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.

OBJECTIVE:The goal of this work is to evaluate the performance of a preclinical Positron Emission Tomography (PET) system, named ScintoTube, which was constructed using a continuous (edgeless) LYSO:Ce scintillator. The PET compatibility tests with high-field Magnetic Resonance Imaging (MRI) scanners are also reported.
Approach: We constructed a preclinical PET system based on a single, continuous-annular LYSO:Ce scintillator, with inner and outer diameters of 64 mm and 80 mm, respectively, and an axial coverage of 96 mm. The system has 24 virtual detectors, being each one composed of a matrix of 9×9 Silicon Photomultipliers (SiPMs). A novel trigger topology was implemented to retrieve the entire Light Distribution (LD) profiles. This information was used to provide the 3D photon impact coordinates which included the Depth Of Interaction (DOI) information.
To evaluate the ScintoTube performance, the NEMA NU 4-2008 protocol was followed. Dead Time and quantification corrections were implemented in the reconstruction process. Moreover, since the system is intended to be used as an insert for high-field MRI scanners, it was evaluated when working under the influence of 7T and 9.4T MRI scanners by acquiring data with different MRI sequences. 
Results: This work successfully implements the edgeless concept for small animal PET insert and demonstrates that it is possible to retrieve homogeneous 3D photon impact positioning accuracy (in the 1 mm range) across the axial Field Of View (FOV) by using an edgeless design. The reported results also validate the capabilities of the PET insert to work under the influence of high-field MRI. There is almost no influence of the MR in our PET insert regarding spatial and energy resolutions and photopeak position. All PET parameter deviations are within the ±5% range when compared to non-MRI case.
Significance: Overall, the obtained results show that the designed ScintoTube is a promising system to be used as an insert with unique capabilities such as fully characterizing the entire LD profiles (3D positioning), suppressing edge effects in the transaxial and axial directions, and improving sensitivity.

Combining Positron Emission Tomography (PET) scanners with Magnetic Resonance (MR) and/or Focused Ultrasound (FUS) equipment has emerged as a leading approach in different medical areas. Such trimodal systems are not commercially available. Aiming to provide simultaneous PET-FUS-MRI imaging, we have designed, assembled, and validated a small-animal preclinical PET insert based on monolithic LYSO crystals of 33 25.4 8 mm 3 . The scanner has outer and inner diameters of 113 mm and 72 mm, respectively, and is compatible with both high-field MRI and commercial FUS systems. This work details the initial performance evaluation of the PET system, following the NEMA NU 4 2008 standards. We have found a spatial resolution of 0.98 ± 0.06 mm at ¼ of the axial Field of View (FOV), and a sensitivity at center of the scanner of 3.1% (30% energy window). The NECR peak is 92 kcps for an activity of 22200 kBq. Regarding image performance, the Recovery Coefficients (RCs) values measured for the Image Quality phantom, were 0.17, 0.38, 0.90, 1.08 and 1.04 for rods with diameters ranging from 1 mm to 5 mm, and Spill-Over-Ratios of 22.9% in water and 15.1% in air. In addition to this, we present for the first time, a simultaneous PET-MRI-FUS study. Using a costum FUS device, we heated a gelatin-filled phantom containing FDG in a specific area. Simultaneously with the heating process, different MR sequences were running with a low-field MRI, while PET data was acquired. As the gelatin melted, the FDG was distributed within the entire phantom volume. This dispersion was observed in the reconstructed PET-MRI images, thus validating our trimodal system.

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.

Semi-monolithic crystals have the potential of combining the timing capabilities of pixelated crystals and the 3D positioning accuracy of monolithic crystals. We present here a preclinical PET scanner consisting of 2 rings with an inner diameter of 106 mm, and an axial length of 52 mm. Each ring consists of 14 semi-monolithic arrays of 22 slabs of $25.6 \times 12 \times 0.97 \mathrm{~mm}^{3}$ each, coupled to $8 \times 8$ SiPMs arrays. A timestamp averaging method, including time skew and time walk corrections, was applied, achieving a detector time resolution as good as 178 ps FWHM. A neural network technique based on a multilayer perceptron architecture has been implemented for the prediction of the annihilation photon impact position inside each detector module along the monolithic direction. The depth of interaction has been determined using the Energy/$I_{\max}$ estimator and calibrated using a Voronoi method. A full NEMA protocol evaluation has been carried out. A spatial resolution of $0.96 \pm 0.07 \mathrm{~mm}$ was achieved at ¼ of the axial Field of View. The sensitivity at the center of the scanner was 3.1% with an energy window of 30%. A NECR value of 10 kcps at a $280 \ \mu \mathrm{Ci}$ was achieved. The Recovery Coefficients (RCs) and Spill-Over-Ratio (SOR) values were obtained for the image quality phantom. RCs of 0.27, 0.71, 0.90, 0.98 and 0.99 were achieved for the 1, 2, 3, 4 and 5 mm rods, respectively. The SOR values were 0.18 for air and 0.27 for water.

