BaSO4 and ESR arrays are 15.2% and 15.3%, respectively. The average DOI resolution of the BaSO4 array is 2.19 mm, which is 24% worse than the 1.76 mm DOI resolution of the ESR array. The timing resolution of both arrays is ~1.6 ns. The LYSO array with the new BaSO4 reflector provided an much better flood histogram in a high resolution dual-ended readout PET detectors as compared to the ESR array, and will be used to develop a small animal PET scanner that can simultaneously achieve uniform high spatial resolution, high sensitivity and low cost.

Study of MRPC technology for BESIII endcap-TOF upgrade

In order to improve the charged particle identification capability, end-cap time-of-flight (ETOF) detector of the Beijing Spectrometer (BESIII) has been upgraded with multi-gap resistive plate chamber (MRPC) technology, aiming at an overall time resolution of 80 ps for minimum-ionization particles to extend the K/ $$\pi $$ separation (2 $$\sigma $$ ) momentum range to 1.4 GeV/c. The previous version of ETOF in BESIII consisted of plastic scintillators. The multi-hit events distort both shape and amplitude of the output signals. MRPC technique was chosen for the BESIII ETOF upgrade as it provides high time resolution and high detection efficiency, is of relatively low cost and is insensitive to neutral particles. Most importantly, the fine segmentation of the MRPC readout stripes can suppress multi-hit events effectively. The final design of MRPC module for ETOF is characterized by double-stack (2 $$\times $$ 6) structure, dual-end readout mode and precision electronics. To batch-produce and test these MRPC modules, a series of tools and production procedures as well as related performance simulation and test methods were developed. Results showed that each MRPC module’s intrinsic time resolution (including the electronics contribution) is around 50 ps and the efficiency is better than 97%. The overall performance of the upgraded ETOF is better than the designed index. The new ETOF has been successfully installed at BESIII and run in 2016.

A Prototype High-Resolution Small-Animal PET Scanner Dedicated to Mouse Brain Imaging.

We developed a prototype small-animal PET scanner based on depth-encoding detectors using dual-ended readout of small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain. The scanner consists of 16 tapered dual-ended-readout detectors arranged in a 61-mm-diameter ring. The axial field of view (FOV) is 7 mm, and the transaxial FOV is 30 mm. The scintillator arrays consist of 14 × 14 lutetium oxyorthosilicate elements, with a crystal size of 0.43 × 0.43 mm at the front end and 0.80 × 0.43 mm at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8 × 8 mm and 13 × 8 mm position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard nuclear-instrumentation-module electronics and a custom-designed multiplexer are used for signal processing. The detector performance was measured, and all but the crystals at the very edge could be clearly resolved. The average intrinsic spatial resolution in the axial direction was 0.61 mm. A depth-of-interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at the center of the FOV was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a FOV that can accommodate the entire mouse brain was approximately 0.6 mm using a 3-dimensional maximum-likelihood expectation maximization reconstruction. Images of a hot-rod microphantom showed that rods with a diameter of as low as 0.5 mm could be resolved. The first in vivo studies were performed using (18)F-fluoride and confirmed that a 0.6-mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with (18)F-FDG were also performed. We developed a prototype PET scanner that can achieve a spatial resolution approaching the physical limits of a small-bore PET scanner set by positron range and detector interaction. We plan to add more detector rings to extend the axial FOV of the scanner and increase sensitivity.

