Thomas Frach

The digital silicon photomultiplier — Principle of operation and intrinsic detector performance

We developed a fully digital implementation of the Silicon Photomultiplier. The sensor is based on a single photon avalanche photodiode (SPAD) integrated in a standard CMOS process. Photons are detected directly by sensing the voltage at the SPAD anode using a dedicated cell electronics block next to each diode. This block also contains active quenching and recharge circuits as well as a one bit memory for the selective inhibit of detector cells. A balanced trigger network is used to propagate the trigger signal from all cells to the integrated time-to-digital converter (TDC). Photons are detected and counted as digital signals, thus making the sensor less susceptible to temperature variations and electronic noise. The integration with CMOS logic has the added benefit of low power consumption and possible integration of data post-processing. In this paper, we discuss the sensor architecture and present first measurements of the technology demonstrator test chip.

The digital Silicon Photomultiplier — A novel sensor for the detection of scintillation light

We developed a fully digital Silicon Photomultiplier (dSiPM) of 3.8 mm × 3.3 mm in size containing 8188 individual Geiger-mode cells. Each detected photon is directly converted into a digital signal in each of the Geiger-mode cells of the sensor. In addition, the complete trigger logic and a time-to-digital converter are integrated into the sensor. To show the performance of the sensor, LYSO crystals of different sizes were coupled to the sensor. The coincidence timing resolution for 3 mm × 3 mm × 5 mm LYSO crystals using a 22Na source amounts to 153 ps FWHM. The energy resolution at 511 keV was determined to be 10.7 % for 4 mm × 4 mm × 22 mm crystals. It is shown that saturation correction can be done without prior need for sensor calibration. The temperature dependence of the photon detection efficiency was found to be -0.6 %/°C including the temperature variation of the light output of LYSO.

The digital silicon photomultiplier — System architecture and performance evaluation

In this paper we present the first fully digital implementation of the Silicon Photomultiplier. The chip design is based on the technology demonstrator chip presented in [3]. The new sensor represents a self-contained detector including a JTAG controller for configuration and test, single-ended and differential clock and test input signals, an integrated acquisition controller and two serial data outputs. The sensor is based on a single photon avalanche photodiode (SPAD) technology integrated in a standard CMOS process flow. Photons are detected directly by sensing the voltage at the SPAD terminal using a dedicated cell electronics block next to each diode. This block also contains active quenching and recharge circuits as well as a one bit memory for the selective activation of individual detector cells. A balanced trigger network is used to propagate the trigger signal from all cells to the two integrated time-to-digital converters. Photons are detected and counted as digital signals, thus making the sensor less susceptible to temperature variations and electronic noise. The resulting data packets are transferred to the readout system through a serial data interface. In this paper, we discuss the new sensor architecture and evaluate its performance.

Arrays of digital Silicon Photomultipliers — Intrinsic performance and application to scintillator readout

Arrays of digital Silicon Photomultipliers (dSiPMs) have been developed. The arrays consist of 4 × 4 chips, each containing 2 × 2 dSiPMs, resulting in 8 × 8 dSiPMs on one array. With a pitch of 4 mm × 4 mm of the dSiPMs, the outer dimension of the array is 32 mm × 32 mm. To show the performance of the detector array, we investigated its intrinsic performance with respect to timing resolution using an external electronic trigger and picosecond laser pulses acting as an optical trigger. In addition, we show results of coupling arrays of 8 × 8 LYSO crystals with 4 mm × 4 mm pitch and 22 mm length to the detector arrays leading to a 1:1 coupling between the individual scintillator crystals and the dSiPM pixels. Two arrays have been operated in coincidence using 22Na as a source for annihilation gamma radiation in order to record energy and coincidence timing resolution.

Performance evaluation of a prototype Positron Emission Tomography scanner using Digital Photon Counters (DPC)

We show performance results of a prototype Positron Emission Tomography scanner based on digital SiPMs, or Digital Photon Counters (DPC), developed by the Philips Digital Photon Counting unit. The scalability of the DPC technology is demonstrated by an excellent system coincidence timing resolution of 266 ps FWHM and an energy resolution of 10.7 % FWHM. Even while using 4 mm × 4 mm × 22 mm LYSO crystals, the spatial resolution is close to 2.4 mm. Although not optimized yet, the image homogeneity is 5.8 %. We show that the system performance is maintained even at highest count rates encountered in PET scans.

