Vera Stankova

Timing performance of the silicon PET insert probe.

Simulation indicates that PET image could be improved by upgrading a conventional ring with a probe placed close to the imaged object. In this paper, timing issues related to a PET probe using high-resistivity silicon as a detector material are addressed. The final probe will consist of several (four to eight) 1-mm thick layers of silicon detectors, segmented into 1 x 1 mm(2) pads, each pad equivalent to an independent p + nn+ diode. A proper matching of events in silicon with events of the external ring can be achieved with a good timing resolution. To estimate the timing performance, measurements were performed on a simplified model probe, consisting of a single 1-mm thick detector with 256 square pads (1.4 mm side), coupled with two VATAGP7s, application-specific integrated circuits. The detector material and electronics are the same that will be used for the final probe. The model was exposed to 511 keV annihilation photons from an (22)Na source, and a scintillator (LYSO)-PMT assembly was used as a timing reference. Results were compared with the simulation, consisting of four parts: (i) GEANT4 implemented realistic tracking of electrons excited by annihilation photon interactions in silicon, (ii) calculation of propagation of secondary ionisation (electron-hole pairs) in the sensor, (iii) estimation of the shape of the current pulse induced on surface electrodes and (iv) simulation of the first electronics stage. A very good agreement between the simulation and the measurements were found. Both indicate reliable performance of the final probe at timing windows down to 20 ns.

Distributed Low-Voltage System for the Front End of the HADES Timing RPC Wall

The power supply system for the front-end electronics of the HADES resistive plate chamber (RPC) detector, installed at GSI (Darmstadt, Germany), is described. The system has a distributed architecture that includes custom low-voltage boards using isolated DC-DC switching converters. These converters are compact and operate at high efficiency. The converters are fitted with input and output electromagnetic interference filters, resulting in very low output noise. Operational tests of the RPC detector demonstrate that the performance of the detector when powered by this system is comparable to that achieved when powered by laboratory bench supplies, proving its suitability for large-scale applications requiring time resolutions better than 100 ps.

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 been designed to provide a very high timing resolution for time-of-flight applications in medical imaging and particle physics. It is dedicated in particular to the EndoToFPET-US project, which is developing an endoscopic PET detector combined with ultrasound imaging for early pancreas and prostate cancer detection. This PET system aims to provide a spatial resolution of 1 mm and a time-of-flight resolution of 200 ps FWHM. The analog frontend of STiC can use either a differential or single ended connection to the SiPM. The time and energy information of the detector signal is encoded into two time stamps. A special linearized time-over-threshold method is used to obtain a linear relation between the signal charge and the measured signal width, improving the energy resolution. The trigger signals are digitized by an integrated TDC module with a resolution of less than 20 ps. The TDC data is stored in an internal memory and transfered over a 160 MBit/s serial link using 8/10 bit encoding. First coincidence measurements using a 3.1 × 3.1 × 15 mm3 LYSO crystal and a S10362-33-50 Hamamtsu MPPC show a coincidence time resolution of less than 285 ps. We present details on the chip design as well as first characterization measurements.

Performance of the MADEIRA PET probe prototype

This paper reports the characterization of a detector module, the building block to be used for the MADEIRA PET probe prototype. The prototype will be used in synchronization with a conventional PET ring, amplifying the basic image with a subset of events with high spatial resolution. For image improvement, the crucial parameters are the spatial and timing resolution of the probe, while the energy resolution can be used in event classification. The final prototype is made of high-resistivity silicon detectors, 1 mm thick with 1040 square pads with a size of 1 mm. The performance was characterized on an evaluation module featuring the same electronics and sensor material, save for sensor geometry where a sensor with 256 square pads with a side of 1.4 mm was used. The pads were read out with VATAGP7, a Gamma Medica-Ideas designed application specific integrated circuit (ASIC). The ASIC provides a logic trigger signal and an analog output for every out of 128 input channels. The measurement of the timing resolution was performed, using a positron source and a timing reference detector. A similar measurement was performed with a test pulse, ie. a charge injection into the ASIC, to study inherent resolution of the firststage electronics. We calibrated the analog output of the ASIC using standard gamma sources (241Am), and we determined the overall energy resolution. The comparators were calibrated by a test pulse, looking at the rate of triggers versus threshold. The gain variation was compensated with internal 3-bit DACs. The evaluation module was successfully characterized, exhibiting energy resolution of 1.5 keV FWHM after gain alignment. The spread of the comparator levels can be decreased below 1 keV. The timing performance allows timing windows of 20 ns to be used. Based on the performance of the evaluation module we conclude that the final prototype can be used as a PET insert probe.

STiC2 - characterization results of a SiPM readout ASIC for time-of-flight applications

STiC is a mixed-mode readout ASIC for silicon-photomultipliers (SiPM) developed in the framework of the EndoToFPET-US project. The chip has been designed in the UMC 0.18 μm technology and aims to provide an optimal timing resolution to Time-of-Flight measurements in medical imaging and particle physics applications. The differential design of the analog input stage rejects noise from the large integrated digital part as well as external sources. The chip allows either a differential or a single-ended readout of the sensors. The time and charge information of the input signal is converted into two time stamps which are digitized by an integrated TDC module with a time binning of 50 ps. A special linearized Time-over-Threshold method has been implemented to provide a linear response to the signal charge in a wide range. The digitized event information is stored in a 64-word FIFO memory and transmitted every 6.4 μs to an external DAQ system using a 160 MBit=s LVDS serial link with 8/10-bit encoding. Characterization measurements show a trigger jitter of σ <; 30 ps for input signals larger than 3 pC injected over a 33 pF capacitor. A measurement using 3.1 × 3.1 × 15mm3 LYSO crystals and Hamamatsu MPPC (S10362-33-50C) sensors to detect coincidence photons from positron annihilations shows an energy resolution of ~ 12% for the 511 keV peak and a coincidence time resolution of 220 ps FWHM. We present details of the ASIC design as well as characterization measurements.

