Due to detector developments in the last decade, the time-of-flight (TOF) method is now commonly used to improve the quality of positron emission tomography (PET) images. Clinical TOF-PET systems based on L(Y)SO:Ce crystals and silicon photomultipliers (SiPMs) with coincidence resolving times (CRT) between 325 ps and 400 ps FWHM have recently been developed. Before the introduction of L(Y)SO:Ce, BGO was used in many PET systems. In addition to a lower price, BGO offers a superior attenuation coefficient and a higher photoelectric fraction than L(Y)SO:Ce. However, BGO is generally considered an inferior TOF-PET scintillator. In recent years, TOF-PET detectors based on the Cherenkov effect have been proposed. However, the low Cherenkov photon yield in the order of ∼10 photons per event complicates energy discrimination-a severe disadvantage in clinical PET. The optical characteristics of BGO, in particular its high transparency down to 310 nm and its high refractive index of ∼2.15, are expected to make it a good Cherenkov radiator. Here, we study the feasibility of combining event timing based on Cherenkov emission with energy discrimination based on scintillation in BGO, as a potential approach towards a cost-effective TOF-PET detector. Rise time measurements were performed using a time-correlated single photon counting (TCSPC) setup implemented on a digital photon counter (DPC) array, revealing a prompt luminescent component likely to be due to Cherenkov emission. Coincidence timing measurements were performed using BGO crystals with a cross-section of 3 mm × 3 mm and five different lengths between 3 mm and 20 mm, coupled to DPC arrays. Non-Gaussian coincidence spectra with a FWHM of 200 ps were obtained with the 27 mm3 BGO cubes, while FWHM values as good as 330 ps were achieved with the 20 mm long crystals. The FWHM value was found to improve with decreasing temperature, while the FWTM value showed the opposite trend.@en

Organ-specific, targeted field-of-view (FoV) positron emission tomography (PET)/magnetic resonance imaging (MRI) inserts are viable solutions for a number of imaging tasks where whole-body PET/MRI systems lack the necessary sensitivity and resolution. To meet the required PET detector performance of these systems, high count-rates and effective spatial resolutions on the order of a few mm, a novel two-axis patterned reflector foil pixelated scintillator crystal array design is developed and its proof-of-concept illustrated in-silico with the Monte Carlo radiation transport modeling toolkit Geant4. It is shown that the crystal surface roughness and phased open reflector cross-sectional patterns could be optimized to maximize either the PET radiation detector's effective spatial resolution, or count rate before event pile up. In addition, it was illustrated that these two parameters had minimal impact on the energy and time resolution of the proposed PET radiation detector design. Finally, it is shown that a PET radiation detector with balance performance could be constructed using ground crystals and phased open reflector cross-sectional pattern corresponding to the middle of the tested range.@en

Typically, a time-of-flight (TOF) PET block detector is built using application-specific integrated circuits (ASICs), since they integrate a high number of channels at a reasonable power consumption and into a small area. However, ASICs’ flexibility is limited and prototyping times are long because a semiconductor fabrication process is required in every design iteration. Alternatively, fast terminal (FT) silicon photomultipliers (SiPMs) require a simplified analog front-end in order to achieve time-of-flight (TOF) accuracy. In addition, field-programmable gate arrays (FPGAs) can allocate time-to-digital converters (TDCs) as well as complex digital readout logics. In this work, we propose building TOF-PET block detectors based on FPGAs, FT-SiPMs, and minimal amount of off-the-shelf components. In this way, TOF-PET accuracy is achieved with a full-flexible and fast prototyping solution. We evaluated the coincidence resolving time (CRT), performance degradations due to channel multiplexing, energy resolution, and scintillator pixel encoding performance of SiPM arrays utilizing the proposed approach. Experimental results show minimal timing degradations, when multiplexing FTs. Moreover, simulation results show a low reduction in the singles count rate of multiplexed channels at typical brain-PET radioactive doses.@en

Photodetectors with excellent time resolution are becoming increasingly important in many applications in medicine, high energy- and nuclear physics applications, biology, and material science. Silicon photomultipliers (SiPM) are a novel class of solid-state photodetectors with good timing properties. While the time resolution of analog SiPMs has been analyzed by many groups, the time resolution of the digital photon counter (DPC) developed by Philips has not yet been fully characterised. Here, the timing capabilities of the DPC are studied using a femtosecond laser. The time resolution is determined for complete dies, single pixels, and individual single photon avalanche diodes (SPADs). The measurements cover a broad dynamic range, from intense illumination down to the single-photon level, and were performed at various temperatures between 0°C and 20°C. The measured single photon time resolution (SPTR) ranges from 101 ps FWHM for the DPC3200 sensor pixel to 247 ps FWHM for the DPC6400 sensor die. An extensive study of the single-SPAD time resolution, ranging from single photon to very high laser intensities (∼1000 photons per pulse), yielded a time resolution of 48 ps FWHM at the single-photon level.@en