Positron Emission Tomography (PET) stands out as a highly specific molecular imaging technique. However, its detection sensitivity remains a challenge. The implementation of Time-of-Flight (TOF) PET technology enhances sensitivity by precisely measuring the time lapse between the annihilation photons. Moreover, by characterizing scattered (Compton) events, the effective sensitivity of PET imaging might significantly be enhanced. In this work, we present the scatter sub-system of a 2 layers preclinical TOF-PET scanner for mice head imaging. The scatter sub-system is composed of 8 identical modules based on analog SiPMs coupled to crystal arrays of 24 × 24 LYSO pixels with 0.95 mm × 0.95 mm × 3 mm dimensions. The system has 29 mm bore and 50.8 mm axial length. An average CTR of 192± 1 ps was obtained for the whole sub-system at the photopeak energy range after energy and timing corrections, and CTR values as good as 155 ps were found for some individual pixels. The transit time spread at the SiPM level was also studied and corrected, achieving a mean value of 41 ps of maximum time difference at the sensor corners with respect to the center. Voronoi diagrams were implemented to correct for position decoding.

Positron Emission Tomography (PET) is a non-invasive imaging modality used to provide information on the internal function of the human or animal body. Recent tendency has been to increase the axial Field of View (FOV) coverage of scanners as a means to increase signal-to-noise ratio (SNR). This has led to the development of total body (TB) PET systems. Further increase in SNR can also be achieved by including time-of-flight (TOF) capabilities. Moreover, retrieving photon depth of interaction (DOI) information allows one to reduce parallax error and obtain a homogeneous FOV spatial resolution.Despite being a challenge, constructing detectors able to simultaneously provide TOF and DOI information results in an increased manufacturing cost due to the large volumes of scintillators and photosensors but also, to the huge number of channels to be digitized. This fact makes TB-PET devices inaccessible for most clinical facilities. To alleviate this problem, the IMAS project focuses on building cost-effective TB-PET technology.In this work, we show the performance of the detectors implemented for the IMAS TB-PET, which is currently being installed at the hospital La Fe in Valencia (Spain). The read-out electronics comprises a highly multiplexed circuitry able to reduce the number of channels by a factor of 4 while maintaining temporal performance in the range of 350 ps, as well as DOI capabilities using thick LYSO scintillator crystals in a semi-monolithic geometry.

Breast cancer causes the greatest number of cancer-related deaths among women. Screening programs using X-ray mammography have made possible to increase survival rates, but this technique presents some limitations. Positron Emission Tomography (PET) has demonstrated to be a potential way to overcome these drawbacks, offering more accurate diagnosis at initial staging and the possibility of monitoring the therapy treatment. However, further improvements in PET technology are still required. The proposed PET system, named DeepBreast, has been specifically designed to efficiently scan the breast in prone position. The system is based on a novel edgeless detector ring composed of curved monolithic scintillators glued together using a high-refractive index compound. This design minimizes the physical gaps between modules boosting the system sensitivity. It also improves the image quality since edge effects are reduced. For the x- and y-photon impact estimation, a Neural Network technique has been successfully implemented. The depth of interaction and energy were calibrated using a method based on Voronoi diagrams. The DeepBreast has been fully constructed and calibrated. Experimental data are being acquired following a modified version of the NEMA protocol for this dedicated system. A spatial resolution of 1.8, 2.0 and 1.5 mm FWHM was achieved at the center of the field of view (cFOV) along the radial, tangential and axial directions, respectively. These values remain almost constant over the entire FOV. A sensitivity of 2.3% was measured at the cFOV. Preliminary reconstructed images of phantoms show the ability of the DeepBreast system to provide high-quality images.