Multichannel DAQ system for SiPM matrices

The use of Silicon Photomultiplier (SiPM) arrays requires the use of multichannel data acquisition (DAQ) systems. For this reason a dedicated DAQ system has been developed for the read-out of several SiPM-based detector layers. The frontend of a system is based on the 64-channel ASIC V ATA64HDR16 from Gamma Medica - Ideas. As first application, the DAQ system will be employed in the construction of Compton telescope for dose monitoring in hadron therapy. However the designed system is suitable for any other devices that need to treat large number of SiPM channels. Tests are presented with SiPM matrix of 16 (4×4) elements each coupled to LYSO and LaBr3 continuous crystals. The complete characterization of the DAQ system and the obtained results are presented.

Simulated One Pass Listmode for fully 3D image reconstruction of Compton camera data

Image reconstruction for Compton camera data can be problematic due to the common trade-off between physically realistic models and speed of computation. In this investigation a novel method of system matrix calculation - Simulated One-Pass Listmode (SOPL) - is extended to incorporate Compton camera data. The method reduces the Cone Surface Response for the Compton camera to an ensemble of Siddon-rays and is conducted in two stages. As part of the ENVISION project for monitoring in hadron therapy, a continuous-crystal Lanthanum Bromide Compton camera has been developed and experimental data acquired. Continuous detection geometries are particularly susceptible to variation in both spatial and spectral resolution over the detection volume and so accurate yet flexible models of detection are particularly important. The SOPL-Compton method was applied via the Maximum Likelihood - Expectation Maximization algorithm to experimental data taken using the prototype device. In this investigation, detection modeling using SOPL-Compton in a two interaction Compton camera is validated and the incorporation of a shift-invariant image-space model confirmed as a useful modification to reduce computational expense. Finally experimental data taken using the prototype LaBr3 Compton camera provide confirmation of the SOPL-Compton approach to system modeling. Results indicate a fast, flexible and accurate algorithm that can easily be extended to alternate and novel detection geometries.

An FPGA based DAQ system for the readout of Madeira PET probe

Madeira project aims to significantly improve three-dimensional (3D) nuclear medicine imaging technologies via a compact photon-sensitive probe interfaced to an external conventional PET ring and placed close to the region of interest. The probe consists of several modules densely packed. Each module is made of two high-resistivity silicon detectors of 1 mm thickness and 1040 square pixels of 1 mm2. The detectors are placed back-to-back at a distance of 0.8 mm. The pads are read out with the VATAGP7 chip, a Gamma Medica — Ideas designed application specific integrated circuit (ASIC). A FPGA based DAQ system has been designed and developed at IFIC-Valencia to read out the probe. The system consists of several DAQ boards working in parallel which control the acquisition process. The DAQ board has trigger and coincidence capabilities to be used in coincidence with a conventional PET scanner. This work describes the characteristics of the system and its architecture, and also the future activities.

A dedicated readout ASIC for Time-of-Flight Positron Emission Tomography using Silicon Photomultiplier (SiPM)

STiC is a mixed-mode Application Specific Integrated Circuit (ASIC) for Silicon Photomultiplier (SiPM) readout with very high timing resolution. It is designed for the Time-of-Flight (ToF) measurement in Positron Emission Tomography (PET) and in high energy physics experiments. STiC is dedicated particularly to the EndoTOFPET-US project, which is developing and building a multi-modal instrument combing ToFPET and ultrasound endoscopy for the development of new biomarkers for pancreas and prostate oncology. A 64-channel prototype (STiC v3) has been developed and produced in 0.18μm UMC CMOS technology. A fully differential analog front-end has been developed to suppress the common-mode noise from both the on-chip digital part and the outer sources, while it allows for both differential and single-ended readout connection schemes with SiPM. A digital-to-analog converter on the input stage tunes the SiPM bias voltage with a range of ~ 900 mV, providing the possibility to compensate the variation on the breakdown voltage of the connected SiPMs. A linearized time-over-threshold method has been implemented to provide better energy resolution than the conventional ToT method. The time and energy information are converted into two time stamps, which are digitized by a build-in TDC with time binning of 50.2 ps. A dedicated digital part has been developed for chip configuration with a Serial Peripheral Interface and for data transmission to an external Data Acquisition System using a 160 MBit/s LVDS serial link with 8/10-bit encoding. Measurements have shown a time jitter <; 20 ps on the analog front-end and a time jitter of ~ 37 ps on the TDC and digital part. A Coincidence Time Resolution of ~ 214 ps FWHM has been obtained by STiC v3 with 3.1 × 3.1 × 15 mm2 LYSO:Ce crystals and Hamamatsu MPPCs (S12643-050CN(x)). Here we present the details of the ASIC design, measurement results as well as the 128-channel front-end module for EndoTOFPET-US project.

Data acquisition system for the readout of SiPM arrays

Silicon photomultipliers (SiPM) are becoming more and more popular in different fields of physics. The use of SiPM arrays requires a data acquisition (DAQ) system capable of reading multiple channels for each event. A DAQ system for the control and the readout of 64-channel detectors was designed and fabricated. Significant improvements in performance and new functionalities have been done. The performance of our DAQ system has been tested in a real application: a Compton telescope prototype. Tests, experimental results and future work are described.