T. C. Meyer

Time based readout of a silicon photomultiplier (SiPM) for time of flight positron emission tomography (TOF-PET)

Time of flight (TOF) measurements in positron emission tomography (PET) are very challenging in terms of timing performance, and should achieve ideally less than 100ps FWHM precision. We present a time-based differential technique to read out SiPMs that has less than 25ps rms electronic jitter. The novel readout is a fast front end circuit (NINO) based on a first stage differential current mode amplifier with 20Ω input resistance. Therefore the amplifier inputs are connected differentially to the SiPMs anode and cathode ports. The leading edge of the output signal provides the time information, while the trailing edge provides the energy information. Based on a Monte Carlo photon-generation model, SPICE simulations were run with a 3×3mm2 SiPM-model, read out with a differential current amplifier. The results of these simulations are presented here and compared with experimental data obtained with a 3×3×15mm3 LSO crystal coupled to a SiPM. The measured time coincidence precision is interpreted by the combined Monte Carlo/SPICE simulation, as well as by Poisson statistics.

Time of flight positron emission tomography towards 100ps resolution with L(Y)SO: an experimental and theoretical analysis

Scintillation crystals have a wide range of applications in detectors for high energy and medical physics. They are recquired to have not only good energy resolution, but also excellent time resolution. In medical applications, L(Y)SO crystals are commonly used for time of flight positron emission tomography (TOF-PET). This study aims at determining the experimental and theoretical limits of timing using L(Y)SO based scintillators coupled to silicon photomultipliers (SiPMs). Measurements are based on the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. Using a 2 × 2 × 3 mm3 LSO:Ce codoped 0.4% Ca crystal coupled to a commercially available SiPM (Hamamatsu S10931-050P MPPC), we achieve a coincidence time resolution (CTR) of 108±5ps FWHM measured at E=511keV. We determine the influence of the data acquisition system to 27±2ps FWHM and thus negligible as compared to the CTR. This shows that L(Y)SO scintillators coupled to SiPM photodetectors are capable of achieving very good time resolution close to the desired 100ps FWHM for TOF-PET systems. To fully understand the measured values, we developed a simulation tool in MATLAB that incorporates the timing properties of the photodetector, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. The simulations are compared with measured data in order to determine their predictive power. Finally we use this model to discuss the influence of several important parameters on the time resolution like scintillation rise- and fall time and light yield, as well as single photon time resolution (SPTR) and the detection efficiency of the SiPM. In addition we find the influence of photon travel time spread in the crystal not negligible on the CTR, even for the used 2 × 2 × 3 mm3 geometry.

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.

Results of wire chamber ageing tests with CH4− and DME-based gas mixtures

Abstract Results are presented of ageing tests performed on single-wire proportional counters under controlled conditions. The rate of the detector ageing with methane mixtures has been found to be independent of the anode and cathode materials used, and also of the purity of the gas. The rate of ageing for DME mixtures, on the other hand, appears to depend on the amount of gas used: it is small when the DME bottle is full, but increases as the cylinder empties. Addition of some water vapour to the Ar-DME mixture provided good lifetime, independently of the amount in the bottle. An explanation of this observation, based on assumptions on the fractional distillation of impurities, is provided.

A Time-Based Front End Readout System for PET & CT

In the framework of the European FP6s BioCare project, we develop a novel, time-based, photo-detector readout technique to increase sensitivity and timing precision for molecular imaging in PET and CT. The project aims to employ Avalanche Photo Diode (APD) arrays with state of the art, high speed, front end amplifiers and discrimination circuits developed for the Large Hadron Collider (LHC) physics program at CERN, suitable to detect and process photons in a combined one-unit PET/CT detection head. In the so-called time-based approach our efforts focus on the systems timing performance with sub-nanosecond time-jitter and -walk, and yet also provide information on photon energy without resorting to analog to digital conversion. The bandwidth of the electronic circuitry is compatible with the scintillators intrinsic light response (e.g. les40ns in LSO) and hence allows high rate CT operation in single-photon counting mode. Based on commercial LSO crystals and Hamamatsu S8550 APD arrays, we show the system performance in terms of timing- and energy resolution as well as its rate behavior (SPICE, simulating a high intensity X-ray beam). If proven viable, this technique may lead to the construction of a compact, radiation tolerant, and cost effective PET/CT detection head in one unit.

