Joshua W Cates

Advances in coincidence time resolution for PET.

Coincidence time resolution (CTR), an important parameter for time-of-flight (TOF) PET performance, is determined mainly by properties of the scintillation crystal and photodetector used. Stable production techniques for LGSO:Ce (Lu1.8Gd0.2SiO5:Ce) with decay times varying from ∼ 30-40 ns have been established over the past decade, and the decay time can be accurately controlled with varying cerium concentration (0.025-0.075 mol%). This material is promising for TOF-PET, as it has similar light output and equivalent stopping power for 511 keV annihilation photons compared to industry standard LSO:Ce and LYSO:Ce, and the decay time is improved by more than 30% with proper Ce concentration. This work investigates the achievable CTR with LGSO:Ce (0.025 mol%) when coupled to new silicon photomultipliers. Crystal element dimension is another important parameter for achieving fast timing. 20 mm length crystal elements achieve higher 511 keV photon detection efficiency, but also introduce higher scintillation photon transit time variance. 3 mm length crystals are not practical for PET, but have reduced scintillation transit time spread. The CTR between pairs of 2.9 × 2.9 × 3 mm(3) and 2.9 × 2.9 × 20 mm(3) LGSO:Ce crystals was measured to be 80 ± 4 and 122 ± 4 ps FWHM, respectively. Measurements of light yield and intrinsic decay time are also presented for a thorough investigation into the timing performance with LGSO:Ce (0.025 mol%).

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%.

Achieving fast timing performance with multiplexed SiPMs.

Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving excellent time resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high bandwidth readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET systems. However, multiplexing SiPMs degrades time resolution because of added dark counts and signal shaping. In this work the relative contribution of dark counts and signal shaping to timing degradation is simulated and a baseline correction technique to mitigate the effect of multiplexing on the time resolution of analog SiPMs is simulated and experimentally verified. A charge sharing network for multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence time resolution of [Formula: see text] ps for a single 3 mm  ×  3 mm  ×  20 mm LYSO scintillation crystals coupled to an array of sixteen 3 mm  ×  3 mm SiPMs that are multiplexed to a single timing channel (in addition to 4 position channels). A [Formula: see text] array of 3 mm  ×  3 mm  ×  20 mm LFS crystals showed an average FWHM coincidence time resolution of [Formula: see text] ps using the same timing scheme. All experiments were performed at room temperature with no thermal regulation. These results show that excellent time resolution for ToF can be achieved with a highly multiplexed analog SiPM readout.

A light sharing, charge multiplexed time-of-flight depth-of-interaction PET detector

Time-of-flight (TOF) and depth of interaction (DOI) measurements are both important capabilities to improve clinical PET imaging. The combination of these two measurements has been shown to improve image quality and uniformity. Current high performance TOF DOI detectors have a high level of complexity, requiring cooling, pulse shape information or increased numbers of photosensors. This works describes and characterizes an approach to TOF DOI using a two layer, 20mm thick, lutetium-yttrium oxyorthosilicate (LYSO) light sharing crystal array. The crystal array was coupled to a single ended SiPM readout with a novel binary encoded multiplexing scheme that has a 4:1 timing channel multiplexing ratio, and two position channels. Flood maps show show excellent crystal separation with a minimum ratio of distance between crystal peaks to standard deviation of crystal peaks of 9.2. The detector achieves 10mm DOI resolution with 180 +/-2ps FWHM coincident time resolution. The top and bottom layers had 170 +/- 2ps, and 185 +/-3ps FWHM coincident timing resolutions respectively. The simplicity of the readout scheme makes it a good candidate for scaling to a practical TOF DOI PET system. The detector presented in this work has single ended readout, no active cooling, a 4:1 timing channel multiplexing ratio, and requires no pulse shape information.

Analog filtering methods improve leading edge timing performance of multiplexed SiPMs

Multiplexing many SiPMs to a single readout channel is an attractive option to reduce the readout complexity of high perfromance time of flight (TOF) PET systems. However, the additional dark counts and shaping from each SiPM cause significant baseline fluctuations in the output waveform, degrading timing measurements using a leading edge threshold. This work proposes a simple analog filtering network to reduce the baseline fluctuations in highly multiplexed SiPM readouts. With 16 SiPMs multiplexed, the FWHM coincident timing resolution for single 3mm × 3mm × 20mm LYSO crystals was improved from 401 +/−4ps to 248 +/− 5ps. With 4 SiPMs multiplexed, using a 20mm length, 2 layer DOI array of LYSO crystals the mean time resolution was improved from 277 +/− 6ps to 217 +/−4ps using a ADCMP572 comparator for timing pickoff. All experiments were performed at room temperature with no active temperature regulation. This results show a promising technique for the construction of high performance multiplexed TOF PET readout systems with simple analog leading edge timing pickoff.

