Emission Tomography System Research Articles

Monitoring Proton Therapy Through in-Beam PET: An Experimental Phantom Study

In this paper, we investigate the use of a positron emission tomography (PET) system to monitor the proton therapy. The monitoring procedure is based on the comparison between the ${ \beta }+$ activity generated in the irradiated volume during the treatment, with the ${ \beta }+$ activity distribution obtained with Monte Carlo (MC) simulation. The dedicated PET system is a dual head detection system; each head is composed of nine scintillating LYSO crystal matrices read out independently with a custom modularized acquisition system. Our experimental data were acquired at the Cyclotron Centre Bronowice, Institute Nuclear Physics in Krakow, Poland, and were simulated with the FLUKA MC code. Homogeneous and heterogeneous plastic phantoms were irradiated with monoenergetic 130 MeV protons. The capabilities of our PET system to distinguish different irradiated materials were investigated, and the proton pencil-beams were used as probes. Our focus was to analyze the activity width and the total activity event number in several cases. Irradiations were performed using either single pencil-beams one at a time, or two pencil-beams during the same data taking. The comparison of 1-D activity profile for experimental data and MC simulation were always in good agreement showing that, the treatment quality assessment in proton therapy can be based on ${ \beta }+$ activity measurements.

Time-based signal sampling using sawtooth-shaped threshold

Energy measurement of scintillation pulses by using time-over-threshold (TOT) has advantages of low readout cost, low power consumption, and less complexity compared with conventional analog-to-digital converter (ADC) approaches. Therefore, TOT is attractive in positron emission tomography (PET) systems based on silicon photomultiplier (SiPM) arrays requiring many readout channels. However, poor energy resolution and linearity of TOT leads to degradation of the overall PET detector and system performance, which is unsuitable for high-performance PET systems. To overcome these limitations, we propose a novel time-based signal-sampling method, sawtooth threshold sampling (STS), for PET scintillation detectors. This method uses a sawtooth-shaped threshold signal generated adaptively to the input scintillation pulse. Based on the time difference between the rising and falling edges of the digital pulse train generated by comparing the input scintillation pulse to the sawtooth signal, we can estimate the input scintillation pulse amplitude at the several time points. We compared several curve-fitting and numerical integration methods for energy estimation from the STS samples. Coincidence data between two identical scintillation detectors composed of one-to-one coupled SiPM and LGSO crystal (3 × 3 × 20 mm3) was measured using the proposed STS circuit. For timing- and energy-resolution measurement, STS (10.5% ± 0.21% and 200 ± 5.8 ps) was superior than simple TOT (16.4% ± 0.52% and 224 ± 8.2 ps) and was similar to high-speed ADC (9.84% ± 0.11% and 193 ± 4.3 ps). In conclusion, the proposed method can be a cost-effective solution for data collection in future SiPM-based PET systems.

Effect of CZT system characteristics on Compton scatter event recovery.

Improving 511 keV photon detection sensitivity is a common goal for positron emission tomography system designers. One attractive approach to increase sensitivity is recovering events that are normally rejected. The kinematics of Compton scattering can be used to recover the line of response through direction difference angle (DDA). The uncertainty of DDA is determined by the energy and spatial resolution of a system. In this work, we evaluated the performance of small animal CZT-based positron emission tomography systems with energy resolution of 1%, 4%, and 6% and different spatial resolution based on prior work for guiding new design efforts. Designs with energy resolution limited by counting statistics and by electronic noise were considered. The influence of modifying the conventional energy window and uncertainty of DDA was investigated. For a system with 4% energy resolution and limited by electronic noise, the figure of merit of noise equivalent count increases by 65% as the lower energy bound increases from 471 keV to 493 keV. If the system-wide energy resolution becomes worse than 4% of the full width half maximum at 511 keV, going to a pixel size finer than 1 mm has very limited effect in reducing total angular uncertainty. For a system with 1% energy resolution, as the spatial resolution improves from 1 mm to 0.5 mm, the contrast-to-noise ratio increases by 9%.