A novel depth-of-interaction block detector for positron emission tomography using a dichotomous orthogonal symmetry decoding concept

Conventionally, a dual-end depth-of-interaction (DOI) block detector readout requires two two-dimensional silicon photomultiplier (SiPM) arrays, one on top and one on the bottom, to define the XYZ positions. However, because both the top and bottom SiPM arrays are reading the same pixels, this creates information redundancy. We propose a dichotomous orthogonal symmetric (DOS) dual-end readout block detector design, which removes this redundancy by reducing the number of SiPMs and still achieves XY and DOI (Z) decoding for positron emission tomography (PET) block detector. Reflecting films are used within the block detector to channel photons going to the top of the block to go only in the X direction, and photons going to the bottom are channeled along the Y direction. Despite the unidirectional channeling on each end, the top readout provides both X and Y information using two one-dimensional SiPM arrays instead of a two-dimensional SiPM array; similarly, the bottom readout also provides both X and Y information with just two one-dimensional SiPM arrays. Thus, a total of four one-dimensional SiPM arrays (4 × N SiPMs) are used to decode the XYZ positions of the firing pixels instead of two two-dimensional SiPM arrays (2 × N × N SiPMs), reducing the number of SiPM arrays per block from 2N2 to 4 N for PET/MR or PET/CT systems. Moreover, the SiPM arrays on one end can be replaced by two regular photomultiplier tubes (PMTs), so that a block needs only 2 N SiPMs + 2 half-PMTs; this hybrid-DOS DOI block detector can be used in PET/CT systems. Monte Carlo simulations were carried out to study the performance of our DOS DOI block detector design, including the XY-decoding quality, energy resolution, and DOI resolution. Both BGO and LSO scintillators were studied. We found that 4 mm pixels were well decoded for 5 × 5 BGO and 9 × 9 LSO arrays with 4 to 5 mm DOI resolution and 16–20% energy resolution. By adding light-channel decoding, we modified the DOS design to a high-resolution design, which resolved scintillator pixels smaller than the SiPM dimensions. Detector pixels of 2.4 mm were decoded for 8 × 8 BGO and 15 × 15 LSO arrays with 5 mm DOI resolution and 20–23% energy resolution. Time performance was also studied for the 8 × 8 BGO and 15 × 15 LSO HR-DOS arrays. The timing resolution for the corner and central crystals is 986 ± 122 ps and 1.89 ± 0.17 μs respectively with BGO, 137 ± 42 ps and 458 ± 67 ps respectively with LSO. Monte Carlo simulations with GATE/Geant4 demonstrated the feasibility of our DOS DOI block detector design. In conclusion, our novel design achieved good performance except the time performance while using fewer SiPMs and supporting electronic channels than the current non-DOI PET detectors. This novel design can significantly reduce the cost, heat, and readout complexity of DOI block detectors for PET/MR/CT systems that don’t require the time-of-flight capability.

Choice of crystal surface finishing for a dual-ended readout depth-of-interaction (DOI) detector

The objective of this study was to choose the crystal surface finishing for a dual-ended readout (DER) DOI detector. Through Monte Carlo simulations and experimental studies, we evaluated 4 crystal surface finishing options as combinations of crystal surface polishing (diffuse or specular) and reflector (diffuse or specular) options on a DER detector. We also tested one linear and one logarithm DOI calculation algorithm. The figures of merit used were DOI resolution, DOI positioning error, and energy resolution. Both the simulation and experimental results show that (1) choosing a diffuse type in either surface polishing or reflector would improve DOI resolution but degrade energy resolution; (2) crystal surface finishing with a diffuse polishing combined with a specular reflector appears a favorable candidate with a good balance of DOI and energy resolution; and (3) the linear and logarithm DOI calculation algorithms show overall comparable DOI error, and the linear algorithm was better for photon interactions near the ends of the crystal while the logarithm algorithm was better near the center. These results provide useful guidance in DER DOI detector design in choosing the crystal surface finishing and DOI calculation methods.

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.

A practical depth-of-interaction PET/MR detector with dichotomous-orthogonal-symmetry decoding.

