Ruud Vinke

Optimizing timing performance of silicon photomultiplier-based scintillation detectors

Precise timing resolution is crucial for applications requiring time-of-flight (ToF) information such as ToF positron emission tomography (PET). Silicon photomulipliers (SiPM) for PET, with their high output capacitance, are known to require custom preamplifiers to maximize timing performance. In this paper, we prove that simple alternative front-end electronics based on a commercial low-noise RF preamplifier can achieve excellent timing resolution. Two radiation detectors with L(Y)SO:Ce scintillators coupled to Hamamatsu SiPMs (MPPC S10362-33-050C) and front-end electronics based on an RF amplifier (MAR-3SM+), have been fabricated. These detectors were used to detect annihilation gamma photons from a Ge-68 source and the output signals were subsequently digitized by a high speed oscilloscope for offline processing. A coincident resolving time (CRT) of 125 ± 2 ps, 147 ± 3 ps and 186 ± 3 ps FWHM with 2 × 2 × 3 mm3 3 × 3 × 5 mm3 and 3 × 3 × 20 mm3 sized L(Y)SO crystals were measured respectively.

Fast Timing Silicon Photomultipliers for Scintillation Detectors

A new silicon photomultiplier is fabricated for fast timing applications by SensL Technologies Ltd. This new family of silicon photomultiplier, herein referred to as fast SPM devices, is fabricated with a third terminal that has a low output capacitance to improve timing performance. Two fast SPMs (an N-on-P type and a prototype P-on-N type) are assessed for energy and timing performances in scintillation detectors. When coupled with L(Y)SO:Ce crystals, the optimal energy resolutions for the 511 keV photon peak are 13.7% and 13.1%, whereas coincidence resolving times (CRTs) of 184±5 and 157±3 ps are attained with 2 × 2 × 3 mm3 crystals for the N-on-P and P-on-N devices, respectively. For longer crystals (3 × 3 × 20 mm3) , more relevant for positron emission tomography, the CRTs are 298±9 and 234±6 ps for the two SPM types, respectively, a significant improvement from standard SPM devices.

Analytical calculation of the lower bound on timing resolution for PET scintillation detectors comprising high-aspect-ratio crystal elements

Excellent timing resolution is required to enhance the signal-to-noise ratio (SNR) gain available from the incorporation of time-of-flight (ToF) information in image reconstruction for positron emission tomography (PET). As the detectors timing resolution improves, so does SNR, reconstructed image quality, and accuracy. This directly impacts the challenging detection and quantification tasks in the clinic. The recognition of these benefits has spurred efforts within the molecular imaging community to determine to what extent the timing resolution of scintillation detectors can be improved and develop near-term solutions for advancing ToF-PET. Presented in this work, is a method for calculating the Cramér-Rao lower bound (CRLB) on timing resolution for scintillation detectors with long crystal elements, where the influence of the variation in optical path length of scintillation light on achievable timing resolution is non-negligible. The presented formalism incorporates an accurate, analytical probability density function (PDF) of optical transit time within the crystal to obtain a purely mathematical expression of the CRLB with high-aspect-ratio (HAR) scintillation detectors. This approach enables the statistical limit on timing resolution performance to be analytically expressed for clinically-relevant PET scintillation detectors without requiring Monte Carlo simulation-generated photon transport time distributions. The analytically calculated optical transport PDF was compared with detailed light transport simulations, and excellent agreement was found between the two. The coincidence timing resolution (CTR) between two 3 × 3 × 20 mm(3) LYSO:Ce crystals coupled to analogue SiPMs was experimentally measured to be 162 ± 1 ps FWHM, approaching the analytically calculated lower bound within 6.5%.