Intercrystal scatter studies for a 1 mm3 resolution clinical PET system prototype

Positron emission tomography (PET) systems designed with multiplexed readout do not usually have the capability to resolve individual intercrystal scatter (ICS) interactions, leading to interaction mispositioning that degrades spatial resolution and contrast. A 3D position sensitive scintillation detector capable of individual ICS readout has been designed and incorporated into a 1 mm3 resolution clinical PET system used for locoregional imaging. Incorporating ICS events increases photon sensitivity by 51.5% compared to using only photoelectric events. A Compton scatter angle error minimization algorithm is used to estimate the first ICS interaction location for accurate line-of-response pairing of coincident photons. An optimal scatter angle error threshold of 15 degrees is used to discard ICS events with a high mismatch between energy-derived and position-derived intercrystal scatter angles. Finally, positioning rather than rejecting ICS events boosts peak contrast to noise ratio by 8.1%, and allows for an equivalent dose reduction of 12% while maintaining equivalent image quality.

Dual-ended readout of bismuth germanate to improve timing resolution in time-of-flight PET

The scintillator bismuth germanate (BGO) has attractive properties for positron emission tomography (PET) systems such as high stopping power, high photo-fraction, and relatively low cost. However, its moderate scintillation light yield and slow rise and decay time compared to lutetium (yttrium) oxyorthosilicate (L(Y)SO) results in a degradation of coincidence timing resolution when scintillation photons are used for timing. Recently, it has been reported that the coincidence timing resolution of BGO can be improved by detecting Cerenkov photons, while scintillation photons still provide energy information. However, the measured coincidence timing spectrum showed much longer tails compared to the single Gaussian distribution. Because of this, TOF PET detectors based on BGO will perform worse than the full width at half maximum (FWHM) of the distribution, which is the most common metric for timing resolution, would suggest. From simulation studies, during the first few picoseconds, BGO generates ~16 Cerenkov photons per photoelectric interaction, following a 511 keV gamma ray interaction, while the probability of producing a scintillation photon during the first few picoseconds is very small. Therefore, when we configure a BGO crystal with dual-ended readout, the first arriving photons among the two opposing SiPMs are most likely Cerenkov photons, and by selecting the appropriate SiPM, an improvement in coincidence timing resolution can be achieved. In this study, both ends of a 3 × 3 × 20 mm3 BGO crystal were coupled to NUV-HD SiPMs. Trigger time differences from the dual-ended readout of BGO were widely distributed due to detecting a mixture of prompt Cerenkov and scintillation photons on both SiPMs. When using trigger times from only a single SiPM, the estimated coincidence timing resolution between two identical BGO detectors was 463 ps FWHM and 1463 ps FWTM. In contrast, when using trigger times from both SiPMs, the estimated coincidence timing resolution was 399 ps FWHM and 936 ps FWTM with no loss of events. Based on a recent report, high-bandwidth amplifiers were implemented and shown to further improve the estimated coincidence timing resolution to 331 ps FWHM and 923 ps FWTM. In summary, the coincidence timing resolution of BGO, most notably the FWTM, was significantly improved using time information from the dual-ended readout.

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.

Comparison of Noise Equivalent Count Rates (NECRs) for the PET Systems With Different Ring Diameter and Electronics

Because the sensitivity of positron emission tomography (PET) system increases in proportion to the inverse of the diameter of the detector ring, brain PET systems are being planned and developed. However, count losses due to the dead-time of the detector blocks increases as the ring diameter decreases. Since the count losses decrease the noise equivalent count rate (NECR) of the system, the NECR of brain PET system may be lower than that of a whole-body PET system in clinical dose range. In this paper, we compared the NECRs of brain PET systems with that of a whole-body PET system using a Monte Carlo simulation. We simulated the PET system for a 27-cm-diameter detector ring, a 54-cm-diameter detector ring, and an 89-cm-diameter detector ring and evaluated the NECRs. In the clinical dose range, the NECR of the small-diameter brain PET system was lower than those of the whole-body PET system unless the electronics was advanced. We conclude that the small-diameter brain PET system with conventional electronics has not so much advantage except for the lower cost and the higher spatial resolution. However, advanced electronics can reduce degradation of NECR, so the small-diameter brain PET system with advanced electronics will be useful for clinical studies.

