Deep learning is increasingly transforming medical imaging by enabling more accurate data interpretation and reconstruction from complex detector signals. In positron emission tomography (PET), accurate localization of photon interactions within detectors is crucial for improving image resolution and diagnostic value. This work focuses on semi-monolithic scintillation detectors, which balance pixelated and monolithic designs, and applies deep learning methods that exploit scintillation light patterns across photosensors to improve interaction positioning. We evaluated how different neural network architectures and combinations of input signals affect positioning accuracy using experimental data from two types of detector arrays: 1) a 1 & times;8 module used in the IMAS total-body scanner and 2) a 1 & times;16 module from a brain-dedicated PET, both coupled to 64-channel photosensor matrices. We compared multilayer perceptron and convolutional neural network, using either reduced 16-channel inputs (obtained via row and column summation) or the full 64-channel configuration, along with energy, time, and engineered features. Results show that deeper architectures and richer inputs improve positioning performance-especially in the depth-of-interaction direction, with gains of around 20%-by better exploiting light-sharing effects between slabs. Finally, we introduced a deep-learning-based signal demultiplexing approach which accurately reconstructs full 64-channel signals from reduced 16-channel measurements with structural similarity index measure above 0.98. This enables the combination of simplified hardware design crucial for data throughput together with the benefits of higher resolution positioning when using 64 signals. This work shows how deep learning, when combined with multiple signal inputs and the learned recovery of full signals from multiplexed data, can enhance the performance of PET instrumentation.

Semi-monolithic scintillation detectors have recently proven to be a promising compromise between pixelated and monolithic crystals. This geometry is particularly suitable for preclinical Positron Emission Tomography (PET) systems, as it provides high spatial resolution while enabling Depth of Interaction (DOI) capabilities. While prior detector developments have almost exclusively focused on lutetium-based scintillators, BGO has re-gained attention for PET applications given its lower cost, higher stopping power, larger photo-fraction and absence of intrinsic radioactivity. In this work, we propose a semi-monolithic detector composed of 44 BGO slabs of 1 mm × 24.2 mm × 10 mm each, designed for preclinical PET systems. We focus on evaluating its spatial and energy performance and directly compare it with a LYSO semi-monolithic block of identical dimensions. The x-monolithic direction and DOI resolutions were evaluated using neural networks, while the y-pixelated direction was evaluated with an analytical method. Average FWHM spatial resolutions of 1.6 ± 0.2 mm and 1.2 ± 0.3 mm were achieved for the BGO and LYSO, respectively, along the x-monolithic direction. Regarding DOI, average FWHM resolution values of 3.0 ± 0.6 mm and 2.0 ± 0.5 mm were obtained for BGO and LYSO, respectively. A FWHM spatial resolution of around 1 mm was estimated for both crystal types along the y-pixelated direction. Finally, mean energy resolutions of 23.7% and 18.5% were found for the BGO and LYSO, respectively. These results show that the proposed BGO block is a viable alternative for high spatial resolution, high sensitivity and low-cost preclinical PET scanners.

Objective.To verify the delivered dose in proton therapyin vivousing positron emission tomography (PET) with millimetric precision.Approach.Proton and ion therapy have gained significant importance in cancer radiotherapy due to their favorable dose distribution and tissue-sparing properties. In conventional gamma radiation therapy some methods ofin vivodose verification are possible with current medical devices. In proton and ion therapy, dose verification is limited, with PET being mainly used for particle range assessments rather than full dose mapping. Prompt gamma techniques are well suited for range verification but are not suitable for full three-dimensional (3D), voxel-wise dose mapping. This study presents initial results forin vivodose verification using F-18 PET imaging during proton therapy. Although the activity concentration of F-18 generated by typical clinical doses (several Gy) is low, PET imaging performed approximately one-hour post-irradiation yields sufficient image quality to derive dose-volume histograms (DVHs), enabling spatial dose verification. We simulated proton treatment in a brain phantom using the Gate and RayStation platforms to assess the production of several positron emitting isotopes. We focused on the production of fluorine-18 (F-18), given its low positron energy and low energy threshold for production, which enables accurate replication of the dose distribution. To evaluate the detectability of the anticipated low activity concentrations (on the order of a fraction of a Bq ml-1) following a 3 Gy proton irradiation, we tested three PET systems: two preclinical scanners based on LYSO detectors and one clinical scanner based on BGO crystals. Finally, we analyzed the DVHs for the simulated and measured dose and activity distributions and compared them with the planned distribution.Main Results.F-18 PET imaging in proton therapy correlates with the delivered dose to within 5% and matches the planned dose fall-off edge within 1 mm, enabling accurate and precisein vivodose verification.Significance.F-18 enables more accurate proton-therapy dose verification than other positron emitters studied (C-11, N-13, and O-15), showing the closest spatial correspondence to the planned dose and activity levels post-therapy that remain detectable on modern BGO-based PET systems.