A Systematic Study to Optimize SiPM Photo-Detectors for Highest Time Resolution in PET

We report on a systematic study of time resolution made with three different commercial silicon photomultipliers (SiPMs) (Hamamatsu MPPC S10931-025P, S10931-050P, and S10931-100P) and two LSO scintillating crystals. This study aimed to determine the optimum detector conditions for highest time resolution in a prospective time-of-flight positron emission tomography (TOF-PET) system. Measurements were based on the time over threshold method in a coincidence setup using the ultrafast amplifier-discriminator NINO and a fast oscilloscope. Our tests with the three SiPMs of the same area but of different SPAD sizes and fill factors led to best results with the Hamamatsu type of 50×50×μm2 single-pixel size. For this type of SiPM and under realistic geometrical PET scanner conditions, i.e., with 2×2×10×mm3 LSO crystals, a coincidence time resolution of 220 ±4 ps FWHM could be achieved. The results are interpreted in terms of SiPM photon detection efficiency (PDE), dark noise, and photon yield.

Aging properties of straw proportional tubes with a XeCO2CF4 gas mixture

Abstract Studies of the behaviour of straw proportional tubes, 4 mm in diameter, under sustained irradiation were carried out with a XeCO 2 CF 4 [50-30-20] gas mixture. No observable gain drop has been found to affect the tubes up to reasonable doses ( ≤ 1 C/cm of wire). The chemical modification of the gas composition, causing the formation of electronegative, relatively stable species during the avalanche processes in a highly irradiated straw tube, is suggested to be responsible for a decrease of the pulse height in a second proportional counter (12.5 mm drift distance) operated with this (radical-enriched) gas mixture. Finally, we have observed that, for a straw tube operated at gain of a few times 10 4 , there is a contribution to the total collected charge coming from few, very large pulses, probably due to occasional transition to a limited streamer regime.

Behaviour of microstrip gas chamber in strong magnetic field

Abstract The behaviour of the microstrip gas chamber has been studied in strong magnetic fields (up to 2.3 T). An almost complete compensation of the effect due to the E × B factor, which is otherwise responsible for a degradation of the spatial resolution, has been obtained by applying a small tilt to the chamber equal to the Lorentz angle. Different. gas mixtures have been studied: an improvement in the resolution has been achieved using gas mixtures with higher cluster density (DME-CO 2 , DME-CF 4 ).

Factors influencing time resolution of scintillators and ways to improve them

The renewal of interest in Time of Flight Positron Emission Tomography (TOF-PET), as well as the necessity to precisely tag events in high energy physics (HEP) experiments at future colliders are pushing for an optimization of all factors affecting the time resolution of the whole acquisition chain comprising the crystal, the photo detector, and the electronics. The time resolution of a scintillator-based detection system is determined by the rate of photoelectrons at the detection threshold, which depends on the time distribution of photons being converted in the photo detector. The possibility to achieve time resolution of about 100ps FWHM requires an optimization of the light production in the scintillator, the light transport and its transfer from the scintillator to the photo detector. In order to maximize the light yield, and in particular the density of photons in the first nanosecond, while minimizing the rise time and decay time, particular attention must be paid to the energy transfer mechanisms to the activator as well as to the energy transition type at the activator ion. Alternatively other light emission mechanisms can be considered. We will show that particularly Cerenkov emission can be used for this purpose. Special emphasis was put on the light transport within the crystal and at its interface with the photo detector. Since light is produced isotropically in the scintillator the detector geometry must be optimized to decrease the optical path-length to the photo detector. Moreover light bouncing within the scintillator, affecting about 70% of the photons generated in currently used crystals, must be reduced as much as possible. We also investigate photonics crystals that are specifically designed to favor specific light propagation modes at the limit of total reflection inside and outside of the crystal, and how they might increase the light transfer efficiency to the photo detector and hence improve time resolution. Examples for the production and deposition of photonics crystals as layers on LYSO and LuYAP crystals are shown here, as well as first results on an improved light extraction due to this method.

Multifractal analysis of minimum bias events in\(\sqrt s \) = 630 GeV\(\bar p\)p collisions

A search for multifractal structures, in analogy with multifractal theories, is performed on UA1 minimum bias events. A downward concave multifractal spectral function,f(α) (where α is the Lipschitz-Hölder exponent), indicates that there are self-similar cascading processes, governing the evolution from the quark to the hadron level, in the final states of hadronic interactions.f(α) is an accurate measure of the bin to bin fluctuations of any observable. It is shown that the most sensitive comparison between data and the Monte Carlo models, GENCL and PYTHIA 4.8 can be made usingf(α). It is found that these models do not fully reproduce the behaviour of the data.