Total-Body Positron Emission Tomography (TB-PET) technology and designs have become very popular in recent years. The advantages of these systems are many, pinpointing the high sensitivity achieved by their long axial FOV and eventually TOF, and the capabilities to simultaneously study the kinetics of multiple organs. Most of TB-PET designs and implementations are based on LYSO crystal pixels without DOI. In this work we present a TB-PET system based on semi-monolithic crystals and, therefore, simultaneously enabling TOF and depth of interaction capabilities. Furthermore, the design named IMAS, makes use of a reduction of signals without compromising performance. We have completed exhaustive simulation studies of the final system geometry, based on 5 rings of 10 cm in the axial direction each, and gaps of about 5 cm, with a total axial length of 71.4 cm. These studies confirm the good performance of the system in terms of spatial resolution, sensitivity and other relevant parameters. Moreover, these days, the full system is being installed at the largest hospital in our region. Very preliminary experimental tests at the lab with the first ring, already predict a spatial resolution of 2.9 mm at the cFOV. We aim to report a full characterization of the scanner during the conference.

This work presents a conceptual design for a Total Body (TB) PET scanner. Although, we show some experimental data obtained with a preliminary detector block configuration, the main focus in this contribution is about system results based on Monte Carlo simulations. The detector blocks are based on an array of semi-monolithic LYSO crystals (slabs). Such slab configuration enhances the Coincidence Time Resolution (CTR) in comparison with monolithic blocks and, moreover, allows one to estimate the impact Depth of Interaction (DOI).In this work, spatial resolution, sensitivity and NECR values were simulated according to the NEMA NU 2 2018 protocol, and compared with other scanners. The parameters used during the simulations are based on experimental tests. In particular, we have experimentally achieved a CTR of 280 ps FWHM and DOI accuracy of 3 mm. To illustrate the system performance, a Derenzo-like phantom has been simulated and reconstructed using an MLEM algorithm. The smallest rods, with 4.8 mm diameter, can be resolved for voxel dimensions of 2 mm. Although comparable results with other commercially available scanners (15 – 25 cm axial length) are found, this system exhibits the unique combination of high TOF and DOI capabilities.

Conventional Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) scanners are usually built using multiple detectors placed in a cylindrical geometry leading to both transaxial and axial gaps between detectors. These undesired gaps decrease system’s sensitivity and degrade spatial resolution towards the edges of the detector. To reduce gaps, we propose to construct a dedicated-breast PET made out of 14 curved LYSO crystals glued together using a high refractive coupling media (n=1.7). The aims of the current work are: i) to reduce edge effects when using glued crystals, and ii) to show the feasibility of the proposed edge-less PET system.We have glued the lateral sides of two cuboid LYSO crystals (n=1.81) of 33.0×25.4×10.0 mm 3 using Meltmount (n=1.7)and acquired data moving a 22 Na pencil beam (~450 µm slit) along the x- and y-axes. Results were compared with the standard case in which the lateral sides are black painted and keep an air gap (0.4 mm) in between the two blocks. The photon interaction coordinates were estimated using a Neural Network (NN) and, the detector performance was evaluated using the mean average error (MAE) parameter. Additionally, we show the design details of a system based on 14 LYSO curved crystals coupled to 12×12 SiPM arrays mounted on a custom flexible board containing also the readout electronics. We have experimentally acquired data and we also simulated (GATE/Geant4 platform) a back-to-back source at the center of the Field of View (cFOV). Data was reconstructed using MLEM with 4 iterations and a voxel size of 0.5 mm.Results show that gluing scintillation crystals with a high refractive index compound and using NN for impact position determination reduces edge effects, achieving spatial resolution of 0.7 mm from the simulated back-to-back source at the cFOV.

Instrumentation research in small animal Positron Emission Tomography (PET) imaging is driven by improving timing, spatial resolution and sensitivity. Conventional PET scanners are built of multiple detectors placed in a cylindrical geometry with gaps between them in both the transaxial and axial planes. These gaps decrease sensitivity and degrade spatial resolution towards the edges of the system field of view (FOV). To mitigate these problems, we have designed and validated an edgeless pre-clinical PET system based on a single LYSO annulus with an inner diameter of 62 mm and 10 outer facets of 26 × 52 mm2 each. The scintillation light is read out using the row and columns of Silicon Photomultipliers (SiPMs) mounted in magnetic-field compatible PCBs. The objective of this work is to provide a calibration method for this system. The particular design of the annulus produces some undesirable effects in the light distributions (LD) at the module joints, which needs to be addressed. Nevertheless, after calibration, the system allows one to properly retrieve both, the energy and 3D photon impact positions.