This work summarizes the design, construction, initial performance evaluation and pilot clinical results of the IMAS system, a long axial field of view (FOV), also known as total-body (TB-), positron emission tomography (PET) prototype scanner. This PET enables for the first time in TB-PET imaging, simultaneously time-of-flight (TOF) and depth-of-interaction (DOI) capabilities. The IMAS detector block is based on LYSO semi monolithic scintillators, with individual slab sizes of 3 mm x 25 mm x 20 mm each. Arrays of 1x8 slabs are coupled to 8x8 Silicon Photomultiplier arrays. A proprietary readout reduces the 64 signals to only 16 outputs, preserving both 3D photon impact positioning and timing accuracy. IMAS has a total of 30,720 channels. PETsys electronics is used for data acquisition. The IMAS geometry is based on 5 rings of 10 cm each, with a 5 cm gap between them. It defines an axial FOV of 71 cm with a bore aperture of 82 cm. We report in this work the pilot tests of the system performance and the first clinical results. We found that the system spatial resolution remained below 4 mm across the entire FOV, even at the off-radial position of 30 cm. A coincidence time resolution with a small size 22Na source of 560 ps FWHM was measured. A sensitivity of 56.54 cps/kBq is in good agreement with previous simulation studies; however, the noise equivalent count rates performance (79 kcps at 3.26 kBq/mL) was significantly lower than expected, likely due to a data transfer bottleneck between the system and the acquisition workstation. Finally, a comparison of one of the imaged patients with a commercial TOF PET/CT scanner is also provided, pinpointing an improved tumor identification for IMAS, and the advantages of TOF and especially DOI capabilities.

Abstract Background Breast cancer causes the largest number of cancer-related deaths among women worldwide. With the aim of improving Positron Emission Tomography (PET) technology for accurate breast cancer diagnosis and staging, we propose a system design based on monolithic crystals with inherent Depth of Interaction (DOI) capabilities and an innovative edgeless detector ring. This approach eliminates the physical gaps between PET detectors, improving the system detection efficiency while potentially enhancing the image quality since edge effects are reduced. We have developed a dedicated breast PET system prototype (DeepBreast) to show the feasibility of this design. The system is composed of 14 curved LYSO monolithic scintillators of 12.5 mm thickness glued side-by-side with a high-refractive index compound. The useful transaxial and axial Field of View (FOV) of the system are 160 mm and 50 mm, respectively. A Neural Network technique was used for the x- and y- photon impact position estimation. The impact DOI and energy values were determined using the Voronoi calibration methodology. An initial experimental evaluation of the DeepBreast system has been performed inspired by the NEMA protocols for whole-body and small-animals PET scanners. Results A nearly flat spatial resolution as a function of radial position was obtained, which indicates the DOI capability of the system to mitigate parallax errors. An average spatial resolution of 1.9 ± 0.1 mm, 1.9 ± 0.1 mm and 1.7 ± 0.1 mm FWHM was achieved at the center of the axial FOV for the radial, tangential, and axial directions, respectively. A maximum sensitivity value of 2% was measured at the center of the FOV. The noise equivalent count rate peak reached 15 kcps at 13.4 MBq. Moreover, percent contrast values of 27.9%, 28.8%, 56.8%, 72.5%, 87.2% and 84.2% were achieved for 4.5 mm, 6 mm, 9 mm, 12 mm, 15 mm and 20 mm cylinders of a larger dedicated IQ phantom, respectively. Conclusions The initial experimental results demonstrate the feasibility of the DeepBreast as an innovative PET scanner for breast cancer imaging.