Conventional dual-end depth-of-interaction (DOI) PET detector readout requires two 2D SiPM arrays; with top and bottom SiPM reading the same pixel, there is information redundancy. We proposed a dichotomous-orthogonal-symmetric (DOS) dual-end DOI readout to eliminate this redundancy to significantly reduce SiPM usage, electronic channels, and heat load. Reflecting films are used within the scintillator array to channel light exiting the top along the X-direction, while light exiting the bottom is channeled along the orthogonal Y-direction. Despite the unidirectional channeling on each end, the top readout can provide X-Y information using two 1-D SiPM arrays; similarly, the bottom readout also provides X-Y information with two 1-D SiPM arrays. Thus four 1-D SiPM arrays (4xN) are used to decode XYZ to replace two 2D SiPM arrays (2NxN); SiPM usage is reduced from 2N**2 to 4N. Monte Carlo simulations (GATE) were carried out to study the XY decoding accuracy, energy resolution, and DOI resolution. Coupling the DOS-DOI design with a channel-decoding scheme, an array of 15x15 LSO (2.4x2.4x20 mm pixels) can be decoded by 18 SiPMs (2 rows of nine 3x3mm SiPM) on top and 18 SiPMs at bottom, thus achieving a 10X reduction in SiPM usage, electronic channels and heat load. For BGO detectors, an 8x8 array (2.4x2.4x20 mm pixels) can be achieved with 6.4X reduction. Simulations show 5-6mm DOI resolution, 0.45-0.96mm XY decoding blurring, 20-24% energy resolution. This study shows the feasibility of the DOS-DOI design. Even comparing to non-DOI detectors, there is a 5X/3X SiPM reduction for LSO/BGO. The proposed detector may yield practical ultrahigh-resolution PET/MR systems with depth-of-interaction with a production cost below current non-DOI systems.

Neutron detector based on Particles of 6Li glass scintillator dispersed in organic lightguide matrix

Most 3He replacement neutron detector technologies today have overlapping neutron–gamma pulse-height distributions, which limits their usefulness and performance. Different techniques are used to mitigate this shortcoming, including Pulse Shape Discrimination (PSD) or threshold settings that suppress all gammas as well as much of the neutrons. As a result, count rates are limited and dead times are high when PSD is used, and the detection efficiency for neutron events is reduced due to the high threshold. This is a problem in most applications where the neutron–gamma separation of 3He detectors had been essential. This challenge is especially severe for neutron coincidence and multiplicity measurements that have numerous conflicting requirements such as high detection efficiency, short die-away time, short dead time, and high stability. 6Li-glass scintillators have excellent light output and a single peak distribution, but they are difficult to implement because of their gamma sensitivity. The idea of reducing the gamma sensitivity of 6Li-glass scintillators by embedding small glass particles in an organic light-guide medium was first presented by L.M. Bollinger in the early 60s but, to the best of our knowledge, has never been reduced to practice. We present a proof of principle detector design and experimental data that develop this concept to a large-area neutron detector. This is achieved by using a multi-component optical medium (6Li glass particles attached to a glass supporting structure and a mineral oil light guide) which matches the indices of refraction and minimizes the absorption of the 395nm scintillator light. The detector design comprises a 10in. long tube with dual end readout with about 3% volume density of 6Li glass particles installed. The presented experimental data with various neutron and gamma sources show the desired wide gap between the neutron and gamma pulse height distributions, resulting in a true plateau in the counting characteristics similar to that of 3He detectors.

Development of an advanced antineutrino detector for reactor monitoring

Here we present the development of a compact antineutrino detector for the purpose of nuclear reactor monitoring, improving upon a previously successful design. This paper will describe the design improvements of the detector which increases the antineutrino detection efficiency threefold over the previous effort. There are two main design improvements over previous generations of detectors for nuclear reactor monitoring: dual-ended optical readout and single volume detection mass. The dual-ended optical readout eliminates the need for fiducialization and increases the uniformity of the detector׳s optical response. The containment of the detection mass in a single active volume provides more target mass per detector footprint, a key design criteria for operating within a nuclear power plant. This technology could allow for real-time monitoring of the evolution of a nuclear reactor core, independent of reactor operator declarations of fuel inventories, and may be of interest to the safeguards community.