During the last years, a technological revolution in both the clinical and pre-clinical PET fields has been produced. Including Time of Flight (TOF) information on Positron Emission Tomography (PET) improves the signal to noise ratio of reconstructed images and consequently the effective sensitivity. In this work we present the inner ring of a dedicated Compton-PET system for mice brain imaging that requires ultra-high spatial and timing resolutions. The main goal of this work is to evaluate the timing and spatial capabilities of the detector block that will be used in the scatter ring of this system.Detector blocks based on pixelated LYSO:Ca crystals of 0.95×0.95 mm 2 and 3 mm thickness, combined with SiPM arrays of 3×3 mm 2 elements are evaluated in terms of energy, spatial and timing resolution. The frontend electronics used for the experiments was the TOFPET2 ASIC from PETsys. Moreover, a pixel identification was carried out based on Voronoi diagrams. A SiPM gain correction was also applied. We reached 130 ps Detector Time Resolution (DTR) considering 9 pixels matching 1 SiPM, and an improved 112 ps at the single crystal pixel level.

Many efforts have been devoted over the years to enhance the capabilities of PET detectors and to develop novel and efficient scanner concepts. Indeed, following several technological advances, the standards in the performance of these detectors have now been greatly expanded. However, in several cases compromises have to be made. In this work, we present our approach to develop an ultra-high spatial and timing resolution PET system, making use of the Compton-PET concept for mouse brain imaging. In particular, we will describe the detector elements of the scatter layer of such PET configuration. For this purpose, scintillators with just 3 mm thickness are tested when coupled to SiPM arrays of 3 mm × 3 mm. The aim, is to explore the optimal detector configuration for sub-100 ps resolution scanners. We make use of detectors based on crystal arrays of 0.95 mm × 0.95 mm pixel (12×12 elements) of the type LYSO(Ca) coupled to SiPMs arrays with 3 mm × 3 mm active area. As it will be discussed, we have been able to reach a detector timing resolution of 100 ps FWHM in some cases, while we were also capable to resolve all crystal pixels of 0.95 mm size.

Abstract Background Prostate cancer (PCa) represents one of the most common types of cancers facing the male population. Nowadays, to confirm PCa, systematic or multiparametric MRI-targeted transrectal or transperineal biopsies of the prostate are required. However, due to the lack of an accurate imaging technique capable to precisely locate cancerous cells in the prostate, ultrasound biopsies sample random parts of the prostate and, therefore, it is possible to miss regions where those cancerous cells are present. In spite of the improvement with multiparametric MRI, the low reproducibility of its reading undermines the specificity of the method. Recent development of prostate-specific radiotracers has grown the interest on using positron emission tomography (PET) scanners for this purpose, but technological improvements are still required (current scanners have resolutions in the range of 4–5 mm). Results The main goal of this work is to improve state-of-the-art PCa imaging and diagnosis. We have focused our efforts on the design of a novel prostate-dedicated PET scanner, named ProsPET. This system has small scanner dimensions defined by a ring of just 41 cm inner diameter. In this work, we report the design, implementation, and evaluation (both through simulations and real data) of the ProsPET scanner. We have been able to achieve < 2 mm resolution in reconstructed images and high sensitivity. In addition, we have included a comparison with the Philips Gemini-TF scanner, which is used for routine imaging of PCa patients. The ProsPET exhibits better contrast, especially for rod sizes as small as 4.5 mm in diameter. Finally, we also show the first reconstructed image of a PCa patient acquired with the ProsPET. Conclusions We have designed and built a prostate specific PET system, with a small footprint and improved spatial resolution when compared to conventional whole-body PET scanners. The gamma ray impact within each detector block includes accurate DOI determination, correcting for the parallax error. The potential role of combined organ-dedicated prostate-specific membrane antigen (PSMA) PET and ultrasound devices, as a prebiopsy diagnostic tool, could be used to guide sampling of the most aggressive sites in the prostate.