Side readout of long scintillation crystal elements with digital SiPM for TOF-DOI PET

PURPOSE Side readout of scintillation light from crystal elements in positron emission tomography (PET) is an alternative to conventional end-readout configurations, with the benefit of being able to provide accurate depth-of-interaction (DOI) information and good energy resolution while achieving excellent timing resolution required for time-of-flight PET. This paper explores different readout geometries of scintillation crystal elements with the goal of achieving a detector that simultaneously achieves excellent timing resolution, energy resolution, spatial resolution, and photon sensitivity. METHODS The performance of discrete LYSO scintillation elements of different lengths read out from the end/side with digital silicon photomultipliers (dSiPMs) has been assessed. RESULTS Compared to 3 × 3 × 20 mm(3) LYSO crystals read out from their ends with a coincidence resolving time (CRT) of 162 ± 6 ps FWHM and saturated energy spectra, a side-readout configuration achieved an excellent CRT of 144 ± 2 ps FWHM after correcting for timing skews within the dSiPM and an energy resolution of 11.8% ± 0.2% without requiring energy saturation correction. Using a maximum likelihood estimation method on individual dSiPM pixel response that corresponds to different 511 keV photon interaction positions, the DOI resolution of this 3 × 3 × 20 mm(3) crystal side-readout configuration was computed to be 0.8 mm FWHM with negligible artifacts at the crystal ends. On the other hand, with smaller 3 × 3 × 5 mm(3) LYSO crystals that can also be tiled/stacked to provide DOI information, a timing resolution of 134 ± 6 ps was attained but produced highly saturated energy spectra. CONCLUSIONS The energy, timing, and DOI resolution information extracted from the side of long scintillation crystal elements coupled to dSiPM have been acquired for the first time. The authors conclude in this proof of concept study that such detector configuration has the potential to enable outstanding detector performance in terms of timing, energy, and DOI resolution.

A method to achieve spatial linearity and uniform resolution at the edges of monolithic scintillation crystal detectors

We have performed Monte Carlo simulations of the scintillation light transport between two adjacent 27.2 mm × 26.7 mm × 20 mm LYSO crystals, optically coupled together using MeItmount coupling medium with a refractive index of 1.7. Each crystal was read out by 4 Hamamatsu S118283344M monolithic SiPM arrays from one crystal side. Event positioning reconstruction results show that this optical coupling technique can substantially reduce the edge-artifacts in these crystals. With realistic values for the multiplication noise of the SiPMs and electronic readout noise, we obtain intrinsic spatial resolutions of ~3.1 mm FWHM at the center and ~3.0 mm FWHM at 0.6 mm from the optical coupling edge of the 20 mm thick monolithic crystals. When reducing the number of readout channels by applying a 4 : 1 multiplexing ratio, a ~4.1 and ~4.3 mm FWHM resolution, respectively, at the center and 0.6 mm from the optical coupling edge was obtained. The simulations further show that a coincidence timing resolution of 350 ps FWHM can be obtained with this crystal geometry, with a 30% SiPM photon detection efficiency (PDE).

Readout Electronics and Data Acquisition of a Positron Emission Tomography Time-of-Flight Detector Module With Waveform Digitizer

A multi-element silicon photomultiplier (SiPM) based time-of-flight (ToF) detector module for positron emission tomography (PET) has been developed. The detector module is based on a 4 × 4 array of LYSO-SiPM elements (Hamamatsu MPPC S10931-050P), with individual bias supply for each element. Each element is read out by a wideband, low-noise RF amplifier to maximize timing performance. All 16 outputs are digitized with a high-speed CAEN V1742 digitizer module (32 + 2 channels, 5 GS/s sampling, 12-bit amplitude resolution, 500 MHz input bandwidth) to acquire raw pulse waveforms for offline timing and energy extraction. As the digitizer has no internal trigger for individual channels, a trigger board has been developed which produces a fast pulse that triggers the digitizer whenever any pixel of the detector detects a signal in coincidence with a reference detector. To assess the performance of the prototype module, a 4 × 4 LYSO scintillator array ( 3×3×5 mm3 elements) was coupled to the SiPM photodetectors and energy/timing resolution measurements were performed using a Ge-68 source. At 1.4 V overvoltage, the energy resolution, not corrected for saturation effects of the SiPM, varied from a minimum of 10.1% to a maximum of 13.3% with an average energy resolution of 11.4 ± 0.8% across the 16 channels. With a reference detector (single 3×3×5 mm3 LYSO crystal coupled to a Hamamatsu MPPC S10362-33), the average coincidence resolving time (CRT) across the detector module was 206 ± 7 ps FWHM at 2.4 V overvoltage-the best reported for a PET block (array) detector based on conventional photodetectors to date.