An investigation on continuous depth-of-interaction detection using a monolithic scintillator with single-ended readout

PET detectors with depth-of-interaction (DOI) capability have been studied to improve imaging resolution widely over the world. Since discrete DOI and continuous DOI detection with dual-ended readout technology have their respective limitations, we in this work focus on the continuous DOI detection with single-ended readout using a monolithic LSO scintillator and a multi-pixel photodetector. Based on a non-linear least squares data fitting method and Geant4 simulation, we studied the relationship between the spatial resolution of gamma positioning and the pixel number of photodetector. The results show that for a pixel number larger than 6x6, the positioning spatial resolution does not become significantly better when increasing the pixel number moreover. Another aspect studied is the effect of crystal thickness on the spatial resolution. Increasing the thickness of crystal leads to higher detection efficiency but lower spatial resolution.

Effects of reflector and crystal surface on the performance of a depth-encoding PET detector with dual-ended readout.

Depth encoding detectors are required to improve the spatial resolution and spatial resolution uniformity of small animal positron emission tomography (PET) scanners, as well as dedicated breast and brain scanners. Depth of interaction (DOI) can be measured by using dual-ended readout of lutetium oxyorthosilicate (LSO) scintillator arrays with position-sensitive avalanche photodiodes. Inter-crystal reflectors and crystal surface treatments play important roles in determining the performance of dual-ended detectors. In this paper, the authors evaluated five LSO arrays made with three different intercrystal reflectors and with either polished or unpolished crystal surfaces. The crystal size in all arrays was 1.5 mm, which is typical of the detector size used in small animal and dedicated breast scanners. The LSO arrays were measured with dual-ended readout and were compared in terms of flood histogram, energy resolution, and DOI resolution performance. The four arrays using enhanced specular reflector (ESR) and Toray reflector provided similar quality flood histograms and the array using Crystal Wrap reflector gave the worst flood histogram. The two arrays using ESR reflector provided the best energy resolution and the array using Crystal Wrap reflector yielded the worst energy resolution. All arrays except the polished ESR array provided good DOI resolution ranging from 1.9 mm to 2.9 mm. DOI resolution improved as the gradient in light collection efficiency with depth (GLCED) increased. The geometric mean energies were also calculated for these dual-ended readout detectors as an alternative to the conventional summed total energy. It was shown that the geometric mean energy is advantageous in that it provides more uniform photopeak amplitude at different depths for arrays with high GLCED, and is beneficial in event selection by allowing a fixed energy window independent of depth. A new method of DOI calculation that improved the linearity of DOI ratio vs depth and simplifies the DOI calibration procedure also was developed and tested. The results of these studies provide useful guidance in selecting the proper reflectors and crystal surface treatments when LSO arrays are used for high-resolution PET applications in small animal scanners or dedicated breast and brain scanners.

A Study of Position-Sensitive Solid-State Photomultiplier Signal Properties.

We present an analysis of the signal properties of a position-sensitive solid-state photomultiplier (PS-SSPM) that has an integrated resistive network for position sensing. Attractive features of PS-SSPMs are their large area and ability to resolve small scintillator crystals. However, the large area leads to a high detector capacitance, and in order to achieve high spatial resolution a large network resistor value is required. These inevitably create a low-pass filter that drastically slows what would be a fast micro-cell discharge pulse. Significant changes in the signal shape of the PS-SSPM cathode output as a function of position are observed, which result in a position-dependent time delay when using traditional time pick-off methods such as leading edge discrimination and constant fraction discrimination. The timing resolution and time delay, as a function of position, were characterized for two different PS-SSPM designs, a continuous 10 mm × 10 mm PS-SSPM and a tiled 2 × 2 array of 5 mm × 5 mm PS-SSPMs. After time delay correction, the block timing resolution, measured with a 6 × 6 array of 1.3 × 1.3 × 20 mm3 LSO crystals, was 8.6 ns and 8.5 ns, with the 10 mm PS-SSPM and 5 mm PS-SSPM respectively. The effect of crystal size on timing resolution was also studied, and contrary to expectation, a small improvement was measured when reducing the crystal size from 1.3 mm to 0.5 mm. Digital timing methods were studied and showed great promise for allowing accurate timing by implementation of a leading edge time pick-off. Position-dependent changes in signal shape on the anode side also are present, which complicates peak height data acquisition methods used for positioning. We studied the effect of trigger position on signal amplitude, flood histogram quality, and depth-of-interaction resolution in a dual-ended readout detector configuration. We conclude that detector timing and positioning can be significantly improved by implementation of digital timing methods and by accounting for changes in the shape of the signals from PS-SSPMs.