In this paper we present a silicon photomultiplier (SiPM) readout system based on a field-programmable gate array (FPGA), which is capable of converting any commercial 16-channel analog SiPM array into a hybrid device with fullydigital readout for application in time of flight positron emission tomography (TOF-PET). In principle this hybrid SiPM array can be implemented with several leading edge discriminators (LEDs) per channel, so that multiple timestamps can be acquired per scintillation pulse allowing to estimate the time of interaction more robustly than if a single timestamp is used. These concepts were studied experimentally in two different ways. In the first approach, we utilized discrete components in combination with an array of time-to-digital converters (TDCs) implemented and validated on the FPGA. The overall resolution of each channel was 56 ps FWHM or better, while crosstalk was undetectable. In the second approach, we utilized a four-channel high-speed acquisition board operating at 5 GSPS and emulated a multiple-LED acquisition by post-processing the digitized waveforms. We present, for the first time, experimental results obtained with the best linear unbiased estimator (BLUE) to estimate the time of interaction from the multiple timestamps.@en

In this paper we present a silicon photomultiplier (SiPM) readout system based on a field-programmable gate array (FPGA), which is capable of converting any commercial 16-channel analog SiPM array into a hybrid device with fullydigital readout for application in time of flight positron emission tomography (TOF-PET). In principle this hybrid SiPM array can be implemented with several leading edge discriminators (LEDs) per channel, so that multiple timestamps can be acquired per scintillation pulse allowing to estimate the time of interaction more robustly than if a single timestamp is used. These concepts were studied experimentally in two different ways. In the first approach, we utilized discrete components in combination with an array of time-to-digital converters (TDCs) implemented and validated on the FPGA. The overall resolution of each channel was 56 ps FWHM or better, while crosstalk was undetectable. In the second approach, we utilized a four-channel high-speed acquisition board operating at 5 GSPS and emulated a multiple-LED acquisition by post-processing the digitized waveforms. We present, for the first time, experimental results obtained with the best linear unbiased estimator (BLUE) to estimate the time of interaction from the multiple timestamps.@en

In time-of-flight (TOF) positron emission tomography (PET), the coincidence resolving time (CRT) has a strong influence on the overall performance. Multichannel digital silicon photomultipliers (MD-SiPMs) are able to obtain several timestamps for gamma photon timemark estimation. Using this feature, the CRT is improved and the system robustness is significantly increased by utilizing multiple photoelectron timestamps. In addition, the PET instrumentation chain is simplified because of the intrinsic digitization and integrated functionality of the MD-SiPM. The main objective of this work is to demonstrate the possibility of building a complete highly-miniaturized PET detector module for endoscopic applications. In addition, we show that it's possible to operate simultaneously several MD-SiPM array chips in order to build a small-animal PET detector modules. We present the implementation of two PET detector modules that are based on MD-SiPMs: a small animal and an endoscopic PET detector modules. The small animal PET detector module consists of 2×4 monolithic MD-SiPM array chips. In addition, this module includes a low-cost field programmable gate array (FPGA), a temperature controlling system and data transfer interfaces. The endoscopic PET detector module comprises a single monolithic array of 9×18 MD-SiPM and a small form-factor FPGA. In this module, a remarkable level of compactness is achieved. Eventually, a thermal characterization and a preliminary radiation measurement are presented.@en

In time-of-flight (TOF) positron emission tomography (PET), the coincidence resolving time (CRT) has a strong influence on the overall performance. Multichannel digital silicon photomultipliers (MD-SiPMs) are able to obtain several timestamps for gamma photon timemark estimation. Using this feature, the CRT is improved and the system robustness is significantly increased by utilizing multiple photoelectron timestamps. In addition, the PET instrumentation chain is simplified because of the intrinsic digitization and integrated functionality of the MD-SiPM. The main objective of this work is to demonstrate the possibility of building a complete highly-miniaturized PET detector module for endoscopic applications. In addition, we show that it's possible to operate simultaneously several MD-SiPM array chips in order to build a small-animal PET detector modules. We present the implementation of two PET detector modules that are based on MD-SiPMs: a small animal and an endoscopic PET detector modules. The small animal PET detector module consists of 2×4 monolithic MD-SiPM array chips. In addition, this module includes a low-cost field programmable gate array (FPGA), a temperature controlling system and data transfer interfaces. The endoscopic PET detector module comprises a single monolithic array of 9×18 MD-SiPM and a small form-factor FPGA. In this module, a remarkable level of compactness is achieved. Eventually, a thermal characterization and a preliminary radiation measurement are presented.@en