PET is a functional imaging method that can visualize metabolic processes and relies on the coincidence detection of emitted annihilation quanta. From the signals recorded by coincident detectors, TOF information can be derived, usually represented as the difference in detection timestamps. Incorporating the TOF information into the reconstruction can enhance the image’s SNR. Typically, PET detectors are assessed based on the coincidence time resolution (CTR) they can achieve. However, the detection process is affected by factors that degrade the timing performance of PET detectors. Research on timing calibrations develops and evaluates concepts aimed at mitigating these degradations to restore the unaffected timing information. While many calibration methods rely on analytical approaches, machine learning techniques have recently gained interest due to their flexibility. We developed a residual physics-based calibration approach, which combines prior domain knowledge with the flexibility and power of machine learning models. This concept revolves around an initial analytical calibration step addressing first-order skews. In the subsequent step, any deviation from a defined expectation is regarded as a residual effect, which we leverage to train machine learning models to eliminate higher-order skews. The main advantage of this idea is that the experimenter can guide the learning process through the definition of the timing residuals. In earlier studies, we developed models that directly predicted the expected time difference, which offered corrections only implicitly (implicit correction models). In this study, we introduce a new definition for timing residuals, enabling us to train models that directly predict correction values (explicit correction models). We demonstrate that the explicit correction approach allows for a massive simplification of the data acquisition procedure, offers exceptionally high linearity, and provides corrections able to improve the timing performance from (371 ± 6) ps to (281 ± 5) ps for coincidences from 430 keV to 590 keV. Furthermore, the novel definition makes it possible to exponentially reduce the models in size, making it suitable for applications with high data throughput, such as PET scanners. All experiments are performed with two detector stacks comprised of 4×4 LYSO:Ce,Ca crystals (each 3.8 mm × 3.8 mm x 20 mm), which are coupled to 4 × 4 Broadcom NUV-MT SiPMs and digitized with the TOFPET2 ASIC.

Objective.Time resolution is crucial in positron emission tomography (PET) to enhance the signal-to-noise ratio and image quality. Moreover, high sensitivity requires long scintillators, which can cause distortions in the reconstructed images due to parallax effects. This study evaluates the performance of a time-of-flight (TOF)-PET module that makes use of a single-side readout of a4×43.1×3.1×15mm3LYSO:Ce matrix with an array of4×4silicon photomultipliers (SiPMs) and a light guide to extract high-resolution TOF and depth of interaction (DOI) information.Approach.This study assesses the performance of the detector prototype using the commercially available TOFPET2 ASIC and SiPMs from various producers. DOI and TOF performance are compared to results using custom-made NINO 32-chip based electronics.Main results.Using a Broadcom NUV-MT array, the detector module read out by the TOFPET2 ASIC demonstrates a DOI resolution of 2.6 ± 0.2 mm full width at half maximum (FWHM) and a coincidence time resolution (CTR) of 216 ± 6 ps FWHM. When read out using the NINO 32-chip based electronics, the same module achieves a DOI resolution of 2.5 ± 0.2 mm and a CTR of 170 ± 5 ps.Significance.The prototype module, read out by commercial electronics and using state-of-the-art SiPMs, achieves a DOI performance comparable to that obtained with custom-made electronics and a CTR of around 200 ps. This approach is scalable to thousands of channels, with only a deterioration in timing resolution compared to the custom-made electronics, which achieve a CTR of 140 ps using a standard non-DOI module.