Brain-Dedicated Emission Tomography Systems: A Perspective on Requirements for Clinical Research and Clinical Needs in Brain Imaging

Research applications of human brain positron emission tomography (PET) imaging have been in place for over 40 years. The unique combination of quantitative PET instrumentation and reconstruction, novel radiotracers, and kinetic modeling has led to the development and application of a huge number of imaging paradigms. These have been applied to the understanding of normal function and pathophysiology, the development of drugs for neuropsychiatric disorders, and the study of drug interactions with the brain. I have had been fortunate to be part of this great endeavor throughout my research career, using brain-dedicated PET scanners, such as the HRRT [item [1] in the Appendix]. Over time, these systems have offered important advantages over whole-body systems. However, due to the limited clinical market for brain PET imaging, scanner manufacturers have focused exclusively on large field-of-view systems for the past two decades [item [2] in the Appendix]. The goal of this perspective is to share a viewpoint on the utility of brain-dedicated PET systems for human research, particularly in the context of novel radiotracers with dynamic studies using kinetic modeling.

Feasibility study of a point-of-care positron emission tomography system with interactive imaging capability.

We investigated the feasibility of a novel positron emission tomography (PET) system that provides near real-time feedback to an operator who can interactively scan a patient to optimize image quality. The system should be compact and mobile to support point-of-care (POC) molecular imaging applications. In this study, we present the key technologies required and discuss the potential benefits of such new capability. The core of this novel PET technology includes trackable PET detectors and a fully three-dimensional, fast image reconstruction engine implemented on multiple graphics processing units (GPUs) to support dynamically changing geometry by calculating the system matrix on-the-fly using a tube-of-response approach. With near real-time image reconstruction capability, a POC-PET system may comprise a maneuverable front PET detector and a second detector panel which can be stationary or moved synchronously with the front detector such that both panels face the region-of-interest (ROI) with the detector trajectory contoured around a patient's body. We built a proof-of-conceptprototype using two planar detectors each consisting of a photomultiplier tube (PMT) optically coupled to an array of 48×48 lutetium-yttrium oxyorthosilicate (LYSO) crystals (1.0×1.0×10.0mm3 each). Only 38×38 crystals in each arrays can be clearly re-solved and used for coincidence detection. One detector was mounted to a robotic arm which can position it at arbitrary locations, and the other detector was mounted on a rotational stage. A cylindrical phantom (102mm in diameter, 150mm long) with nine spherical lesions (8:1 tumor-to-background activity concentration ratio) was imaged from 27 sampling angles. List-mode events were reconstructed to form images without or with time-of-flight (TOF) information. We conducted two Monte Carlo simulations using two POC-PET systems. The first one uses the same phantom and detector setup as our experiment, with the detector coincidence re-solving time (CRT) ranging from 100 to 700ps full-width-at-half-maximum (FWHM). The second study simulates a body-size phantom (316×228×160mm3 ) imaged by a larger POC-PET system that has 4×6 modules (32×32 LYSO crystals/module, four in axial and six in transaxial directions) in the front panel and 3×8 modules (16×16 LYSO crystals/module, three in axial and eight in transaxial directions) in the back panel. We also evaluated an interactive scanning strategy by progressively increasing the number of data sets used for image reconstruction. The updated images were analyzed based on the number of data sets and the detector CRT. The proof-of-concept prototype re-solves most of the spherical lesions despite a limited number of coincidence events and incomplete sampling. TOF information reduces artifacts in the reconstructed images. Systems with better timing resolution exhibit improved image quality and reduced artifacts. We observed a reconstruction speed of 0.96×106 events/s/iteration for 600×600×224 voxel rectilinear space using four GPUs. A POC-PET system with significantly higher sensitivity can interactively image a body-size object from four angles in less than 7min. We have developed GPU-based fast image reconstruction capability to support a PET system with arbitrary and dynamically changing geometry. Using TOF PET detectors, we demonstrated the feasibility of a PET system that can provide timely visual feedback to an operator who can scan a patient interactively to support POC imaging applications.

