Digital Silicon Photomultipliers Research Articles

Wavelength discrimination (WLD) TOF-PET detector with DOI information

Depth-of-interaction (DOI) encoding can contribute to improving spatial resolution uniformity and sensitivity in positron-emission-tomography (PET) scanners. In addition, time-of-flight (TOF) PET scanners with DOI encoding have received considerable interest because of their potential for improving the spatial resolution, sensitivity, and image quality of the overall system. In this study, a new DOI detector configuration utilizing scintillators’ emission wavelength is proposed, and experimental results on the energy, timing, and DOI performance of the detector are provided.The DOI information from the proposed phoswich-type detector can be acquired at the detector level without complex signal processing by utilizing a single optical filter with customized optical properties. For this, we used either a short pass filter (SPF) or a long pass filter (LPF) that allows light photons of a specific wavelength to pass. The two-layered phoswich detector was configured with two scintillators with different photon-emission spectra. In this study, we used Ce:GAGG (3 mm × 3 mm × 10 mm) and LYSO:Ce (3 mm × 3 mm × 10 mm) as the top and bottom layer scintillators, respectively. A digital silicon photomultiplier (dSiPM) was used as the photosensor and for data acquisition. The phoswich detector was placed in the center of two dSiPM pixels, where one of the dSiPM pixels was covered with the optical filter, and the light guide was placed on the other pixel. The detector was tested for energy, timing, and DOI encoding performance.When an SPF was used, the energy resolutions of 16.2% and 11.8% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer) respectively without correcting for saturation effect. With a small (3 mm × 3 mm × 5 mm) LYSO crystal as the reference detector, CRTs (coincidence-resolving times) of 338 ps and 244 ps were recorded for the top and bottom layers respectively. The detector configuration also provides an excellent DOI-separation figure-of-merit (FoM) value of 1.9.In the case of LPF, the energy resolutions of 12.0% and 12.9% were achieved for the Ce:GAGG (top layer) and LYSO:Ce (bottom layer), respectively. CRTs (coincidence resolving times) of 314 ps and 263 ps were recorded for the top and bottom layers, respectively. The DOI-separation FoM value of 1.5 was achieved in this setup.Results show that the proposed method can provide excellent discrete DOI positioning accuracy without compromising the timing performance of the detector.

Embedded time of arrival estimation for digital silicon photomultipliers with in-pixel TDCs

Including precise time-of-flight measurements in the reconstruction of positron emission tomography (PET) images reduces the required radiotracer dose, scan time and improves the contrast-to-noise ratio. However, major improvements of photodetectors and scintillators are still required to fully exploit the projected 10 ps FWHM coincidence timing resolution (CTR). On the photodetector side, single photon avalanche diode (SPAD) detectors based on digital architectures promise better CTR by timestamping multiple scintillation photons from a single PET event with an array of in-pixel time-to-digital converters (TDCs). The digital nature of these architectures allow the introduction of data processing schemes to optimally combine the produced timestamps for a scintillation event with a time of arrival estimator. This processing is typically performed on an external computing platform, which requires the transfer of large quantities of raw data and leads to a higher dead time owing to the large number of TDCs. To circumvent this issue, we propose a photodetector architecture with in-pixel TDCs that embeds the data processing scheme, reducing the dataset produced by the TDC array to a single timestamp per scintillation event. Hardware-in-the-loop simulations of the fabricated CMOS 65 nm post-processing circuit coupled with simulated SPADs and 1 × 1 × 10 mm3 LYSO scintillators demonstrate a CTR improvement from 160 ps to 126 ps, a bandwidth reduction from 34.7 Mbit/s to 0.5 Mbit/s and a serial transmission dead time reduction from 69 μs to 1μs.