Investigation of optical transmittance and light response uniformity of 600-mm-long BGO crystals

Optical transmittance and light response uniformity were investigated for 25mm×25mm×600mm BGO crystals, the longest BGO single crystals reported so far. The long BGO crystals exhibit good optical transmittance within the wavelength range concerned and an overall energy resolution of 19.68% for 0.662MeV γ-rays. The light response uniformity of one long BGO, SIC-BGO-125, was measured in the single-end or dual-end readout mode combined with different reflecting materials. It was suggested that the light response uniformity of the long rectangular BGO is primarily affected by internal absorption, the reflectivity of reflecting materials and the readout mode. The best uniformity of −1.4±0.8% can be achieved when the crystal is measured only from the tail end and wrapped with ESR. Reflecting materials of high reflectivity can significantly improve the light response uniformity, but further improvement is limited by the internal absorption of the crystal.

Optical Simulation of Dual-Ended Readout of Axially-Oriented 100 mm Long LYSO Crystals for Use in a Compact PET System

Monte Carlo simulations of optical photon transport were performed using DETECT2000 for detectors using dual-ended readout of axially-oriented 100 mm long LYSO crystals. In order to modulate the light transport in the crystals and improve the axial-positioning resolution, systematic etched band patterns were simulated on the long surfaces of the crystals. Roughed surface bands with a width of 1 mm and inter-band spacing of 5 mm over the central 8 cm of the crystal were simulated on the long surfaces of initially polished 3 mm×2 mm×100 mm and 2 mm×2 mm×100 mm crystals. This band pattern was simulated for cases of bands on one surface, bands on two opposite surfaces, and bands on all four surfaces. Each crystal was modeled as Teflon-wrapped and read out at both ends using position sensitive photomultiplier tubes (PSPMTs). Optical photons were simulated as originating at nine central-axis locations, separated by 10 mm, along the crystal length. The ratio of two PMT signals was used to determine the axial-positioning resolution while their sum was used to determine the energy resolution. The DETECT2000 simulation parameters required to give good agreement with measured data were determined in order to benchmark the simulation for this detector geometry. These simulation parameters were then used to examine the effects of alternative banding pattern designs. The best axial-positioning resolution obtained in this work was 3.5 mm FWHM for the case of etched bands along the central 8 cm of the crystal with band size of 4 mm and inter-band-spacing of 20 mm on two opposite surfaces.

Studies on fast triggering and high precision tracking with Resistive Plate Chambers

We report on studies of fast triggering and high precision tracking using Resistive Plate Chambers (RPCs). Two beam tests were carried out with the 180GeV/c muon beam at CERN using glass RPCs with gas gaps of 1.15mm and equipped with readout strips with 1.27mm pitch. This is the first beam test of RPCs with fine-pitch readout strips that explores precision tracking and triggering capabilities. RPC signals were acquired with precision timing and charge integrating readout electronics at both ends of the strips. The time resolution was measured to be better than 600ps and the average spatial resolution was found to be 220μm using charge information and 287μm only using signal arrival time information. The dual-ended readout allows the determination of the average and the difference of the signal arrival times. The average time was found to be independent of the incident particle position along the strip and is useful for triggering purposes. The time difference yielded a determination of the hit position with a precision of 7.5mm along the strip. These results demonstrate the feasibility using RPCs for fast and high-resolution triggering and tracking.

