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PURPOSE Detectors for positron emission tomography (PET) typically use two types of scintillation crystals, pixelated or monolithic. A variant of these types of scintillators are the so-called semi-monolithic crystals. They consist of a monolithic crystal segmented in one direction in pieces called slabs. These scintillators have the potential to successfully combine the benefits of pixelated and monolithic configurations, providing good timing and spatial resolutions as well as the capacity to decode the depth of interaction (DOI) information. In this work, the timing performance of a detector based on semi-monolithic crystals was studied in depth. The energy response was also evaluated. METHODS The semi-monolithic detector consists of 1 × 24 LYSO slabs of 25.4 × 12 × 0.95 mm3 each. The bottom surface of the slabs is coupled to an array of 8 × 8 silicon photomultipliers (SiPMs) of 3 × 3 mm2 active area, 50 μm cell size and 3.2 mm pitch. The 64 output signals were independently readout by the TOFPET2 ASIC. In order to achieve the best coincidence time resolution (CTR), four different time walk corrections were tested. Additional work investigated the best method of combining the timestamps belonging to the same event. RESULTS The resolvability of the slabs in the measured flood maps improves with the thickness of a light guide placed in between the scintillators and photosensors. The energy resolution does not change significantly with values as good as 13.7%. Regarding the CTR, values of 335.8, 363, 369.8, and 402.5 ps have been obtained for the whole detector for no light guide, 0.5, 1.0, and 1.5 mm thickness light guide cases, respectively. These values further improve to 276.1, 302.6, 305.6 and 336.2 ps, respectively, when energy-weighted averaging of timestamps is applied. CONCLUSIONS We have shown both an excellent timing resolution and good energy resolution for a PET detector based on semi-monolithic crystals. The use of light guides of different thicknesses does not significantly affect the energy resolution of the whole detector, but the timing capabilities slightly worsen with the increasing thickness of the light guide.

The applicability extent of hadron therapy for tumor treatment is currently limited by the lack of reliable online monitoring techniques. An active topic of investigation is the research of monitoring systems based on the detection of secondary radiation produced during treatment. MACACO, a multi-layer Compton camera based on LaBr3 scintillator crystals and SiPMs, is being developed at IFIC-Valencia for this purpose. This work reports the results obtained from measurements of a 150 MeV proton beam impinging on a PMMA target. A neural network trained on Monte Carlo simulations is used for event selection, increasing the signal to background ratio before image reconstruction. Images of the measured prompt gamma distributions are reconstructed by means of a spectral reconstruction code, through which the 4.439 MeV spectral line is resolved. Images of the emission distribution at this energy are reconstructed, allowing calculation of the distal fall-off and identification of target displacements of 3 mm.

A detector block based on semi-monolithic scintillators consists of a monolithic crystal segmented in one direction in different pieces called slabs. In this configuration, optical photons are preferred to only propagate in the monolithic direction, but not in the segmented direction. Semi-monolithic crystals are intended to overcome the limitations of monolithic and pixelated crystals, offering both accurate time of flight (TOF) and depth of interaction (DOI) information, as well as good spatial resolution. In the work presented here, the characterization of two single LYSO slabs of different dimensions coupled to two different photodetector arrays has been carried out. The dimensions of the slabs are 50×18×3 mm3and 25×18×3 mm3with all faces polished. The faces of the slabs have been laterally wrapped with ESR and a retroreflector film added to the entrance. The employed photodetectors consist of a linear array of 1×16 SiPMs and a matrix of 8×8 SiPMs, in both cases with elements of 3x3 mm2active area each, 50 μm cell size and 3.2 mm pitch. The readout electronics was based on the TOFPET2 ASIC in all cases. The interaction position has been calculated with analytical methods. The preliminary spatial resolution is around 2 mm FWHM both in the long monolithic axis and in the DOI directions. The coincidence time resolution (CTR) of the detectors under study with a reference detector of 3×3×10 mm3ranges from 368 to 295 ps FWHM (50 and 25 mm slabs, respectively). This value improves to 318–260 ps FWHM CTR if energy weighted averaging of the timestamps belonging to the same event is applied.