A 256 Pixelated SPAD readout ASIC with in-Pixel TDC and embedded digital signal processing for uniformity and skew correction

Abstract Coincidence timing resolution (CTR) of 10 ps FWHM would drastically increase the contrast of images in pre-clinical and clinical positron emission tomography (PET) due to the capability to localize the annihilation site along the line of response with unprecedented accuracy. Currently, the scintillators and the photodetectors with their electronic readout are the two limiting factors to reach such a timing resolution. On the photodetector side, the single photon timing resolution (SPTR) must be improved below 4 ps RMS to reach a CTR of 10 ps FWHM. To this end, we propose to use a 3D digital silicon photomultiplier (3DdSiPM) where each single photon avalanche diode (SPAD) has its own quenching circuit (QC) and time-to-digital converter (TDC). To reach an SPTR of 4 ps RMS in an array of SPAD requires to correct each readout pixel individually to minimize the impact of TDC least significant bit (LSB) non-uniformities and SPAD-to-SPAD skew. For this purpose, we developed an array of 256 pixels of SPAD readout circuits optimized for high timing precision with embedded digital signal processing. The latter is a code-to-time conversion where each pixel is individually corrected for TDC LSB variation and skew. Measurement results on single pixels (QC and TDC) show timing jitter down to 8 ps RMS. The uncorrected array timing jitter is about 87 ps RMS and is reduced to 18 ps RMS after skew correction and TDC LSB correction.

PennPET Explorer: Design and Preliminary Performance of a Whole-Body Imager.

We report on the development of the PennPET Explorer whole-body imager. Methods: The PennPET Explorer is a multiring system designed with a long axial field of view. The imager is scalable and comprises multiple 22.9-cm-long ring segments, each with 18 detector modules based on a commercial digital silicon photomultiplier. A prototype 3-segment imager has been completed and tested with an active 64-cm axial field of view. Results: The instrument design is described, and its physical performance measurements are presented. These include sensitivity of 55 kcps/MBq, spatial resolution of 4.0 mm, energy resolution of 12%, timing resolution of 256 ps, and a noise-equivalent count rate above 1,000 kcps beyond 30 kBq/mL. After an evaluation of lesion torso phantoms to characterize quantitative accuracy, human studies were performed on healthy volunteers. Conclusion: The physical performance measurements validated the system design and led to high-quality human studies.

Dark Count Resilient Time Estimators for Time-of-Flight PET

A better timing resolution for positron emission tomography (PET) detectors adds time-of-flight information to increase the image contrast. The timing resolution can be improved by timestamping multiple photons from a scintillation event and combining the information using the mean or the Gauss–Markov estimator. To circumvent the performance degradation incurred by these estimators at higher dark count rates (DCRs), two dark count resilient algorithms were studied independently in the literature: the filtered Gauss–Markov and the artificial neural networks. This paper presents parametric simulations that compare them under different DCR conditions. We studied two different types of scintillator-based digital silicon photomultipliers: a typical $1\times 1\times 10$ mm3 LYSO:Ce crystal paired with a 180 ps single photon timing resolution (SPTR) photodetector and a fast $1\times 1\times 3$ mm3 LYSO:Ce:rCa crystal paired with a 35 ps SPTR photodetector. For a single photon avalanche diode with DCR over 1 Hz/ $\mu {\mathrm{ m}}^{2}$ , simulations show a significant coincidence timing resolution (CTR) gain when using the filtered Gauss–Markov estimator, improving the CTR of the typical detector by 10% and the fast detector by 33% over the unfiltered Gauss–Markov estimator. In all evaluated cases, the filtered Gauss–Markov estimator yields the lowest CTR.

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.

Dark count rate and band to band tunneling optimization for single photon avalanche diode topologies

This paper proposes two optimal designs of single photon avalanche diodes (SPADs) minimizing dark count rate (DCR). The first structure is introduced as p+/pwell/nwell, in which a specific shallow pwell layer is added between p+ and nwell layers to decrease the electric field below a certain threshold. The simulation results show on average 19.7% and 8.5% reduction of p+/nwell structure’s DCR comparing with similar previous structures in different operational excess bias and temperatures respectively. Moreover, a new structure is introduced as /nwell/pwell, in which a specific shallow nwell layer is added between n+ and pwell layers to lower the electric field below a certain threshold. The simulation results show on average 29.2% and 5.5% decrement of /nwell structure’s DCR comparing with similar previous structures in different operational excess bias and temperatures respectively. It is shown that in higher excess biases (about 6 volts), the n+/nwell/pwell structure is proper to be integrated as digital silicon photomultiplier (dSiPM) due to low DCR. On the other hand, the p+/pwell/nwell structure is appropriate to be utilized in dSiPM in high temperatures (above 50 °C) due to lower DCR value.