3123 Objectives: Multimodal PET/MR imaging is a powerful molecular imaging tool widely used in research and clinical practice for a broad variety of applications. Small animal PET inserts are an instrumentation tool that can be used in combination with already installed MRI systems. This work shows the initial performance and MRI compatibility of a small animal PET insert for mouse applications, based on monolithic LYSO scintillation crystals. Methods: The PET system is based on LYSO cuboids of 25x33x10 mm, laterally black painted and include a retroreflector layer at the entrance face. This layer improves the detectability of the gamma ray position interaction. The crystals are coupled to custom made 10x8 SiPM arrays with 3x3 mm active area each. 24 crystals are distributed in 3 rings defining an axial and transaxial FOV of 100 mm and 40 mm, respectively. The SiPM photosensor and readout electronics are mounted on high performance boards that avoid the generation of eddy current by the switching gradient fields.This work shows the results carried out within a Bruker BioSpec 94/20 MRI imaging instrument using a 35 mm volume RF coil. The PET insert includes an RF shielding based on carbon fiber structures. We report here the average performance of the detector blocks, and some of the most important parameters regarding PET imagingand MR compatibility. Results: After calibration of each detector block using a novel approach based on Voronoi diagrams [1], the average energy resolution of the whole system is about 13.5%, together with an impact precision determination in 2D below 1.5 mm, and about 2 mm depth of interaction (DOI) resolution.Regarding MR compatibility, the PET insert has been run under several MRI sequences including EPI with high duty cycles. No significant change in MRI performance was observed when compared to the case without the PET. The PET detector performance is not affected by interferences that could arise from different MRI imaging sequences, concerning impact determination, energy or any other parameter [2,3]. Therefore, the reconstructed images did not exhibit any degradation.The initial performance evaluation resulted in a peak sensitivity of nearly 12% at CFOV, and a NECR mouse peak of 481 kcps at 25.7 MBq . The PSF-corrected spatial resolution has been determined to be about 0.7 mm FWHM for all three space components. Using DOI allows to reach similar spatial resolution performance for the entire FOV. The good spatial resolution of the device allows to distinguish 0.8mm rods of a micro-Derenzo phantom when using 0.25 mm voxels, 1 mm virtual pixels and 25 iterations (MLEM). Conclusions: This works provides design principles needed for the realization of small PET/MR inserts for mice. The initial study has shown that the requirements for simultaneous PET/MR mouse imaging applications can be reached, thus high spatial PET imaging resolution and the absence of any interference effects for PET and MRI.The PET insert reaches an unprecedented image performance resolving well 0.8 mm Derenzo rods while simultaneously imaging MRI with EPI sequences. The high spatial resolution is obtained across the entire PET FOV, thus helping researchers for a more reliable quantification during their investigations. Bibliography: [1] M. Freire, IEEE TRPMS, 2019, doi: 0.1109/TRPMS.2019.2947716. [2] A.J. Gonzalez, IEEE TRPMS 3, 343, 2019. [3] W. Gsell, to be submitted to EJNMMI, 2020.

In this manuscript, detector blocks for Positron-Emission-Tomography (PET) are being evaluated for their integration in a TOF-PET system. In this contribution, detector concepts based on analog SiPM photosensors have been tested along with different types of scintillation materials. Firstly, pixelated crystals have been evaluated showing accurate results in terms of spatial, energy and timing resolution. Then, monolithic blocks were coupled to large SiPM photosensors arrays to determine their capabilities to resolve the gamma impacts in terms of position and timing. The read-out and the digitation of all signals were performed by a commercially available Application-Specific-Integrated-Circuit (ASIC) named TOFPET2. The obtained results as well as some methodologies that imrove the timing performance of the monolithic blocks, are being described in this paper.

Hybrid molecular and anatomical imaging devices, especially when simultaneously working, have shown to provide advantages over sequential acquisitions. In particular, we present in this preliminary study, the working performance of a brain positron emission tomography (PET) insert, within a 3T magnetic resonance imaging (MRI) system. To our knowledge, this is the largest PET system based on monolithic LYSO blocks. It consists of 60 scintillator blocks of $50\ \text{mm} \times 50\ \text{mm} \times 20\ \text{mm}$ arranged in 3 rings of 20 detector modules each. An effective field of view (FOV) of 240 mm in diameter and 154 mm axially is defined. The crystals included a retroreflector layer at the entrance face, and are coupled to custom arrays of $12 \times 12$ silicon photomultipliers (SiPM), $3\ \text{mm} \times 3\ \text{mm}$ each. Frontend electronics provide ${X}$ and ${Y}$ projections of the scintillation light by sampling each row and column of the SiPM arrays. The insert is thermally stabilized by using temperature-controlled air to about 27 °C. The PET insert has been installed at the Klinikum rechts der Isar (Munich) and tested within the whole-body Siemens biograph mMR, a 3T MRI combined with a PET scanner. A system sensitivity of almost 7% for an energy window of 350–650 keV was measured using a small size source at the center of the FOV (CFOV). Current system evaluation showed a spatial resolution at the CFOV of 1.7 mm using iterative algorithms, being below 2 mm within a centered diameter of 120 mm. Rods of a small Derenzo phantom of 2.5 mm were clearly resolved, independently of all tested MRI sequences including echo-planar imaging, ultrashort time echo, MPrage, and T2-flair or arterial spin labeling. The PET insert did not show any count rate degradation also under those sequences for a variety of MR imaging acquisitions.