Timing-performance evaluation of Cherenkov-based radiation detectors

With the upgradation of detector components, such as scintillators and photodetectors, the PET-image signal-to-noise ratio of time-of-flight positron emission tomography (TOF-PET) systems has been improved, compared to those of ordinary nonTOF-PET systems. A TOF-PET with an ultrahigh time resolution, for example a coincidence time resolution (CTR) better than few tens of picoseconds, can not only improve the image quality, but also remove the image reconstruction process, significantly impacting medical imaging. Therefore, it is crucial to develop a high-time resolution PET detector. We focus on the prompt emission of Cherenkov radiation, owing to the instantaneousness of which, a high time resolution can be expected. One of the candidates for the Cherenkov radiator is lead fluoride (PbF2) due it has excellent properties, including transparency toward the ultraviolet region, high refractive index (n = 1.82), and high density (7.77 g/cm3). Moreover, it does not contain radioisotopes, unlike lutetium-based scintillators, which are commonly used in the currently available TOF-PET detectors. In this work, we experimentally investigate the timing performance of PbF2-based Cherenkov detectors, breaking down the timing performance into physical components. 3×3×5 mm3 and 9.6×9.6×5 mm3 PbF2 crystals are used as Cherenkov radiators; both are attached to a microchannel plate photomultiplier tube (MCP-PMT) because the single channel MCP-PMT is one of the best photodetectors in terms of the SPTR, which is 25 ps full width at half maximum (FWHM). All the surfaces, except the end surface where the MCP-PMT is connected, are wrapped in black tape to suppress the reflections of the Cherenkov photons in the crystal. The CTR is measured by placing a detector pair face-to-face, using an 22Na point source, and an oscilloscope at 20 GS/s with a set bandwidth of 4.2 GHz. A CTR of 46.9 ps FWHM, corresponding to a position resolution of 7.0 mm, is obtained, consistent with our simulation results.

Timing performance of organic scintillators coupled to silicon photomultipliers

This work compares the time resolution obtained from coincidence measurements using combinations of four types of silicon photomultipliers (SiPMs), five types of organic scintillators, and two timing algorithms. Organic scintillators in combination with SiPMs have shown to have equivalent or better time resolution and pulse shape discrimination (PSD) capability than with photo-multiplier tubes (PMTs). In addition, SiPMs are more compact and require low operating voltage. These factors have made them suitable for the development of compact neutron scatter cameras (NSCs) and potentially low-cost positron emission tomography (PET) systems. The following measurements analyze combinations of organic scintillators and SiPMs that could be used for these applications. Measurements and analyses assessed the time resolution of standard-output and fast-output pulses of the various scintillator and SiPM combinations. The coincidence measurements were performed using a Na-22 source to characterize time resolution where all coincident events above about a 40 keVee threshold were analyzed. Uncertainties were determined by applying bootstrapping to the acquired data sets. The full width at half maximum (FWHM) of the time difference distribution for each combination is reported as the time resolution of the system. For standard-output pulses, we found that using a digital implementation of analogue constant fraction discrimination (DIACFD) improved timing resolution greater than a single standard deviation for six of the fourteen tested combinations compared to using digital constant fraction discrimination (DCFD). Conversely, for fast-output pulses, DCFD with a moving average filter produced the best timing resolution. Among the tested non-PSD capable organic scintillators, EJ-228 coupled to a MicroFJ-SMA-60035 SensL SiPM yielded the best time resolution of 303 ± 4 ps for standard-output pulses and 227 ± 4 ps for fast-output pulses. The best time resolution found for PSD capable organic scintillators was obtained using Inrad Optics stilbene coupled to a MicroFJ-SMA-60035 SensL SiPM which yielded a time resolution of 428 ± 6 ps for standard-output pulses and 293 ± 4 ps for fast-output pulses.