A theoretical study of digital silicon photomultiplier utilization in diffuse optical imaging systems.

Digital silicon photomultiplier (dSiPM) is introduced for diffuse optical imaging (DOI) applications instead of conventional photomultiplier tubes and avalanche photodiodes (APDs) as a state-of-the-art detector. According to the low-level light regime in DOI applications, high sensitivity and high dynamic range (DR) image sensors are needed for DOI systems. dSiPM is proposed as a developing detector which can detect low-level lights. Also, an accurate equation is obtained for calculating the DR of dSiPMs. Different dSiPMs and the corresponding benefits are studied for DOI applications. Furthermore, a 120 dB DR dSiPM is chosen for use in DOI systems. It is shown that dSiPMs can be utilized in DOI configurations such as time domain (TD), frequency domain (FD) and continuous wave (CW) systems. Ultimately, by utilizing dSiPM in DOI systems, the DOI method can be used for thoracic imaging due to the high DR and signal-to-noise ratio (SNR) of the detector.

Monte Carlo Simulation-Based Maximum-Likelihood Position Estimate for Monolithic Scintillation Detectors

In this paper, we propose a Monte Carlo simulation-based maximum-likelihood position estimate (MLPE) for the localization of photons in monolithic scintillator detectors. The difference between the new method and the existing MLPEs is that the new method does not require repeated experiments to generate the look-up table (LUT), which contains the detector response for two-dimensional space; instead, a single Monte Carlo simulation generates the LUT. The LUT generated from the simulation was then directly applied to experimental measurements with a Gaussian noise model-based MLPE algorithm. Using the new method, monolithic scintillator detector, which was assembled from monolithic NaI(Tl) and digital SiPM, a differential linearity 0.1 mm and a spatial resolution of 2.0 mm full width at half maximum (FWHM) offering 64.1% and 60.9% improvements, respectively compared to the results obtained over the center-of-gravity method.

Status and perspectives of solid state photon detectors

Abstract Development of solid state photon detectors is a mature field of engineering and technology based on well-established grounds of solid state physics, and, in the same time, a frontier area of research and innovations faced with dramatic challenges. The ultimate challenge for the modern developments is a detection of optical signals at a quantum level – resolving arrival time and spatial location of individual photons – to realize a formula “every photon counts”. To succeed, the developments are focused on improvements in three directions: threshold sensitivity and photon number resolution, fast timing and time resolution, and fine granularity imaging with fast readout. There are many inherent trade-offs to be resolved in each direction. Development of Silicon Photomultiplier (SiPM) is considered as one of the most promising innovations toward “near ideal” photon detector. SiPMs of various designs have been developed in the 1990s–2000s in Russia, and their unique performance in the photon number and time resolution has been demonstrated and recognized in the mid-2000s. Now SiPMs are widely implemented in nuclear medicine, high energy physics, astrophysics , and Cherenkov light detection. However, developers of Geiger Mode APD or SPAD arrays based on active quenching also found new approaches and opportunities for considerable improvements using modern CMOS technology, namely: reduction of a dead space occupied by electronics, multiplexing readout architecture, backside illumination, and 3D integration of photosensor and electronic layers (3D digital SiPM). Detection of Cherenkov light is one of the most challenging applications for photodetectors . Superior photon number resolution starting from single photons, picosecond-scale time resolution, and large-area imaging are typical requirements, and all these highly demanded capabilities are contradictory. This report presents overview and analysis of the state-of-art in the modern solid state photon detectors as well as their potential and perspectives to meet the quantum imaging challenge.