Abstract Background Designing positron emission tomography (PET) scanners involves several significant challenges. These include the precise measurement of the time of arrival of signals, accurate integration of the pulse shape, maintaining low power consumption, and supporting the readout of thousands of channels. To address these challenges, researchers and engineers frequently develop application-specific integrated circuits (ASICs), which are custom-designed readout electronics optimized for specific tasks. As a result, a wide range of ASIC solutions has emerged in PET applications. However, there is currently no comprehensive or standardized comparison of these ASIC designs across the field. Methods In this paper, we evaluate the requirements posed to readout electronics in the field of PET, give an overview of the most important ASICs available for PET applications and discuss how to characterize their essential features and performance parameters. We thoroughly review the hardware characteristics of the different circuits, such as the number of readout channels provided, their power consumption, input and output design. Furthermore, we summarize their performance as characterized in literature. Results While the ASICs described show common trends towards lower power consumption or a higher number of readout channels over the past two decades, their characteristics and also their performance assessment by the developers, producers and vendors differ in many aspects. To cope with the challenge of selecting a suitable ASIC for a given purpose and PET application from the varying information available, this article suggests a protocol to assess an ASIC’s performance parameters and characteristics. Conclusion ASICs developed for PET applications are versatile. With novel benchmarks set for the impact of scintillator and photosensor on the time-of-flight performance, the pressure on ASICs to deliver higher timing resolution and cope with an even higher data rate is enormous. Latest developments promise new circuits and improvements in time-of-flight performance. This article provides an overview on existing and emerging readout solutions in PET over the past 20 years, which is currently lacking in literature.

Objective.Integrating time-of-flight (ToF) measurements in radiography and computed tomography (CT) enables an approach for scatter rejection in imaging systems that eliminates the need for anti-scatter grids, potentially increasing system sensitivity and image quality. However, present hardware dedicated to the time-correlated measurement of x-rays is limited to a single pixel physically too large for the desired spatial resolution. A switch to highly integrated electronics and detectors is needed to progress towards detector arrays capable of acquiring images, while offering a timing resolution below 300 ps FWHM to achieve scatter rejection comparable to current anti-scatter grids.Approach.Using off-the-shelf scintillators, photodetectors and readouts designed for ToF positron emission tomography (PET) provides a preliminary evaluation of available highly integrated readout systems supporting detector arrays for ToF scatter rejection. The TOFPET2c ASIC from PETSys offers an established development platform necessary for fast and reliable results, with no known limitation regarding time-correlated detection of medical imaging x-rays (20-140 keV).Main results.Reliable photon detection down to 31 keV was achieved, reaching energy resolutions from 23% to 92% FWHM throughout the desired energy range. Optimal detector timing resolution (DTR) from 250 ps FWHM at 130 keV to 678 ps FWHM at 30 keV was reached. Strong time walk effects were observed, showing a time shift of 642 ps up to 1740 ps between events spanning the energies used in x-ray medical imaging.Significance.The TOFPET2c ASIC has shown its potential for ToF scatter rejection, but meets the time resolution requirement of 300 ps FWHM only for limited energies (110-140 keV). This significant timing degradation observed at lower energies limits the use of the TOFPET2c ASIC for ToF scatter rejection, but offers significant advancements regarding the understanding of the phenomenon arising from the time-correlated detection of medical imaging x-rays.

Resolving the depth of interaction (DOI) of a $\gamma $ -photon in the scintillator is necessary to correct for parallax errors in organ-dedicated and large-scale time-of-flight positron emission tomography (TOF-PET) scanners or enable the precise recovery of Compton-scattered $\gamma $ -photons. Doubling the number of readout channels and moving toward more complex detector designs are methods to encode DOI, often associated with high costs. We propose a DOI-capable TOF-PET detector unit concept confining light-sharing to two detector channels, where the high benefit lies in scalability and the prospect of Compton recovery between adjacent units. We evaluate these scalable, DOI-capable unit concepts, realizing DOI encoding between two LYSO:Ce,Ca crystals ( 3x3x 20 mm(3); Taiwan Applied Crystals) one-to-one coupled to two Broadcom AFBR-S4N33C013 silicon-photomultipliers (SiPMs) read out with the TOFPET2 ASIC. The best-performing unit employing a triangular reflector sheet and optical glue between the two crystals and mounted on two FBK NUV-MT SiPMs results in a DOI resolution of about 3 mm (RMSE) based on the energy ratio digitized by the two channels while maintaining a coincidence time resolution (CTR) of 226 ps (FWHM) with TOFPET2 ASIC readout, applying a linear DOI correction. Using HF readout, the CTR of the proposed detector unit was improved to 141 ps (FWHM).