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.

Simulation study on system configuration refinement for the PET–laparoscope system—an intraoperative tool for surgical navigation in gastric cancer surgery

Positron emission tomography is used to diagnose gastric cancer and identify lymph node metastasis preoperatively. However, locating these lymph node metastases during surgery is difficult and usually leads to unnecessary node removal and results in a deterioration of a patient’s quality of life. To improve the quality of life, intraoperative identification of lymph node metastasis is necessary. A positron emission tomography system consisting of an external detector array connected to the patient’s surgical bed and a movable detector probe which is inserted into the patient’s stomach along with a laparoscopic instrument has been proposed to identify lymph node metastasis intraoperatively. This study presents the image reconstruction of a system involving data acquisition using a movable detector. A simulation-based investigation on the refinement of the detector design is also presented. The image is reconstructed using a modified filtered back projection algorithm. In the simulation experiments, the proposed algorithms and detector setup successfully reconstructed images of radiation sources with better than 7-mm spatial resolution in a 2-min scan.

A Sensor Network Architecture for Digital SiPM-Based PET Systems

Digital silicon photo-multipliers (SiPMs) have emerged in the recent past as a viable low cost alternative to photomultiplier tubes in positron emission tomography systems, providing multiple timestamps, energy and scintillation coordinates at high spatial granularity as well as MRI compatibility. The rich but large datasets generated by digital SiPM sensors have posed a data preprocessing and acquisition challenge, at the sensor, module and system level when a multitude of such sensors are to be used. In this paper, we present a sensor network-based approach for data acquisition, scalable to multiring configurations, whereby each module acts as an autonomous sensing and computing unit, capable of determining in real time basic information for each scintillation event and communicating it to its peers. The proposed architecture is equally applicable to modules based on analog SiPMs with local digitization. Coincidence detection can then take place in the ring itself, in a deferred and distributed manner to ensure scalability and allow to fully process only the fraction of the total events which corresponds to true coincidences. Simulations and experimental results show that it is indeed possible to handle the system level challenges associated with digital SiPMs at data rates compatible with realistic configurations, including event packet transfers and real-time coincidence detection, using Gb/s serial communication links for internode communication. The downside of the proposed architecture is represented by the need, at module level, for additional connectivity and processing power. We also address possible solutions for network-based clock synchronization, in particular a hybrid scheme, combining a hard-wired ring clock distribution network with a network-based clock offset estimator. The latter was tested in an 8-node system, performing synchronization in real time with a worst-case phase estimator stability (standard deviation of the clock phase offset estimation between two nodes) of about 160 ps, and substantial room for improvement (at least $5\boldsymbol \times $ according to simulations).

Studies on sub-millimeter LYSO:Ce, Ce:GAGG, and a new Ce:GFAG block detector for PET using digital silicon photomultiplier