Effects of SiPM multiplexing on timing performance

Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving fast timing resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high speed readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET readout systems. In this work the effect of multiplexing on the timing resolution of analog SiPMs is modeled, simulated and tested. The simulations and experiments show that baseline fluctuations from cumulative uncorrelated dark noise are the most important cause of timing degradation, but their effects can be mitigated with baseline correction. A charge sharing network for position sensitive multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence timing resolution of 232 +/-2ps for single 3mm × 3mm × 20mm LYSO scintillation crystals with 16 SiPMs multiplexed to a single timing channel (in addition to 4 position channels). Measurements with a 4×4 array of 3mm × 3mm × 20mm LFS crystals show excellent crystal separation with a minimum ratio of distance between crystal peaks to standard deviation of crystal peaks of 11.9. The mean FWHM coincidence timing resolution of the 4×4 LFS array was 278 +/-7ps. All experiments were performed at room temperature with no thermal regulation. These results show that fast timing resolution for ToF can be achieved with highly multiplexed analog readout.

A multiplexed TOF and DOI capable PET detector using a binary position sensitive network

Time of flight (TOF) and depth of interaction (DOI) capabilities can significantly enhance the quality and uniformity of positron emission tomography (PET) images. Many proposed TOF/DOI PET detectors require complex readout systems using additional photosensors, active cooling, or waveform sampling. This work describes a high performance, low complexity, room temperature TOF/DOI PET module. The module uses multiplexed timing channels to significantly reduce the electronic readout complexity of the PET detector while maintaining excellent timing, energy, and position resolution. DOI was determined using a two layer light sharing scintillation crystal array with a novel binary position sensitive network. A 20 mm effective thickness LYSO crystal array with four 3 mm  ×  3 mm silicon photomultipliers (SiPM) read out by a single timing channel, one energy channel and two position channels achieved a full width half maximum (FWHM) coincidence time resolution of 180  ±  2 ps with 10 mm of DOI resolution and 11% energy resolution. With sixteen 3 mm  ×  3 mm SiPMs read out by a single timing channel, one energy channel and four position channels a coincidence time resolution 204  ±  1 ps was achieved with 10 mm of DOI resolution and 15% energy resolution. The methods presented here could significantly simplify the construction of high performance TOF/DOI PET detectors.

Timing performance of fast LGSO scintillators coupled to novel SiPMs

Stable production techniques for high lutetium content LGSO (Lu1.8Gd0.2SiO5:Ce) with decay times varying from ~30-40 ns have been established over the past decade, and the decay time can now be accurately controlled with varying cerium concentration (0.025-0.075 mol%). This material is promising for time-of-flight positron emission tomography (TOF-PET), as it has similar light output and equivalent stopping power for 511 keV annihilation photons compared to industry standard LSO:Ce and LYSO:Ce, and the decay time is improved by more than 30% with proper Ce concentration. This work investigates the achievable coincidence timing resolution (CTR) with 90% Lu LGSO:Ce(0.025 mol%) when coupled to a new silicon photomultiplier (SiPM). The CTR measured between pairs of 2.9×2.9×3 mm3 and 2.9×2.9×20 mm3 LGSO:Ce crystals was 80±4 and 122 ±4 ps FWHM, respectively. Measurements of light out output and energy resolution with these crystals are also presented.

Direct conversion semiconductor detectors in positron emission tomography

Semiconductor detectors are playing an increasing role in ongoing research to improve image resolution, contrast, and quantitative accuracy in preclinical applications of positron emission tomography (PET). These detectors serve as a medium for direct detection of annihilation photons. Early clinical translation of this technology has shown improvements in image quality and tumor delineation for head and neck cancers, relative to conventional scintillator-based systems. After a brief outline of the basics of PET imaging and the physical detection mechanisms for semiconductor detectors, an overview of ongoing detector development work is presented. The capabilities of semiconductor-based PET systems and the current state of these devices are discussed.

A dual layer fast timing detector for time-of-flight PET

This work presents the design of scintillation detectors for clinical PET using off-the-shelf components that achieve both excellent time resolution and DoI information. The detector consists of a dual layer stack of 3×3×20 mm3 crystals, 1:1 coupled to SiPM arrays. Each pixel contains an LYSO:Ce and LSO:Ce,Ca(0.4%) crystal directly bonded to one another with an optical epoxy, and layer identification was performed via pulse shape discrimination. Layer interaction discrimination was explored through parameterization of crystal orientation, reflector and coupling materials, SiPM selection, and preamplifier choice - as all these affect pulse shape discrimination capabilities. The final design selection achieved a layer identification identification error rate of 9%. The achievable coincidence timing resolution between two identical two-layer detectors was found to be 224 ps FWHM. Layer-gated coincidence timing resolutions of 222 and 220 ps FWHM were measured for LSO:Ce,Ca(0.4%) and LYSO:Ce in coincidence with the same layers on an identical detector. The detector design has been integrated into a data acquisition system to assess imaging performance with this design.