Objective. Modern PET scanners offer precise TOF information, improving the SNR of the reconstructed images. Timing calibrations are performed to reduce the worsening effects of the system components and provide valuable TOF information. Traditional calibration procedures often provide static or linear corrections, with the drawback that higher-order skews or event-to-event corrections are not addressed. Novel research demonstrated significant improvements in the reachable timing resolutions when combining conventional calibration approaches with machine learning, with the disadvantage of extensive calibration times infeasible for a clinical application. In this work, we made the first steps towards an in-system application and analyzed the effects of varying data sparsity on a machine learning timing calibration, aiming to accelerate the calibration time. Furthermore, we demonstrated the versatility of our calibration concept by applying the procedure for the first time to analog readout technology. Approach. We modified experimentally acquired calibration data used for training regarding their statistical and spatial sparsity, mimicking reduced measurement time and variability of the training data. Trained models were tested on unseen test data, characterized by fine spatial sampling and rich statistics. In total, 80 decision tree models with the same hyperparameter settings, were trained and holistically evaluated regarding data scientific, physics-based, and PET-based quality criteria. Main results. The calibration procedure can be heavily reduced from several days to some minutes without sacrificing quality and still significantly improving the timing resolution from ( 304 ± 5 ) ps to ( 216 ± 1 ) ps compared to conventionally used analytical calibration methods. Significance. This work serves as the first step in making the developed machine learning-based calibration suitable for an in-system application to profit from the method’s capabilities on the system level. Furthermore, this work demonstrates the functionality of the methodology on detectors using analog readout technology. The proposed holistic evaluation criteria here serve as a guideline for future evaluations of machine learning-based calibration approaches.

The addition of a time-of-flight (ToF) measurements to radiography and computed tomography (CT) opens the door to an anti-scatter grid-free approach to scatter rejection in imaging systems, potentially increasing system sensitivity and image quality. Previously developed hardware limited to a few channels showed that the ToF scatter rejection is possible, but lacked in scale and density. A medium-scale ToF scatter rejection detection module was developed, allowing for the evaluation of off-the-shelf ToF ASICs, as well as the future development of dedicated devices. The TOFPET2c ASIC designed by PETSys showed good potential for the first detector, offering high integration and pixel count. The evaluation of the TOFPET2c ASIC at X-ray energies (20 keV to 140 keV) showed a 414 ps FWHM detector timing resolution. Such a value is slightly over the 300 ps FWHM limit sufficient to reach scatter-rejection similar to anti-scatter grids and requires further investigation. An energy resolution bellow 50% was also reached for all desired energies.

Last year’s SNMMI, Siemens Healthineers presented the Biograph Vision.X PET/CT scanner, currently delivering the industry’s leading time-of-flight (TOF). Although recent TOF-research explores novel materials, electronics, and calibration concepts, system developments are often evaluated against the Biograph Vision.X benchmark. In this work, we explored what coincidence time resolution (CTR) can be achieved with conventional readout electronic (TOFPET2c ASIC) and clinical detector blocks using our previously established residual physics timing calibration. It is the first time we apply the concept to a detector read out with analog SiPMs and the TOFPET2c ASIC. Our method relies on channel-individual information in combination with machine learning (ML) models, which we characterized in this work regarding their functionality and capability to improve the detectors’ CTR. We compare the timing performance against an approach based on a purely analytical calibration as it is applicable to general system architectures. The results show that the residual physics calibration concept offers significant advances demonstrated by the improvement of CTR from $335 \pm 5 \mathrm{ps}$ (analytical cal) to around $219 \pm 1 \mathrm{ps}$ (analytical cal + ML) for coincidences within an energy window from 300 keV to 700 keV.