Assessment of photodiodes as a light detector for PET scanners

Current PET systems based on pixelated scintillator arrays coupled to photomultiplier tubes suffer from pile-up and electronics dead time at high count rates. With a pixelated readout, i.e. one-to-one coupling of a scintillator crystal to a photo detector, these effects can be strongly reduced. Recent developments of high light output scintillators like LYSO and LaBr3 in combination with very low noise amplifiers based on modern CMOS processes make it possible to use high quantum efficiency blue-sensitive PIN photodiodes as a light detector. To explore the potential of this approach, a model of the signal detection chain was implemented. It comprises the scintillation light pulse and its quantum noise, optical coupling, charge conversion in the diode, noise sources of the integrating amplifier, shaper circuits for the energy and timing channel, and the discriminator for the timing channel. The model is verified using off-the-shelf PIN photodiodes and a dedicated CMOS preamplifier excited by a picosecond laser as well as scintillator pulses. The model predicts that with high light output scintillators, high quantum efficiency photodiodes and optimized preamplifiers, a pixelated PET readout with very good energy resolution and sufficient timing resolution can be realized. To complement the study, an APD-based readout is also considered and the related signal to noise issues are discussed

Assessment of the spatial resolution of PET scanners using a Geant4-based Monte Carlo tool

In this paper, the spatial resolution of PET scanners is assessed by means of a Geant4-based Monte Carlo tool. To obtain a high level of accuracy, all relevant contributions like /spl beta//sup +/ range and gamma noncollinearity, crystal size and material, intercrystal scatter, and scanner geometry are implemented in detail. For a system based on Anger logic, the scintillator light yield, the photon statistics and the PMT characteristics are also taken into account. In the simulation, Monte Carlo and analytical algorithms share a common setup, which allows for the necessary computation speed to generate clinical count statistics without compromising accuracy. In the first part, the major contributions to the spatial resolution are analyzed separately. The impact on the total system resolution is then illustrated by investigating three scanner designs with different gantry sizes, crystal sizes, crystal materials and /spl beta//sup +/ tracers.

RESCUE - Reduction of MRI SNR Degradation by Using an MR-Synchronous Low-Interference PET Acquisition Technique

Devices aiming at combined Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) to enable simultaneous PET/MR image acquisition have to fulfill demanding requirements to avoid mutual magnetic- as well as electromagnetic-field-related interferences which lead to image quality degradation. Particularly Radio-Frequency (RF)-field-related interferences between PET and MRI may lead to MRI SNR reduction, thereby deteriorating MR image quality. RF shielding of PET electronics is therefore commonly applied to reduce RF emission and lower the potential coupling into MRI RF coil(s). However, shields introduce eddy-current-induced MRI field distortions and should thus be minimized or ideally omitted. Although the MRI noise floor increase caused by a PET system might be acceptable for many MRI applications, some MRI protocols, such as fast or high-resolution MRI scans, typically suffer from low SNR and might need more attention regarding RF silence to preserve the intrinsic MRI SNR. For such cases, we propose RESCUE, an MRI-synchronously-gated PET data acquisition technique: By interrupting the PET acquisition during MR signal receive phases, PET-related RF emission may be minimized, leading to MRI SNR preservation. Our PET insert

Exact modeling of analog pulses for PET detector modules

Hyperion\ II^D

Rate-dependence of the key performance parameters in an Anger logic based PET detector

using Philips Digital Photon Counting (DPC) sensors serves as the platform to demonstrate RESCUE. To make the DPC sensor suitable for RESCUE to be applied for many MRI sequences with acquisition time windows in the range of a few milliseconds, we present in this paper a new technique which enables rapid DPC sensor operation interruption by dramatically lowering the overhead time to interrupt and restart the sensor operation. Procedures to enter and leave gated PET data acquisition may imply sensitivity losses which add to the ones occurring during MRI RF acquisition. For the case of our PET insert, the new DPC quick-interruption technique yields a PET sensitivity loss reduction by a factor of 78 when compared to the loss introduced with the standard start/stop procedure. For instance, PET sensitivity losses related to overhead time are 2.9% in addition to the loss related to PET gating being equal to the MRI RF acquisition duty cycle (14.7%) for an exemplary T1-weighted 3D-FFE MRI sequence. MRI SNR measurement results obtained with one