The spatial, timing, and energy resolutions of a detector affect the image quality of a positron emission tomography (PET) system. These parameters are in turn dependent on various factors, such as the choice of scintillator, photodetectors, reflector material, and surface treatment (rough or polished) of the scintillators. In this study, we investigated the performances of sub-millimeter scintillator array, LYSO:Ce (polished and rough surfaces with BaSO4reflector or enhanced specular reflector (ESR)), a gadolinium aluminum gallium garnet (Ce:GAGG, rough surface with BaSO4reflector), and a new gadolinium fine aluminum gallate (Ce:GFAG, rough surface with BaSO4reflector) detectors. The outer dimension of each scintillator block was ∼12 × 12 mm with a 12 × 12 matrix of 0.9 ×0.9× 6 mm3 crystal elements. These blocks were optically coupled to a digital silicon photomultiplier (dSiPM, DPC-3200-22-44) with a 1 mm thick optical guide. Experiments were conducted at a sensor temperature of ∼15 °C, and a two-dimensional position histogram for 22Na gamma photons showed that all pixels were clearly resolved for all block detectors (peak-to-valley ratios ranging from 5.3 to 11.1). The average energy resolutions for the LYSO (ESR, rough), LYSO (ESR, polished), LYSO (BaSO4, rough), LYSO (BaSO4, polished), GAGG (BaSO4, rough), and GFAG (BaSO4, rough) arrays were measured as 11.3%, 10.2%, 18.7%, 10.3%, 10.0%, and 13.2% full width at half maximum (FWHM), respectively. The coincidence resolving times (with 3 × 3 × 5 mm3 LYSO crystal as reference) for the LYSO (ESR, rough), LYSO (ESR, polished), LYSO (B aSO4, rough), LYSO (BaSO4, polished), GAGG (BaSO4, rough), and GFAG (BaSO4, rough) arrays were 209 ps, 222 ps, 197 ps, 230 ps, 321 ps, and 276 ps, respectively. In conclusion, the new GFAG scintillator may be a promising candidate for future high-resolution time-of-flight (ToF) PET systems, considering the tradeoff between its performance and potential to be grown at a lower cost.

Shared-photodetector readout to improve the sensitivity of positron emission tomography

Sensitivity is an important performance characteristic of positron emission tomography (PET) systems. Improved sensitivity can be used to reduce injected dose, reduce scan time, or improve the signal-to-noise ratio and temporal resolution for dynamic studies. One way to improve the sensitivity of PET scanners is to reduce the gaps between detector modules. In this paper, a new signal processing method, named the shared-photodetector readout method, is proposed and evaluated. In this method, the signals generated in nearest neighbor photodetectors adjacent to the detector module of interest, were used to help identify the interaction location in the detector module of interest. Using this method, scintillator array-based detector modules with almost 100% packing fraction can be built, and the edge crystals can be clearly resolved, even when the crystals are small compared to the photodetector size. To evaluate this signal processing concept in one dimension, a detector block with four dual-ended readout detector modules, was designed. The detector block consisted of eight 4 × 4 arrays of SensL MicroFJ-30035 SiPMs coupled to both ends of a 14 × 56 array of 0.9 × 0.9 × 20 mm3 LYSO elements with a pitch size of 0.96 mm and a length of 20 mm. Performance in terms of energy resolution, flood histogram, timing resolution and depth-of-interaction resolution obtained using the shared-photodetector readout method were compared to those obtained using a conventional readout method. The results show that better over-all performance was achieved using the shared-photodetector readout method, especially at the edges and corners of the array.

Development of broad-band high-reflectivity multilayer film for positron emission tomography system

The use of non-ideal reflective materials and low-precision manual manufacturing technologies is a fundamental technical obstacle blocking the positron emission tomography (PET) systems from achieving better performances. We propose to address that long-standing obstacle with advanced multilayer dielectric coating technologies. We designed an broad-band multilayer high-reflectivity (HR) film that can be coated directly on the surface of ultra-precision polished lutetium-yttrium oxyorthosilicate (LYSO) scintillators. The film consists of 48 layers of TiO2/SiO2/HfO2 which are deposited on LYSO scintillator crystal alternately using the electron beam evaporation method. The overall thickness of the HR film is about 3μm. The HR film combines 3 quarter wavelength reflective films, with the central wavelengths of 365 nm, 430 nm and 570 nm respectively, to match the emission spectrum of the LYSO scintillator. The optical experimental results show that the HR film achieved an excellent average reflectivity of 99.50% at 8̂ incident angle for incident lights with wavelengths between 360 to 620 nm. The average reflectivity at 60̂ incident angle is higher than 90%. The results of the hardness experiments and the adhesive strength experiments show that the HR film has an excellent mechanical strength. The HR coating technology developed in this study is very attractive because it allows to “print” high-performance reflectors on a scintillator directly with high-precision, instead of manually gluing reflective films on the scintillator. Thus, we conclude that the HR film provides a viable solution to the long standing technical bottleneck that limits the development of high-performance detectors for advanced PET imaging.