The Hyperion PET detector platform comprises sensor tiles, which are connected through flexible cables to the mainboards of the PET detector modules. The versatility of analog SiPM / ASIC combinations will be used to design sensor tiles for multiple PET/MRI projects. SiPMs and digitizing ASICs are integrated into the detector platform. The first implementation will have a form factor of $49 \times 49 \mathrm{~mm}^{2}$ and use 4-mm SiPMs. The resulting 144 SiPMs shall be read out by a single TOFPET ASIC with 64 input channels to improve the required space, power consumption and costs. Several channel compression schemes with a factor of at least 2.25 are under investigation: Row-column-sum and matrix-head circuits with active components such as amplifiers as well as different passive connection schemes using the light-spread of a crystal over multiple sensors. Simple patterns allow straightforward algorithms for crystal identification, whereas other more optimized patterns employ more advanced processing algorithms, like neural networks, for this task. Simulations show that the probability to find the correct crystal is only reduced by $1.6 \%$ to $4.6 \%$ compared to the uncompressed data from all 144 channels. Test PCBs were designed for SiPMs and for the compression schemes as interposing PCBs. At the same time, a sensor tile is being designed with a TOFPET2 ASIC and an FPGA on the electronics side and just two connectors for the SIPMs on the other side. As such, the digitizing ASIC, the compression schemes, and the SiPMs can be freely combined.

For organ-dedicated and large axial field of view positron emission tomography scanners, depth-of-interaction (DOI) encoding is indispensable to correct parallax errors at oblique angles. The sensitivity of these systems can be boosted effectively by additionally taking advantage of the time-of-flight (TOF) gain in signal-to-noise ratio due to a high coincidence time resolution (CTR). Ultimately, TOF and DOI resolution can not be treated independently due to the DOI-dependent impact of the photon travel time spread (PTS) on the TOF resolution. With a sufficient DOI resolution, the impact of the PTS on the CTR can be corrected. We present a TOF- and DOIcapable detector block that maintains a high CTR while allowing for a DOI resolution of 3.4 mm to 4.7 mm (RMSE), which results in a binning of the crystal into at least two DOI layers. We report the CTR achieved with different system-applicable readout electronics (NINO and TOFPET2 ASIC) and provide benchmarks for the CTR improvement with custom high-frequency readout electronics. With these, the CTR of a single TOF-DOI unit of 141 ps (FWHM) can be maintained on the detector-block level.

Abstract Background Repetitive PYP shortages in the US have many labs alternating between Tc-99m labelled PYP and HDP for the diagnosis of cardiac amyloid. Studies have not identified a cutoff for the H/CL ratio with HDP when used to diagnose ATTR cardiac amyloid due to issues with soft tissue HDP uptake. Purpose We sought to assess the use of the commonly used H/CL ratio cutoff of 1.5 with HDP in the diagnosis of ATTR cardiac amyloid. Methods In a retrospective study, consecutive patients undergoing cardiac amyloid imaging with HDP were assessed. All were imaged and interpreted utilizing planar, SPECT, and CT imaging at 2 hours post injection. Patients were divided into positive and negative studies based on grade 2 or 3 myocardial uptake on fused SPECT/CT images. A final clinical diagnosis of ATTR cardiac amyloidosis, including testing for monoclonal gammopathy, was confirmed by chart review. Planar images were analyzed for counts in the heart and contralateral lung to calculate a H/CL ratio. Results A total of 274 patients were imaged with HDP during the study period. A total of 44 positive studies (16.1% of the total) (mean age 81.3 years, 75% male) and 66 negative studies (mean age 73.4 years, 59% male) were analyzed. The mean H/CL ratio for positive HDP studies was 1.9 ± 0.38 and 1.09 ± 0.12 for negative studies (p<0.0001). 100% (66/66) of the negative studies had a H/CL ratio below 1.5 and 43/44 (97.7%) of positive studies had a H/CL ratio above 1.5. The one outlier had a H/CL ratio of 1.45 (88 yo female with moderate LVH, heart failure with mild LV dysfunction, LBBB, and paroxysmal atrial fibrillation). Conclusion Utilization of a H/CL ratio on HDP planar imaging with a similar cut-off to PYP would also seem to be reasonable as the vast majority of positive ATTR patients had a H/CL ratio above 1.5 with HDP. Quantification of HDP would seem to be feasible and accurate.Heart/Contralateral Lung Ratio