Semi-monolithic detectors, a hybrid configuration combining the benefits of pixelated arrays and monolithic blocks, present a compelling and cost-effective solution for positron emission tomography (PET) scanners with both time-of-flight (TOF) and depth-of-interaction (DOI) capabilities. In this work, we evaluate four LYSO-based semi-monolithic arrays with various surface treatments, read out with the PETsys TOFPET2 ASIC, to identify the optimal configuration for a novel brain PET scanner. The chosen array, featuring ESR on all surfaces except for the black-painted lateral pixelated ones, achieved 15.9 ± 0.6 % energy resolution and 253 ± 15 ps detector time resolution (DTR). neural network with multilayer perceptron architectures were used to estimate the annihilation photon impact position, yielding average accuracies of 3.7 ± 1.1 mm and 2.6 ± 0.7 mm (FWHM) along the DOI and monolithic directions, respectively. The comparative analysis of the four arrays also prompted an investigation into light sharing in semi-monolithic detectors, supported by a GATE-based simulation framework which was designed to complement the experimental results and confirm the observed trends in time resolution. By refining the detector design based on semi-monolithic geometry and optimized surface crystal treatment to enhance positioning accuracy, this study contributes to the development of a next-generation brain PET scanner, with competitive performance but at a moderate cost.

Traditional PET detectors based on pixelated scintillation crystals with single-ended readout do not provide depth of interaction (DOI) information in an easy and cost-effective way. In this work, we propose a PET detector with single-ended readout and 1:1 coupling, based on arrays of naked pixelated crystals that are glued in one direction, and optically separated in the other one. We have named this approach as pseudo-slab. In this configuration, some of the optical photons will propagate in the glued direction, generating a light distribution from which DOI information can be retrieved. We have characterized four different detector configurations, all of them consisting of a linear array of 1x8 LYSO crystals of 3x3x20 mm(3 )each, with an optical glue of approximately 70 mu m in between them. The top and bottom faces are polished, and with a different number of unpolished lateral surfaces (2 versus 4) and different wrappings (Enhanced Specular Reflector versus BaSO4 ). The results obtained for the four detector configurations show energy resolutions ranging from 8.5% to 9.8% and coincidence time resolutions (with a reference pixel) below 290 ps for all cases using only the fastest timestamp and close to 230 ps when energy-weighted averaging of multiple timestamps is applied (corresponding to 182 ps detector time resolution). Regarding DOI performance, all configurations provide DOI information, showing a better performance with more number of unpolished faces and also when using BaSO4 as a reflector.

The integration of Time-of-Flight (ToF) information into Positron Emission Tomography (PET) image reconstruction enhances both signal-to-noise ratio and the localization of annihilation events. A critical component contributing to the accuracy of ToF-PET is the scintillator. To overcome the time resolution limitations in conventional scintillators, the metascintillator approach has been proposed. The metascintillator is an engineered composition of small units that combines and optimizes various features within a single scintillator heterostructure.In this work, metascintillator-based brain PET systems were simulated using the GATE toolkit and compared with designs based on bulk LYSO or BGO. Sensitivity, noise equivalent count rate (NECR) and scatter fraction were evaluated following NEMA guidelines. To match the peak sensitivity of a system utilizing a 15 mm bulk BGO, the metascintillator-based scanners using BGO/BaF2, BGO/EJ232, LYSO/BaF2 and LYSO/EJ232 must possess thicknesses of 23.2 mm, 22.5 mm, 29.7 mm and 31.1 mm, respectively. With ToF gain, the scanner utilizing a 25 mm thick LYSO-EJ232 metascintillator exhibited the most promising NECR curve, peaking at 1180 cps at 1600MBq. This work takes a significant step towards harnessing the information gain facilitated by the integration of metascintillator-based detectors in PET imaging.

Proton range verification (PRV) in proton therapy is an unmet clinical need. Prompt-gamma imaging (PGI) using thick collimators is a PRV modality that has obtained the most success to-date. The gamma detectors in such approach consist of scintillation crystals coupled to photodetectors. In this work, we report the development and use of detectors made of monolithic pure Cherenkov emitter crystals for the same purpose. We demonstrate for the first time the ability of such detector configuration to provide spatial resolution information in one direction using measurements from a collimated slit. The detector consisted of a PbF2 crystal with dimensions 25 × 25 × 10 mm3 coupled to a S13361-3050AE-08 array of 8×8 SiPMs from Hamamatsu. The SiPM array was connected to a row-column readout, with 8+8 channels, and triggered on the sum of the columns. Three different event reconstruction algorithms were tested: center of gravity (CoG), rise to the power (RTP), and neural-network (NN). The NN yielded the best spatial resolution, with 3.7±0.9 mm full width half maximum (FWHM) in average for all positions. CoG and RTP also showed a consistent shift with the change of position of the slit, although with more modest results, between 4 mm and 7 mm in average for all positions. This is the first characterization of monolithic pure Cherenkov emitters for Multi-MeV gamma imaging. Results are promising for this detector concept, showing that it can offer an alternative for collimated PGI in PRV with potential of sustaining high count rates, with effective background rejection, and low production costs based on the cost of primary components of the crystals.