Use of systematic surface roughing to enhance the spatial resolution of the dual-ended readout of axially-oriented 100 mm long LYSO crystals

In this work we explored the effects of using systematic band patterns of surface roughing to modulate the light transport in a dual-ended readout detector for PET imaging that uses 100 mm long LYSO crystals oriented in the axial direction. The long surfaces of 3 × 2 × 100 mm3 LYSO crystals, initially polished on all sides, were systematically roughed in a band pattern with 0.8 mm bands of roughed surface spaced at 5 mm increments over the central 8 cm of the crystal length. Cases of one to four surfaces of the crystal roughed with this pattern were explored. Two configurations of bands were examined for the opposite surfaces of the crystals: (i) bands created in the same axial location (coinciding bands) and (ii) band locations interleaved (interleaving bands). Each crystal was then wrapped in Teflon and read out at both ends using position sensitive photomultiplier tubes (PSPMTs). An electronically collimated 511 keV photon beam was oriented perpendicular to the long direction of the crystal and data acquired at discrete locations along the crystal length. The ratio of the two PMT signals was used to find the axial position of interaction and axial-positioning resolution in the light sharing direction of the dual-ended readout detector while their sum was used to determine the light output and energy resolution. The axial-positioning resolution corrected for the beam width improved from an average of 8.7 mm FWHM with no surface treatment to 5.9, 3.9 or 3.4 mm FWHM for coinciding band patterns on one, two or four surfaces, respectively. The banding patterns resulted in degraded energy resolution, with a decrease from 11.1% FWHM for the untreated crystal to 12.0%, 13.5% and 13.6% for the patterns on one, two and four surfaces, respectively. When the axial-positioning resolution is corrected for the beam width, we estimated the best resolution for the case of four banded surfaces to be 3.4 mm FWHM in this study. The same axial-positioning resolution was obtained when interleaving bands were created on only two opposite surfaces with a better energy resolution of 12.2%.

Sci-Fri AM: Imaging - 02: High resolution detectors for PET mammography.

With high specificity for malignant breast lesions, dedicated-breast molecular imaging systems such as positron emission mammography (PEM) have potential to improve the sensitivity of cancer in women with radio-dense breasts and to reduce the false-positive rate of breast screening when used as a diagnostic adjunct. For high signal-to-noise ratio and to minimize the patient dose, scintillation detectors in a PEM system must have high annihilation photon detection efficiency. This efficiency can be increased by accepting annihilation photons from wider incident angles and by using depth-of-interaction (DOI) measurement within a scintillation crystal to minimize parallax blurring. We have developed a dual-ended readout block (DERB) detector that uses asymmetry of signals from photodetectors on either end of a scintillation array to measure DOI and uses Anger Logic with light sharing to identify interacting crystal elements while minimizing the number of photodetectors required. A prototype DERB detector was constructed from two arrays of silicon photomultipliers (SiPM), two glass optical diffusers, and an array of LYSO scintillation crystals. Assembled, each of the 2 × 2 SiPM arrays detect photons that are dispersed via the optical light diffusers originating from either end of 3 × 3 scintillation crystal elements. We evaluated the ability of the detector to identify the crystal index, resolve DOI, and discriminate energy. The DERB detector was able to clearly identify interacting crystal elements, to measure DOI with ̃5mm resolution in 2mm × 2mm × 20mm crystals, and to achieve an average energy resolution of ̃20%. The DERB detector characteristics suggest that it can be used to reduce the parallax effect in PEM systems without increasing the number of required photodetectors. Further investigation is warranted to improve performance with high optical photon detection efficiency photodetectors.