Cross-luminescence (CL) has been shown to be one of the promising scintillation mechanisms to overcome timing limitations in TOF-PET, TOF-CT or high energy physics. However, well-known CL-emitters (e.g. BaF 2 ) have so far not been established in practice due to their disadvantages, mainly the emission in the deep-UV, calling for the need to develop new CL-emitting scintillators. One promising candidate is CsZnCl-based materials with a decay time ~1-2ns and light emission ~300nm. In this work, Cs 2 ZnCl 4 and Cs 3 ZnCl 5 are examined regarding their scintillation emission time profile and coincidence time resolution (CTR) read out with commercially available and research SiPMs. With 2x2x3mm³ crystals and a time-correlated single-photon counting (TCSPC) setup decay times of 1.79ns for Cs 2 ZnCl 4 and 1.03ns for Cs 3 ZnCl 5 were measured with no long tails in the scintillation emission time profiles, validating previously published measurements. Additionally, a prompt emission with significant abundance of 5% and 6% was discovered. Assuming a light yield of 1980ph/MeV for Cs 2 ZnCl 4 and 1460ph/MeV for Cs 3 ZnCl 5 , this relates to 122 (Cs 2 ZnCl 4 ) and 73 (Cs 3 ZnCl 5 ) ph/MeV energy deposit. CTR measurements with a high-frequency setup reached 60ps (FWHM) using Cs 2 ZnCl 4 coupled to FBK VUV SiPMs with silicon oil (Dow Corning 200), which is on par with state-of-the-art LYSO:Ce,Ca glue-coupled to NUV-MT SiPMs at 58ps. We find that the CTR of Cs 2 ZnCl 4 , air-coupled to VUV SiPMs, at 78ps (FWHM) is comparable to BaF 2 at 75ps (FWHM). With their fast decay time and high CTR, these crystals seem to be perfectly suitable for high-rate time-of-flight applications further supported by the non-hygroscopicity for Cs 2 ZnCl 4 and only slight hygroscopicity for Cs 3 ZnCl 5 . Further measurements and simulations are planned to explore the prompt emission and to investigate light transport and sensitivity.

The TOFPET ASIC from PETsys Electronics is a promising candidate to use for SiPM signal digitization in PET systems. Our group is currently integrating the ASIC in the MRI-compatible Hyperion PET detector platform. During this process, the calibration of the ASIC’s time- and charge-to-digital converters to register the timestamp and energy of an SiPM signal have to be considered both in firmware and in software routines. We present the calibration process we will use in our systems and compare to the PETsys-provided routines used in their hard-, firm-, and software. The TDC calibration method we propose uses randomly generated triggers. We could show that the method is reliable and even produces slightly better timing accuracy compared to the vendor provided method and does not rely on generating test pulses with specific phase shifts relative to the clock edge of the reference signal.