Objective.A key challenge in PET systems is collecting large amounts of data with the most accurate information-time, energy, and position-to produce high-resolution images while limiting the number of channels to reduce costs and improve data collection efficiency. The new ultra-high-performance brain (UHB) scanner under development aims to tackle this issue, using a semi-monolithic detector that combines pixelated arrays and monolithic designs, along with signal multiplexing techniques.Approach.We assessed the time, energy, and positioning performance of the multiplexing circuit (summing signals along rows and columns) and compared it to the standard readout, both using TOFPET2 ASIC.Main Results.While time resolution worsens by about 15%, energy and positioning resolution-more crucial in small diameter scanners-are unaffected by signal summation. Overall, a pair of detector modules (2 × 2 arrays each) features an energy resolution of 16.9 ± 1.3% and 405 ± 29 ps coincidence time resolution. Positioning accuracy-estimated using multilayer perceptron neural network-is 1.9 ±0.4 mm and 3.0 ±0.7 mm along the monolithic and depth-of-interaction direction, respectively.Significance.This study demonstrates that this channel reduction readout effectively maintains high performance while allowing for reduced costs and enhanced scalability.

Most preclinical PET scanners are based on pixelated detectors without Depth of Interaction (DOI) capabilities, which is crucial to correct for parallax errors. Semi-monolithic crystals have the potential to combine the timing capabilities of pixelated crystals and the 3D positioning accuracy of monolithic scintillators. In this work, we present a preclinical PET prototype consisting of 2 rings defining an inner diameter of 106 mm and an axial length of 52 mm. Each ring contains 14 arrays of 1 × 22 LYSO slabs of 0.97 mm × 25.6 mm × 12 mm, coupled to 8 × 8 SiPMs arrays. The sensitivity, spatial resolution, count rate performance and image quality were studied using the NEMA NU 4-2008 protocol. All images were reconstructed using the MLEM algorithm with 0.5 mm voxel size, including DOI information and normalization correction. A mean spatial resolution for all measured positions and across the three directions (axial, transaxial and radial) of 1.61 ± 0.19 mm was obtained at the center of the FOV. A peak sensitivity of 3.5% was obtained at the center of the scanner, for an energy window (EW) between 358 keV -664 keV. The noise equivalent count rate peak reached 106.9 kcps for an activity of 11.7 MBq using the mouse-sized phantom, the same EW window and a time coincidence window of 10 ns. For the image quality phantom, contrast recovery coefficients of 0.20, 0.62, 0.75, 0.81 and 0.85 were found for the 1, 2, 3, 4 and 5 mm rods, respectively. Spill-over-ratio values of 0.10 for air-filled and 0.23 for water-filled cylinders were measured. Also, according to the Rayleigh criterion, 1.5, 1.2 and 1 mm hot spots of a micro-Derenzo phantom were well distinguished.

Background: Proton and ion therapy have gained significant importance in radiation therapy cancer treatment due to their favorable dose distribution and tissue-sparing properties. In conventional gamma radiation therapy some methods of in vivo dose verification are possible with current medical devices. Proton and ion therapy dose verification is limited, mainly using PET for particle range. Prompt gamma methods offer low spatial resolution. This study presents initial results for in-vivo dose verification with PET imaging of F-18 during proton therapy. Although the activity concentration of F-18 generated by typical clinical doses (several Gy )is low, PET imaging performed approximately one hour post-irradiation yields sufficient image quality to derive dose-volume histograms (DVH), enabling spatial dose verification. Purpose: To verify the applied dose in proton therapy in vivo using Positron Emission Tomography with millimetric precision. Materials and Methods: We simulated proton treatment in a brain phantom using Gate and RayStation platforms to assess the production of several positron emitting isotopes. We focused on the production of fluorine-18 (F-18), given its low positron energy, which enables accurate reproduction of the dose distribution. To evaluate the detectability of the anticipated low activity concentrations (on the order of a few Bq/mL) following a 3 Gy proton irradiation, we tested three PET systems: two preclinical scanners based on LYSO detectors and one clinical scanner based on BGO crystals. Finally, we have analyzed the dose-volume histograms for simulated and measured dose and activity distributions and compared them with the planned ones. Results: F-18 PET imaging in proton therapy correlates with delivered dose within 5% error and matches the planned dose fall-off edge within 1 mm, enabling accurate and precise in vivo dose verification. Conclusion: The dose verification in proton therapy using F-18 Positron Emission Tomography allows higher precision of dose than other positron emitters like C-11, N-12 or O-15. ### Competing Interest Statement The method described in the manuscript of dose verification using β+ isotopes in proton and He-4 radiation therapy is patent pending (EP25382365). Marcin Balcerzyk, Marta Freire and Antonio Gonzalez are inventors in this patent. ### Funding Statement The publication was funded in part by Fundacion Vital Fundazioa. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors

Abstract Background The renewed interest in BGO scintillators for TOF-PET is driven by the improved Cherenkov photon detection with new blue-sensitive SiPMs. However, the slower scintillation light from BGO causes significant time walk with leading edge discrimination (LED), which degrades the coincidence time resolution (CTR). To address this, a time walk correction (TWC) can be done by using the rise time measured with a second threshold. Deep learning, particularly convolutional neural networks (CNNs), can also enhance CTR by training with digitized waveforms. It remains to be explored how timing estimation methods utilizing one (LED), two (TWC), or multiple (CNN) waveform data points compare in CTR performance of BGO scintillators. Results In this work, we compare classical experimental timing estimation methods (LED, TWC) with a CNN-based method using the signals from BGO crystals read out by NUV-HD-MT SiPMs and high-frequency electronics. For $${2 \times 2 \times 3}\,\hbox {mm}^{3}$$ 2 × 2 × 3 mm 3 crystals, implementing TWC results in a CTR of 129 ± 2 ps FWHM, while employing the CNN yields 115 ± 2 ps FWHM, marking improvements of 18 % and 26 %, respectively, relative to the standard LED estimator. For $${2 \times 2 \times 20}\,\hbox {mm}^{3}$$ 2 × 2 × 20 mm 3 crystals, both methods yield similar CTR (around 240 ps FWHM), offering a $$\sim$$ ∼ 15 % gain over LED. The CNN, however, exhibits better tail suppression in the coincidence time distribution. Conclusions The higher complexity of waveform digitization needed for CNNs could potentially be mitigated by adopting a simpler two-threshold approach, which appears to currently capture most of the essential information for improving CTR in longer BGO crystals. Other innovative deep learning models and training strategies may nonetheless contribute further in a near future to harnessing increasingly discernible timing features in TOF-PET detector signals.

Metascintillators are scintillator topologies whose principle is based on sharing the energy of an impinging gamma ray between their composing materials. These can be a dense crystal such as LYSO or BGO to retain a high gamma absorption probability, and a fast-emitting compound such as BC-422, EJ232 or BaF 2 contributing to its light production kinetics. In this work, we look into the details of metascintillator pulse formation as modelled by a double bi-exponential shape. We analyze the extent of energy sharing through analysis, simulation and experiment in a coincidence timing resolution (CTR) measurement setup, using 3×3×15 mm 3 metascintillators, compared against a LYSO:Ce:Ca 3×3×5 mm 3 reference detector. Features of individual pulses allow us to choose the photoelectric interactions and provide insight into the extent of energy sharing for each gamma interaction. We evaluate the quality of energy sharing surrogates for different metascintillator designs. Different populations of photoelectric interactions are defined based on the extent of energy sharing. These populations have different production rates of fast photons in the first picoseconds and hence significantly different timing capabilities. We benchmark this selection by using the features to apply a timewalk correction. For a 3:1 volume ratio BGO:EJ232 metascintillator, improvement rises up to ∼25% for the whole photopeak, from 280.1 ps to 204.7 ps, while the 10% events with higher production in the fast emitter show a ∼50% improvement, from 106 ps to 54.7 ps. These results are compared with other tested metascintillators. This shows that while metascintillators with comparable light yield components still provide the best alternative, it is possible through simple pulse analysis to measure and isolate the photoelectric interactions in every metascintillator with two components.

This study focuses on advancing metascintillators to break the 100 ps barrier and approach the 10 ps target. We exploit nanophotonic features, specifically the Purcell effect, to shape and enhance the scintillation properties of the first-generation metascintillator. We demonstrate that a faster emission is achievable along with a more efficient conversion efficiency. This results in a coincidence time resolution improved by a factor of 1.6, crucial for TOF-PET applications.

