EndoTOFPET-US Project Research Articles

A Silicon Photomultiplier Readout ASIC for Time-of-Flight Applications Using a New Time-of-Recovery Method

Silicon photomultiplier timing chip (STiC) is a 64-channel mixed-mode application-specific integrated circuit in the $0.18-\mu \text{m}$ CMOS technology from Univited Microelectronics Corporation (UMC) for silicon photomultiplier (SiPM) readout with very high timing resolution. It is designed for time-of-flight measurements in positron emission tomography and high-energy physics experiments. In order to achieve the best timing performance without compromising the charge/energy measurement, a novel time-based signal processing technique called “time-of-recovery (ToR)” method has been developed and implemented in the analog front end of the chip. This technique converts the incoming charge into a digital pulse with a linearized time-over-threshold width. Measurements have shown a time jitter smaller than 20 ps for the analog front end and smaller than 40 ps for the time-to-digital converter and the digital part. A coincidence time resolution of 214-ps full-width at half-maximum (FWHM) has been obtained with STiC using $3.1{\times }3.1{\times }15$ mm3 LYSO:Ce crystals and Hamamatsu multi pixel photon counters (S12643-050CN(X)). The measured energy resolution for a 511-keV photon is 11.2% FWHM after correcting for SiPM saturation effects. In this paper, we report on the details of the ToR method and how it is embedded within the STiC design; results of performance measurements as well as the 128-channel front-end module used for the EndoTOFPET-US project are also presented.

EndoTOFPET-US – A Miniaturised Calorimeter for Endoscopic Time-of-Flight Positron Emission Tomography

In the scope of the EndoTOFPET-US project, a novel multimodal device for Ultrasound (US) Endoscopy and Positron Emission Tomography (PET) is being developed. The project aims at detecting and quantifying morphologic and functional markers and developing new biomarkers for pancreas and prostate oncology. Exploiting the Time-of-Flight (TOF) information of the gamma rays allows for a more sensitive, more precise and lower radiation- dose imaging and intervention on small internal structures. The detection of the gamma rays is realised with the help of scintillator crystals with Silicon Photomultiplier (SiPM) read-out, aiming at a coincidence time resolution of 200 ps and a spatial resolution of ≈ 1 mm. For the endoscopic detector, digital SiPMs are utilised for the first time in an instrument planned for clinical applications. The functionality of the instrument as well as the challenges that accompany the high miniaturisation of the endoscopic detector and the asymmetric and variable geometry of the system, are presented. The demands on the system involve the fields of scintillating crystallography, ultra-fast photon detection, highly integrated electronics, system integration as well as image reconstruction.The single detector components have been fully characterised and are performing up to specifications. Two dedicated ASIC chips have been developed for the project. The first PET images have been acquired with a test setup that consists solely of hardware and software developed within the collaboration and demonstrate that the data acquisition and reconstruction chain is operational. In this talk, the characterisation of the single components and the status of the detector integration and comissioning is presented.

STIC3 – Silicon Photomultiplier Timing Chip with picosecond resolution

The diagnostic of pancreas and prostate cancer is a challenging task due to the background noise coming from the closer organs. The EndoToFPET-US project aims to combine the synergy between metabolic and anatomical (ultrasound) image in order to improve the precision in the tumor localization. The goal of the project is to develop a Positron Emission Tomography (PET) system that provides a time-of-flight resolution of 200ps FWHM for improving the signal to noise ratio and further to improve the medical image quality. In order to achieve this purpose an ASIC has been designed for very high timing resolution in time-of-flight (ToF) applications. In this paper we present the ASIC performance and the first characterization measurements with the 64-channels prototype version (STiC3). Measurements are performed with LYSO scintillator crystal and a Multi Pixel Photon Counter (MPPC). Measurements with the chip show an analog-front-end stage jitter of 35ps for the first photo-electron equivalent charge and reach 18ps for the third photo-electron. Coincidence time resolution (CTR) of 240ps FWHM is measured with 3.1×3.1×15mm3 LYSO crystal and 50μm pixel pitch MPPC. Further optimization including the Time-to-Digital Converter (TDC) non-linearity corrections and setup fine tuning are ongoing for achieving the desired CTR of 200ps FWHM.

On the comparison of analog and digital SiPM readout in terms of expected timing performance

In time of flight positron emission tomography (TOF-PET) and in particular for the EndoTOFPET-US Project (Frisch, 2013 [1]), and other applications for high energy physics, the multi-digital silicon photomultiplier (MD-SiPM) was recently proposed (Mandai and Charbon, 2012 [2]), in which the time of every single photoelectron is being recorded. If such a photodetector is coupled to a scintillator, the largest and most accurate timing information can be extracted from the cascade of the scintillation photons, and the most probable time of positron emission determined. The readout concept of the MD-SiPM is very different from that of the analog SiPM, where the individual photoelectrons are merely summed up and the output signal fed into the readout electronics. We have developed a comprehensive Monte Carlo (MC) simulation tool that describes the timing properties of the photodetector and electronics, the scintillation properties of the crystal and the light transfer within the crystal. In previous studies we have compared MC simulations with coincidence time resolution (CTR) measurements and found good agreement within less than 10% for crystals of different lengths (from 3mm to 20mm) coupled to SiPMs from Hamamatsu. In this work we will use the developed MC tool to directly compare the highest possible time resolution for both the analog and digital readout of SiPMs with different scintillator lengths. The presented studies reveal that the analog readout of SiPMs with microcell signal pile-up and leading edge discrimination can lead to nearly the same time resolution as compared to the maximum likelihood time estimation applied to MD-SiPMs. Consequently there is no real preference for either a digital or analog SiPM for the sake of achieving highest time resolution. However, the best CTR in the analog SiPM is observed for a rather small range of optimal threshold values, whereas the MD-SiPM provides stable CTR after roughly 20 registered photoelectron timestamps in the time estimator.