Fast timing in ToF-PET improves the signal-to-noise ratio for better patient comfort through either a lower dose or a shorter scan time. Recent clinical PET scanners using lutetium-based crystals, like LYSO, reach a coincidence time resolution (CTR) of around 200 ps. Their CTR is mainly limited by the scintillation process. Thus, crystals with a significant emission of Cherenkov light, most prominently BGO, are being investigated as an alternative. BGO cannot reach its full potential due to a too low bandwidth of the electronics and too low time resolution of the photosensors. We investigate the segmentation of the photosensor into an array of individually-read-out $\mu \mathrm{SiPMs}$ to allow a better use of the Cherenkov photon’s prompt time information through an effectively higher bandwidth. In this work, we simulated the optical photon production with Geant4 and applied a signal model with a leading-edge threshold to determine timestamps and calculate the CTR from the first $\mu$ SiPM timestamp. Detectors with either 3 mm or 20 mm BGO or LYSO crystals were examined with varying photosensor segmentations. Segmenting the photosensor resulted in a significant improvement of the CTR for BGO crystals of both lengths, reducing it to $29 \pm 15 \mathrm{ps}$ (FWHM) for 3 mm and $80 \pm 21 \mathrm{ps}$ (FWHM) for 20 mm. BGO benefits from the segmentation due to its higher Cherenkov-to-scintillation ratio, while the same does not apply to LYSO with its lower Cherenkov and higher scintillation light yield. Adding noise factors, we validated the simulation of a full SiPM with measurements. For the 20 mm BGO crystal, we found that the CTR of the highest segmentation deteriorated slightly, but stayed in the range of 100 ps. For a more realistic model, further simulations with more noise factors are planned. Finally, first measurements with test structures will be used to verify our simulation results.

Time resolution plays a key role in positron emission tomography (PET) by enhancing the signal-to-noise ratio and ultimately improving the quality of the image. In previous studies, an array of 16 LYSO crystals (measuring 3.1x3.1x15 mm 3 ) coupled to Hamamatsu S13361-3050AE-04 SiPMs and readout by a custom-made NINO board achieved a coincidence time resolution (CTR) below 160 ps. However, the spatial resolution of the array was affected by parallax error due to the unknown depth of interaction (DOI) of the incident gamma-ray photon. The DOI information was successfully extracted with 3 mm resolution using a light-sharing mechanism and the NINO board. Nevertheless, the board is not commercially available and cannot be scaled to a full PET detector. To address this limitation, this study examines the commercially available TOFPET2 ASIC (application specific integrated circuits) from PETsys Electronics S.A. using light-sharing DOI-capable modules and compares it to the custom-made NINO board. The SiPM array coupled to the matrix of 16 LYSO crystals using TOFPET2 PETsys demonstrates a DOI resolution of 3.6 ± 0.3 mm and a CTR value of 224 ± 3 ps. This approach is scalable to a full PET detector. Finally, potential paths to further improve DOI resolution and CTR are discussed.

Ultrafast high-frequency (HF) readout concepts have advanced the performance limits of single-channel time-of-flight positron emission tomography (TOF-PET) detectors to sub-100 ps coincidence time resolution (CTR).We have implemented a 16-channel version of an adapted HF readout concept, including pulse discrimination and a linearized time-over-threshold (TOT) method, fully compatible with a high precision time-to-digital converter (TDC) developed at CERN, the picoTDC. The functionality of the circuit has been initially tested using the TOFPET2 ASIC as back-end electronics, to emulate the TDC. The implemented pulse discrimination has shown the capability to mitigate influences of baseline shifts and noise on the TOFPET2 front end, resulting in a CTR of 105 ps using 2x2x3 mm 3 LYSO:Ce,Ca crystals and Broadcom NUV-MT SiPMs and in the absence of side peaks in the coincidence time difference spectra. For a 4x4 matrix of 3.8x3.8x19 mm 3 LYSO:Ce,Ca crystals one-to-one coupled to 16 Broadcom NUV-MT SiPMs, the average CTR was improved from 252 ps (FWHM) to 240 ps (FWHM). The ongoing integration with the picoTDC is expected to improve the CTR due to its state-of-the-art resolution of below 10 ps and will further set the path towards a 10 ps readout solution.