Gradient Tree Boosting-Based Positioning Method for Monolithic Scintillator Crystals in Positron Emission Tomography

Monolithic crystals are considered as an alternative for complex segmented scintillator arrays in positron emission tomography systems. Monoliths provide high sensitivity, good timing, and energy resolution while being cheaper than highly segmented arrays. Furthermore, monoliths enable intrinsic depth of interaction capabilities and good spatial resolutions (SRs) mostly based on statistical calibrations. To widely translate monoliths into clinical applications, a time-efficient calibration method and a positioning algorithm implementable in system architecture such as field-programmable gate arrays (FPGAs) are required. We present a novel positioning algorithm based on gradient tree boosting (GTB) and a fast fan beam calibration requiring less than 1 h per detector block. GTB is a supervised machine learning technique building a set of sequential binary decisions (decision trees). The algorithm handles different sets of input features, their combinations and partially missing data. GTB models are strongly adaptable influencing both the positioning performance and the memory requirement of trained positioning models. For an FPGA-implementation, the memory requirement is the limiting aspect. We demonstrate a general optimization and propose two different optimization scenarios: one without compromising on positioning performance and one optimizing the positioning performance for a given memory restriction. For a 12 mm high LYSO-block, we achieve an SR better than 1.4 mm FWHM.

Estimating the NEMA characteristics of the J-PET tomograph using the GATE package

A novel whole-body positron emission tomography (PET) system based on plastic scintillators is developed by the J-PET Collaboration. It consists of plastic scintillator strips arranged axially in the form of a cylinder, allowing the cost-effective construction of the total-body PET system. In order to determine the properties of the scanner prototype and optimize its geometry, advanced computer simulations were performed using the GATE (Geant4 application for tomographic emission) software.The spatial resolution, sensitivity, scatter fraction and noise equivalent count rate were estimated according to the National Electrical Manufacturers Association norm, as a function of the length of the tomograph, the number of detection layers, the diameter of the tomographic chamber and for various types of applied readout. For the single-layer geometry with a diameter of 85 cm, a strip length of 100 cm, a cross-section of 4 mm × 20 mm and silicon photomultipliers with an additional layer of wavelength shifter as the readout, the spatial resolution (full width at half maximum) in the centre of the scanner is equal to 3 mm (radial, tangential) and 6 mm (axial). For the analogous double-layer geometry with the same readout, diameter and scintillator length, with a strip cross-section of 7 mm × 20 mm, a noise equivalent count rate peak of 300 kcps was reached at 40 kBq cc−1 activity concentration, the scatter fraction is estimated to be about 35% and the sensitivity at the centre amounts to 14.9 cps kBq−1. Sensitivity profiles were also determined.

Large surface gamma cameras for medical imaging: characterization of the bismuth germanate blocks

The CLaRyS collaboration focuses on the development of gamma detectors for medical applications, in particular for what concerns the range monitoring in ion beam therapy. Part of the research program aims to implement two gamma-camera clinical prototypes, a multi-collimated camera and a Compton camera. A common absorber detector has been designed for the two prototypes, based on bismuth germanate (BGO) blocks, 3.5×3.8×3 cm3, assembled in various geometrical configurations to meet the application requirements. The surface of each block is streaked in a matrix of 8×8 pseudo pixels, which makes possible a position reconstruction via Anger logic from the signals collected by four read-out photo-multiplier tubes. The whole set of available blocks comes from a dismantled positron emission tomography system by Siemens, so that each single block must be tested and characterized in terms of space, time and energy response. We present in this work the implemented characterization method, which leads to a complete estimation of the block response via gamma source irradiations and data analysis devoted to a custom calibration for the imaging performance optimization of each detector module. A reference set of blocks has been completely characterized and showed very homogeneous responses: the average energy resolution is 25% FWHM at 511 keV and 20% FWHM at 1275 keV, the time resolution ranges between 3.9 and 5.3 ns FWHM and the spatial resolution has been verified to be limited to the pseudo-pixel size.