Multipurpose, Fully Integrated 128 $\times$ 128 Event-Driven MD-SiPM With 512 16-Bit TDCs With 45-ps LSB and 20-ns Gating in 40-nm CMOS Technology

A multipurpose monolithic array of 2 $ {\times }$ 2 multichannel digital silicon photomultipliers (MD-SiPMs) fabricated in 40-nm CMOS technology is presented. Each MD-SiPM comprises 64 $ {\times }$ 64 smart pixels connected to 128 low-power 45-ps sliding-scale time-to-digital converters (TDCs). The system can operate in two different modes: 1) event-driven and 2) frame-based. The first is suited for positron emission tomography (PET) and the second for synchronous applications like LiDAR. The design includes electronics to capture gamma events by means of a scintillator. The digital readout is fully embedded in the sensor and it is reconfigurable by SPI. Data packets are sent following a simple protocol compatible with an external FIFO, therefore making use of an FPGA optional. Every MD-SiPM can deliver up to 64M time-stamps/s. The sensor can be arranged in any type of configuration through a dedicated synchronization input and can be used to operate jointly with an event generator, such as a pulsed laser, which is useful in many applications. Inherently compatible with 3-D-stacking technology, the sensor can serve as front-end electronics when it is used with a different SPAD silicon tear.

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).

PET performance evaluation of the small-animal Hyperion IID PET/MRI insert based on the NEMA NU-4 standard

The Hyperion IID PET insert is the first scanner using fully digital silicon photomultipliers for simultaneous PET/MR imaging of small animals up to rabbit size. In this work, we evaluate the PET performance based on the National Eletrical Manufacturers Association (NEMA) NU 4-2008 standard, whose standardized measurement protocols allow comparison of different small-animal PET scanners. The Hyperion IID small-animal PET/MR insert comprises three rings of 20 detector stacks with pixelated scintillator arrays with a crystal pitch of 1 mm, read out with digital silicon photomultipliers. The scanner has a large ring diameter of 209.6 mm and an axial field of view of 96.7 mm. We evaluated the spatial resolution, energy resolution, time resolution and sensitivity by scanning a 22Na point source. The count rates and scatter fractions were measured for a wide range of 18F activity inside a mouse-sized scatter phantom. We evaluated the image quality using the mouse-sized image quality phantom specified in the NEMA NU4 standard, filled with 18F. Additionally, we verified the in-vivo imaging capabilities by performing a simultaneous PET/MRI scan of a mouse injected with 18F-FDG. We processed all measurement data with an energy window of 250 keV to 625 keV and a coincidence time window of 2 ns. The filtered-backprojection reconstruction of the point source has a full width at half maximum (FWHM) of 1.7 mm near the isocenter and degrades to 2.5 mm at a radial distance of 50 mm. The scanner’s average energy resolution is 12.7% (ΔE/E FWHM) and the coincidence resolution time is 609 ps. The peak absolute sensitivity is 4.0% and the true and noise-equivalent count rates reach their peak at an activity of 46 MBq with 483 kcps and 407 kcps, respectively, with a scatter fraction of 13%. The iterative reconstruction of the image quality phantom has a uniformity of 3.7%, and recovery coefficients from 0.29, 0.91 and 0.94 for rod diameters of 1 mm, 3 mm and 5 mm, respectively. After application of scatter and attenuation corrections, the air- and water-filled cold regions have spill-over ratios of 6.3% and 5.4%, respectively. The Hyperion IID PET/MR insert provides state-of-the-art PET performance while enabling simultaneous PET/MRI acquisition of small animals up to rabbit size.

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.

Impact of a Bayesian penalized likelihood reconstruction algorithm on image quality in novel digital PET/CT: clinical implications for the assessment of lung tumors

BackgroundThe aim of this study was to evaluate and compare PET image reconstruction algorithms on novel digital silicon photomultiplier PET/CT in patients with newly diagnosed and histopathologically confirmed lung cancer. A total of 45 patients undergoing 18F-FDG PET/CT for initial lung cancer staging were included. PET images were reconstructed using ordered subset expectation maximization (OSEM) with time-of-flight and point spread function modelling as well as Bayesian penalized likelihood reconstruction algorithm (BSREM) with different β-values yielding a total of 7 datasets per patient. Subjective and objective image assessment with all image datasets was carried out, including subgroup analyses for patients with high dose (> 2.0 MBq/kg) and low dose (≤ 2.0 MBq/kg) of 18F-FDG injection regimen.ResultsSubjective image quality ratings were significantly different among all different reconstruction algorithms as well as among BSREM using different β-values only (both p < 0.001). BSREM with a β-value of 600 was assigned the highest score for general image quality, image sharpness, and lesion conspicuity. BSREM reconstructions resulted in higher SUVmax of lung tumors compared to OSEM of up to + 28.0% (p < 0.001). BSREM reconstruction resulted in higher signal-/ and contrast-to-background ratios of lung tumor and higher signal-/ and contrast-to-noise ratio compared to OSEM up to a β-value of 800. Lower β-values (BSREM450) resulted in the best image quality for high dose 18F-FDG injections, whereas higher β-values (BSREM600) lead to the best image quality in low dose 18F-FDG PET/CT (p < 0.05).ConclusionsBSREM reconstruction algorithm used in digital detector PET leads to significant increases of lung tumor SUVmax, signal-to-background ratio, and signal-to-noise ratio, which translates into a higher image quality, tumor conspicuity, and image sharpness.