Improving time-of-flight (TOF) is an effective way to boost the signal-to-noise ratio in positron emission tomography (PET), however, the scanner sensitivity and resolution must be maintained. The use of axially aligned long LYSO:Ce,Ca or BGO fibers employing double-sided readout has the potential to reduce parallax errors through depth-of-interaction (DOI) estimation and to allow a reduction in the number of readout channels required, resulting in cost benefits. Due to orientation, these fibers may also facilitate the integration of TOF-PET with magnetic resonance imaging (MRI) in hybrid imaging systems.In a first step, we tested LYSO:Ce,Ca and BGO scintillation fibers of 3x3x20 mm 3 and 3x3x100 mm 3 to determine the best achievable DOI and coincidence time resolution (CTR). Using Broadcoam NUV-HD SiPMs of 3x3 mm 2 size we measured for the 100 mm fiber an average CTR of (137±1) ps FWHM and an average DOI resolution within the fiber of (12.3±0.5) mm FWHM. The 3x3x20 mm 3 crystal showed a sub-100 ps CTR of (98±1) ps and a DOI resolution of (8.5±0.2) mm, whereas in both cases the spatial resolution is defined to 3 mm in all other directions. Newest SiPMs from Broadcom, with metal in trench technology (NUV-MT), further show to improve these values. Measuring with 3x3x20 mm 3 BGO we obtain (220±5) ps FWHM in standard single-sided readout, but observed a non-negligible influence of optical crosstalk in double-sided readout, caused by the opposing SiPM.This study indicates that long scintillation fibers can achieve excellent CTR and DOI resolution, almost independent on the fiber length, due to a time-based estimation. Using 100 mm long fibers, an electronic channel reduction of a factor 2.5 is achieved, which can lead to lower production costs and to advantages in building organ-dedicated or total-body/large-scale PET scanners.

The Biograph Vision, developed by Siemens Healthineers, is regarded as the gold standard for time-of-flight (TOF) performance in clinical positron emission tomography (PET) imaging. When comparing new multi-channel readout circuits, it is essential to account for potential disparities in material and assembly of custom detector blocks. This ensures that the sole influence of readout electronics on timing performance can be accurately assessed. In this context, we evaluate the TOFPET2 ASIC, designed by PETsys Electronics S.A., in conjunction with a Biograph Vision detector block. We examine the coincidence time resolution (CTR) and energy resolution. Furthermore we evaluated the scintillator for DOI capabailities, not available in the clinical product, using the channel-individual readout capabilities.

Functional imaging techniques like positron emission tomography (PET) are an essential tool in an aging society. Despite impressive advances in microelectronics, photodetectors and scintillation materials, PET is still awaiting a breakthrough in terms of reduced cost and increased performance. Large potential is seen in ultraprecise time-of-flight (TOF), aiming at coincidence time resolutions (CTRs) better than 30 ps. However, state-of-the-art TOF-PET systems are still far away from this goal, achieving typical CTRs of 214 ps (FWHM). Several proposals have been put forth, whereas the most promising is to use prompt photon emission, e.g. Cherenkov radiation in BGO crystals, which are cheap to produce, thus contributing to drastic cost cutting. However, Cherenkov detection is challenging due to its limited photon yield, which in turn requires a very high photon detection efficiency (PDE), low dark count rate (DCR) and extremely fast and innovative electronic readout schemes. Recent analog silicon photomultipliers (aSiPMs) meet the first two targets, but not the latter.In the Digilog project we envisage to unite the best of these two worlds, combining high PDE, low DCR and an exceptional SPTR. To reach this goal, we will segment state-of-the-art aSiPMs into smaller clusters, called µSiPMs. A balanced segmentation of the electronic readout will make it possible to efficiently detect the first scintillation and Cherenkov photons, with a manageable granularity at system level. The µSiPM signals will feature photon-density time walk correction and photon counting. We envision to create 3D-stacked sensors where the electronics will be housed in a CMOS bottom-tier and the µSiPMs in the top-tier chip. Preliminary measurements on first µSiPM test-structures already reached PDE and DCR close to their commercial counterparts, while an SPTR of 25 ps FWHM, close to our sub-20 ps goal, has been achieved.