Comparison of end/side scintillator readout with digital-SiPM for ToF PET

Side-readout of scintillation light from crystal elements in PET is an alternative to conventional end-readout configurations, with the benefit of being able to provide fine depth-of-interaction (DOI) information and good energy resolution while achieving excellent timing resolution required for time-of-flight PET. In this study, the performance of discrete LYSO scintillation elements read out from the end/side with digital silicon photomultipliers (dSiPM) has been assessed. Compared to 3 × 3 × 20 mm3 LYSO crystals read out from their ends that gave a coincidence resolving time (CRT) of 162 ± 7 ps FWHM and saturated energy spectra, a side-readout configuration achieved an excellent CRT of 144 ± 2 ps FWHM after correcting for timing skews within the dSiPM and an energy resolution of 11.8 ± 0.2% without requiring energy saturation correction. On the other hand, with smaller 3 × 3 × 5 mm3 LYSO crystals that can also be tiled/stacked to provide DOI information, a timing resolution of 134 ± 6 ps was attained but produced highly saturated energy spectra.

Timing performance comparison of P-on-N and N-on-P silicon photomultipliers

We have investigated the timing performance of P-on-N and N-on-P silicon photomultipliers (SiPM) in light of their use in time-of-flight (TOF) positron emission tomography (PET) detectors. Polished 3 mm × 3 mm × 5 mm and 3 mm × 3 mm × 20 mm LYSO:Ce scintillators were coupled to STmicroelectronics P-on-N and N-on-P SiPMs. The SiPMs were read out with low-noise RF transimpedance amplifiers and fed to a waveform digitizer for offline time pickoff analysis. For the P-on-N devices coincidence resolving times (CRTs) of 234 ps and 189 ps FWHM were obtained with the 3 mm × 3 mm × 20 mm and 3 mm × 3 mm × 5 mm crystals, respectively. For the Non-P devices CRTs of 280 ps and 214 ps FWHM, respectively, were obtained with above crystal types. The P-on-N devices are promising for TOF-PET due to their high photon detection efficiency (PDE) and low transit time spread (TTS).

Electrical delay line multiplexing for pulsed mode radiation detectors.

Medical imaging systems are often composed of a large number of radiation detectors to provide high resolution imaging. For example, whole-body Positron Emission Tomography (PET) systems are typically composed of thousands of scintillation crystal elements, which are coupled to photosensors. PET systems would greatly benefit from methods to reduce the number of data acquisition channels, such that the cost and complexity can be kept at a minimum. In this paper we present an electrical delay line multiplexing scheme that can significantly reduce the number of readout channels, while the signal integrity is preserved for good time resolution performance. A 4 × 4 LYSO crystal array, with each crystal element having 3 mm × 3 mm × 5 mm dimensions, was coupled to 16 Hamamatsu MPPC S10931-050P SiPM elements. For proof-of-concept, 4 SiPM elements of the array were connected to the multiplexing stage. Results show that each SiPM element could be accurately identified. The method is flexible to allow multiplexing configurations across different block detectors, and is scalable to an entire ring of detectors.

A method to achieve spatial linearity and uniform resolution at the edges of monolithic scintillation crystal detectors for PET

We have performed Monte Carlo simulations of the scintillation light transport between two adjacent 27.2 mm × 26.7 mm × 20 mm LYSO crystals, optically coupled together using MeItmount coupling medium with a refractive index of 1.7. Each crystal was read out by 4 Hamamatsu S118283344M monolithic SiPM arrays from one crystal side. Event positioning reconstruction results show that this optical coupling technique can substantially reduce the edge-artifacts in these crystals. With realistic values for the multiplication noise of the SiPMs and electronic readout noise, we obtain intrinsic spatial resolutions of ~3.1 mm FWHM at the center and ~3.0 mm FWHM at 0.6 mm from the optical coupling edge of the 20 mm thick monolithic crystals. When reducing the number of readout channels by applying a 4 : 1 multiplexing ratio, a ~4.1 and ~4.3 mm FWHM resolution, respectively, at the center and 0.6 mm from the optical coupling edge was obtained. The simulations further show that a coincidence timing resolution of 350 ps FWHM can be obtained with this crystal geometry, with a 30% SiPM photon detection efficiency (PDE).