Quenching Circuit and SPAD Integrated in CMOS 65 nm with 7.8 ps FWHM Single Photon Timing Resolution

This paper presents a new quenching circuit (QC) and single photon avalanche diode (SPAD) implemented in TSMC CMOS 65 nm technology. The QC was optimized for single photon timing resolution (SPTR) with a view to an implementation in a 3D digital SiPM. The presented QC has a timing jitter of 4 ps full width at half maximum (FWHM) and the SPAD and QC has a 7.8 ps FWHM SPTR. The QC adjustable threshold allows timing resolution optimization as well as SPAD excess voltage and rise time characterization. The adjustable threshold, hold-off and recharge are essential to optimize the performances of each SPAD. This paper also provides a better understanding of the different contributions to the SPTR. A study of the contribution of the SPAD excess voltage variation combined to the QC time propagation delay variation is presented. The proposed SPAD and QC eliminates the SPAD excess voltage contribution to the SPTR for excess voltage higher than 1 V due to its fixed time propagation delay.

Optimization of the Signal-to-Background Ratio in Prompt Gamma Imaging Using Energy and Shifting Time-of-Flight Discrimination: Experiments With a Scanning Parallel-Slit Collimator

Much attention is currently being paid to imaging prompt gamma (PG) rays for in vivo proton range monitoring in proton therapy. PG imaging using a collimator is affected by neutron-related background. We study the effectiveness of background reduction experimentally, using a scanning parallel-slit PG collimator as a simplified model of a multislat PG camera. The analysis is focused on the falloff region of the PG intensity profile near the Bragg peak, which is the typical region of interest for proton range estimation. Background reduction was studied for different energy windows, with and without a shifting time-of-flight window that takes into account the proton velocity within the phantom. Practical methods are put forward that apply to cyclotron-based pencil beams. The parallel-slit collimator was placed in front of arrays of cerium-doped lutetium yttrium silicate-coupled digital silicon photomultipliers, used to measure energy and time spectra together with intensity profiles of prompt events emitted from a polymethylmethacrylate phantom irradiated with a 160-MeV proton pencil beam. The best signal-to-background ratio of ~1.6 was similar to that obtained previously with a knife-edge-slit collimator. However, the slope-over-noise ratio in the PG-profile falloff region, was ~1.2 higher for the present collimator, given its better resolution.

Sub-3 mm, near-200 ps TOF/DOI-PET imaging with monolithic scintillator detectors in a 70 cm diameter tomographic setup