Positron EmissionTomography (PET) constitutes the molecular imaging technique of choice for the diagnosis, treatment follow-up, and understanding of several diseases, including many neurological conditions. Focusing on its application in neurology, dedicated PET systems aim to increase both clinical and physical sensitivity.This abstract presents the geometrical design and preliminary results of the novel 4D-PET, a high-sensitivity organ-specific PET system dedicated to studying the human brain. The 4D-PET system will allow accurate imaging for brain studies due to its expected high sensitivity, high 3D spatial resolution by implementing a Neural Network architecture for the 3D photon impact position estimation- and boosted signal-to-noise ratio (SNR) -enabled by including precise photon time of flight (TOF) information-. We report the preliminary performance results obtained at super-module level (experimental validation) and system level (simulated data), demonstrating the superior performance of the 4D-PET. The experimental results yielded a spatial resolution (FWHM) in the pixelated axis of approximately ½ the slab thickness ~0.8 mm. Regarding the monolithic and DOI axis, the Neural Network prediction reported average FWHM values of 2.1±1.0 mm and 3.4±1.8 mm, respectively. The timing measurements reported an average CTR value of 359±7 ps. Regarding the simulations results, a sensitivity of >15% at the center of the field of view was obtained. Reconstructed phantom images show good image quality and spatial resolution. Similarly. the calculated Contrast Recovery Coefficient (CRC) values improved when using TOF information compared to the non-TOF case.These results demonstrate that, the unique 4D-PET configuration allows for a simultaneous 3D-impact positioning of the gamma-ray and its arrival time estimation thus, making it possible to visualize small critical structures of the brain for neurophysiology and neuropsychiatry applications.

This article summarizes the evolution of dedicated prostate PET instrumentation. It starts by introducing prostate cancer, as well as the most common diagnostic and staging methods that are used in the clinics. Then, it describes the key aspects of PET detectors and their assembly in full PET scanners highlighting the most suitable geometries for prostate examination, and a review on the existing prostate dedicated PET. Finally, the next steps for extending the use of PET in the daily diagnose, staging, and image-guided biopsy of patients with prostate cancer are discussed.

Positron Emission Tomography (PET) constitutes the molecular imaging modality of choice for the study and diagnosis of different medical conditions. Current research focuses on increasing the sensitivity of PET systems to provide faster imaging or/and a reduction of the dose administered to the patient. With this objective, different approaches have been proposed such as constructing larger axial coverage PET systems and/or enabling accurate time-of-flight (TOF)-PET scanners. Implementing large coverage scanners is a major investment (high cost) challenge. Moreover, enabling accurate TOF capabilities requires fast and low-noise electronics. To mitigate these limitations, BGO-based systems may be an option since BGO scintillators are cheaper than Lubased scintillators, and their emission spectrum contains promptly-emitted photons (Cherenkov light, ($\sim 20$ above BGO’s absorption band and $\sim 9$ above LSO’s absorption band)) which can be exploited to boost TOF performance. However, detecting these Cherenkov photons is challenging and requires the implementation of specific readout able to discriminate the events based on their dynamics, at a reasonable cost. In this work we show our first steps towards designing and implementing a scalable readout for large coverage BGO-PET. Our readout is based on a balun transformer to split the photodetector signals combined with a two-channel method for event discrimination.

Total-Body Positron Emission Tomography (TB-PET) technology and designs have become very popular in the recent years. These systems are very attractive because of their high sensitivity resulting from their extended axial Field of View (FOV) and potential Time of Flight (TOF) capabilities, allowing for the simultaneous study of the kinetics of multiple organs. Most of TB-PET designs and implementations are based on LYSO crystal pixels without Depth of Interaction (DOI) capabilities. In this work we present a TB-PET system, named IMAS, based on semi-monolithic crystals to simultaneously enable TOF and DOI capabilities. Our design makes use of a reduction of signals without compromising performance. The system geometry is based on 5 rings of 10 cm in the axial direction each, and gaps of about 5 cm, resulting in a total axial length of 71.4 cm. The system has been constructed and installed (June 2023) at the largest hospital in Valencia named La Fe. Very preliminary experimental tests already predict an almost homogeneous spatial resolution below 4 mm in the whole FOV (as far as at 30 cm off-radial), outperforming any other scanner with a long axial FOV. The system sensitivity is $7.6 \%$ with a source at the Center of the FOV (CFOV). The detectors reached a TOF of about 350 ps FWHM. We aim to report a full characterization of the scanner during the conference.