A fully digital approach for MR compatible Time-of-Flight PET techniques.

Time-of-Flight PET (TOFPET) is likely to bring a significant improvement in PET image quality that could have a high impact in PET/MR acquisition and image reconstruction procedures. Silicon photomultipliers (SiPM) are known to be compatible with high magnetic fields. However, commercial SiPMs do not take advantage of their digital nature and are working in an analog mode, which make them sensitive to noisy environments. In order to be immune to the hostile environment of an MRI system we propose a fully digital approach, based on the Multidigital SiPM (MD-SiPM) we have developed in the frame of the FP7 funded EndoTOFPET-US project. In parallel we propose to exploit transient phenomena in scintillators generating a few hundred photons in the picosecond range. We will review the different processes at work and evaluate if some of the transient phenomena taking place during the fast thermalization phase of hot electron-hole pairs produced by the conversion of the 511 KeV gamma rays can be exploited to extract a sub-100ps time tag. Simulation results show how a multidigital approach makes the readout system much more robust and immune to a noisy environment. Characterization data and performance evaluation of our MD-SiPM will be shown as well as first unpublished results on the potential of exploiting transient phenomena in scintillators. The potential of Time-of-Flight techniques in PET/MRI is presented. The merit of fully digital photon counting methods is discussed. The possibility to exploit ultrafast transient phenomena in specifically optimized scintillators, which can generate up to a few hundreds of photons in less than 10 ps, opens the way to coincidence timing resolution of better than 100ps at a system level, with a high potential for improving the image mapping and overall image quality of a combined PET/MRI machine.

Development of EndoTOFPET-US, a multi-modal endoscope for ultrasound and time of flight positron emission tomography

The EndoTOFPET-US project aims at delevoping a multi-modal imaging device that combines Ultrasound with Time-Of-Flight Positron Emission Tomography into an endoscopic imaging device. The goal is to obtain a coincidence time resolution of about 200 ps FWHM and sub-millimetric spatial resolution for the PET head, integrating the components in a very compact detector suitable for endoscopic use. The scanner will be exploited for the clinical test of new bio-markers especially targeted for prostate and pancreatic cancer as well as for diagnostic and surgical oncology. This paper focuses on the status of the Time-Of-Flight Positron Emission Tomograph under development for the EndoTOFPET-US project.

STiC — a mixed mode silicon photomultiplier readout ASIC for time-of-flight applications

STiC is an application specific integrated circuit (ASIC) for the readout of silicon photomultipliers. The chip has beendesigned to provide a very high timing resolution for time-of-flight applications in medical imaging and particle physics. Itis dedicated in particular to the EndoToFPET-US project, which is developing an endoscopic PET detector combined with ultrasoundimaging for early pancreas and prostate cancer detection. This PET system aims to provide a spatial resolution of 1 mmand a time-of-flight resolution of 200 ps FWHM. The analog frontend of STiC can use either a differential or single endedconnection to the SiPM. The time and energy information of the detector signalis encoded into two time stamps. A special linearized time-over-threshold method is used to obtain a linear relation between thesignal charge and the measured signal width, improving the energy resolution. The trigger signals are digitized by an integratedTDC module with a resolution of less than 20 ps. The TDC data is stored in an internal memory and transfered over a160 MBit/s serial link using 8/10 bit encoding. First coincidence measurements using a3.1 × 3.1 × 15 mm3 LYSO crystal and a S10362-33-50 Hamamtsu MPPC show a coincidence time resolution of less than285 ps. We present details on the chip design as well as first characterization measurements.

Combining endoscopic ultrasound with Time-Of-Flight PET: The EndoTOFPET-US Project

The EndoTOFPET-US collaboration develops a multimodal imaging technique for endoscopic exams of the pancreas or the prostate. It combines the benefits of high resolution metabolic imaging with Time-Of-Flight Positron Emission Tomography (TOF PET) and anatomical imaging with ultrasound (US). EndoTOFPET-US consists of a PET head extension for a commercial US endoscope and a PET plate outside the body in coincidence with the head. The high level of miniaturization and integration creates challenges in fields such as scintillating crystals, ultra-fast photo-detection, highly integrated electronics, system integration and image reconstruction. Amongst the developments, fast scintillators as well as fast and compact digital SiPMs with single SPAD readout are used to obtain the best coincidence time resolution (CTR). Highly integrated ASICs and DAQ electronics contribute to the timing performances of EndoTOFPET. In view of the targeted resolution of around 1mm in the reconstructed image, we present a prototype detector system with a CTR better than 240ps FWHM. We discuss the challenges in simulating such a system and introduce reconstruction algorithms based on graphics processing units (GPU).

EndoTOFPET-US: a novel multimodal tool for endoscopy and positron emission tomography

The EndoTOFPET-US project aims to develop a multimodal detector to foster the development of new biomarkers for prostate and pancreatic tumors. The detector will consist of two main components: an external plate, and a PET extension to an endoscopic ultrasound probe. The external plate is an array of LYSO crystals read out by silicon photomultipliers (SiPM) coupled to an Application Specific Integrated Circuit (ASIC). The internal probe will be an highly integrated and miniaturized detector made of LYSO crystals read out by a fully digital SiPM featuring photosensor elements and digital readout in the same chip. The position and orientation of the two detectors will be tracked with respect to the patient to allow the fusion of the metabolic image from the PET and the anatomic image from the ultrasound probe in the time frame of the medical procedure. The fused information can guide further interventions of the organ, such as biopsy or in vivo confocal microscopy.