Recently, a monolithic scintillator detector for time-of-flight (TOF)/depth-of-interaction (DOI) positron emission tomography (PET) was developed. It has a detector spatial resolution of ~1.7 mm full-width-at-half-maximum (FWHM), a coincidence resolving time (CRT) of ~215 ps FWHM, and ~4.7 mm FWHM DOI resolution. Here, we demonstrate, for the first time, the imaging performance of this detector in a 70 cm diameter PET geometry. We built a tomographic setup representative of a whole-body clinical scanner, comprising two coaxially rotating arms, each carrying a detector module, and a central, rotating phantom table. The fully automated setup sequentially acquires all possible lines of response (LORs) of a complete detector ring, using a step-and-shoot acquisition approach. The modules contained 2 × 2 detectors, each detector consisting of a 32 mm × 32 mm × 22 mm LYSO crystal and a digital silicon photomultiplier (dSiPM) array. The system spatial resolution was assessed using a Na-22 point source at different radial distances in the field-of-view (FOV). Using 2D filtered back projection (2D FBP, non-TOF), tangential and radial spatial resolutions of ~2.9 mm FWHM were obtained at the center of the FOV. The use of DOI information resulted in almost uniform spatial resolution throughout the FOV up to a radial distance of 25 cm, where the radial and tangential resolution are ~3.3 mm FWHM and ~4.7 mm FWHM, respectively, whereas without DOI the resolution deteriorates to ~9 mm FWHM. Additional measurements were performed with a Na-22 filled Derenzo-like phantom at different locations within the FOV. Images reconstructed with a TOF maximum-likelihood expectation-maximization (TOF ML-EM) algorithm show that the system is able to clearly resolve 3 mm diameter hot rods up to 25 cm radial distance. The excellent and uniform spatial resolution, combined with an energy resolution of 10.2% FWHM and a CRT of ~212 ps FWHM, indicates a great potential for monolithic scintillators as practical high-performance detectors in TOF/DOI-PET systems.

Optimization of DPC dSiPM-Based DOI Compton Camera by Monte Carlo Simulation

In this paper, a Compton camera capable of estimating depth of interaction (DOI) in the detector was designed and optimized using Geant4. The proposed DOI Compton camera consists of two pixelated CsI(Tl) scintillators coupled with digital silicon photomultipliers [Digital Photon Counter (DPC)-3200-22]. The parameters affecting the performance of a Compton camera were evaluated for optimization in terms of image sensitivity and image resolution with a measure of figure of merit. The considered parameters were: 1) type of the reflector surface; 2) thickness of the scintillator; and 3) the distance between the scatter and the absorber detectors. The simulation results showed that a ground white surface is better than a polished white one. The optimal thickness and the scatter–absorber distance were found to be 15 mm and 4 cm, respectively. With the optimized parameters, performance of the DOI Compton camera was estimated for a 137Cs source at 1 m using Geant4. The image resolutions were 6.06° full-width at half-maximum with the list-mode maximum likelihood expectation maximization algorithm. It took 20 s to take Compton images for the gamma ray sources of $0.1~\mu $ Sv/h intensity with the DOI Compton camera. It was shown that there was a clear improvement by employing DOI estimation algorithm in combination with the use of thick scintillators for a Compton camera.

Novel inter-crystal scattering event identification method for PET detectors

Here, we propose a novel method to identify inter-crystal scattering (ICS) events from a PET detector that is even applicable to light-sharing designs. In the proposed method, the detector observation was considered as a linear problem and ICS events were identified by solving this problem. Two ICS identification methods were suggested for solving the linear problem, pseudoinverse matrix calculation and convex constrained optimization. The proposed method was evaluated based on simulation and experimental studies. For the simulation study, an 8 × 8 photo sensor was coupled to 8 × 8, 10 × 10 and 12 × 12 crystal arrays to simulate a one-to-one coupling and two light-sharing detectors, respectively. The identification rate, the rate that the identified ICS events correctly include the true first interaction position and the energy linearity were evaluated for the proposed ICS identification methods. For the experimental study, a digital silicon photomultiplier was coupled with 8 × 8 and 10 × 10 arrays of 3 × 3 × 20 mm3 LGSO crystals to construct the one-to-one coupling and light-sharing detectors, respectively. Intrinsic spatial resolutions were measured for two detector types. The proposed ICS identification methods were implemented, and intrinsic resolutions were compared with and without ICS recovery. As a result, the simulation study showed that the proposed convex optimization method yielded robust energy estimation and high ICS identification rates of 0.93 and 0.87 for the one-to-one and light-sharing detectors, respectively. The experimental study showed a resolution improvement after recovering the identified ICS events into the first interaction position. The average intrinsic spatial resolutions for the one-to-one and light-sharing detector were 1.95 and 2.25 mm in the FWHM without ICS recovery, respectively. These values improved to 1.72 and 1.83 mm after ICS recovery, respectively. In conclusion, our proposed method showed good ICS identification in both one-to-one coupling and light-sharing detectors. We experimentally validated that the ICS recovery based on the proposed identification method led to an improved resolution.