Fast timing in gamma-ray detectors is pivotal to enable accurate time-of-flight (TOF) in Positron Emission Tomography (PET). While electronics already can reach intrinsic timing capabilities well below 50 ps, and photosensors also provide accurate timing performances, scintillators lag behind. Herein, in order to achieve sufficient sensitivity, dense inorganic crystals such as LYSO or BGO with thicknesses of 10–30 mm are required. The response of such crystals is relatively slow, about 40 and 300 ns, respectively. Alternative to these crystals, cost-effective organic scintillators have very fast responses (2–3 ns rise times) but lack sufficient stopping power for medical applications. In this work, we investigate combining the favorable characteristics of high density but slow crystals such as BGO and LYSO, with low density but very fast plastics. Taking advantage of scintillation mechanics, where gamma rays deposit their energy within just few hundred µm from initial interaction point, we propose a new geometrical arrangement, where thin layers of crystal and plastic are placed in an alternating configuration. Simulation results show that for slab thicknesses of less than 200 µm, the majority of events are initiated in the crystal and shared between crystal and plastic. We have also analyzed light transport along the layers for a variety of different treatments to the scintillator walls for high aspect ratio detector designs. This topology allows one not only timing resolution improvement through production of a big amount of quasi-prompt photons on top of the standard scintillation pulse, but also precise determination of the 3D localization of the gamma interaction point without compromising the stopping power and, therefore, detection sensitivity of the system. Pulses have been analyzed through their quad-exponential model and a theoretical investigation of approaches to harness the superb timing of a variable population of quasi-prompt photons has been made.

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.

The IRIS group at IFIC Valencia is developing a three-layer Compton camera for treatment monitoring in proton therapy. The system is composed of three detector planes, each made of a L a B r 3 monolithic crystal coupled to a SiPM array. Having obtained successful results with the first prototype (MACACO) that demonstrated the feasibility of the proposed technology, a second prototype (MACACO II) with improved performance has been developed, and is the subject of this work. The new system has an enhanced detector energy resolution which translates into a higher spatial resolution of the telescope. The image reconstruction method has also been improved with an accurate model of the sensitivity matrix. The device has been tested with high energy photons at the National Accelerator Centre (CNA, Seville). The tests involved a proton beam of 18 MeV impinging on a graphite target, to produce 4.4 MeV photons. Data were taken at different system positions of the telescope with the first detector at 65 and 160 mm from the target, and at different beam intensities. The measurements allowed successful reconstruction of the photon emission distribution at two target positions separated by 5 mm in different telescope configurations. This result was obtained both with data recorded in the first and second telescope planes (two interaction events) and, for the first time in beam experiments, with data recorded in the three planes (three interaction events).

The IRIS group of IFIC-Valencia is developing a Compton telescope for treatment monitoring in hadron therapy. The system is composed of three detector layers, each of them consisting of a monolithic LaBr 3 scintillator crystal coupled to Silicon Photomultiplier (SiPM) arrays. A first version of the prototype was developed within the European project ENVISION. Tests with point-like sources and in-beam were carried out with successful results, demonstrating the feasibility of the proposed technology. The limitations of the system were also identified. A new version of the prototype is under development with the aim of improving the performance of the telescope detector planes. To this end, the initial tests of a detector plane based on a new SiPM array have been carried out. The first characterization, coincidence and imaging tests with this new version of the detector plane are presented. In addition, the suitability of CeBr 3 scintillator crystals for this application is being investigated.

Two of the limitation factors of current PET scanners are their low sensitivity and lack of high image contrast. In order to improve these parameters, Compton interactions, which are usually discarded or unavoidably averaged, must be distinguished and included in the image reconstruction process. To achieve this, one of our proposals is a multi-layer detector approach employing thin scintillation layers with photodetectors coupled to all four lateral sides of the crystal. In the work presented here, a characterization of detectors based on LYSO and CeBr 3 crystals with lateral sides readout has been performed. Both crystals have an active volume of 51×51×3 mm3, resulting in a very high aspect ratio of 17. The CeBr 3 crystal also includes a 5 mm thick quartz glass frame due to encapsulation constraints. The obtained results show an energy resolution of 12.1% and 9.1% FWHM for the LYSO and CeBr 3 detectors, respectively. Pilot spatial resolutions ranging from 2.7 mm up to 4.3 mm for the LYSO crystal and from 4.1 mm up to 11.2 mm for the CeBr 3 crystal were observed. These results suggest that the quartz glass frame surrounding the CeBr 3 crystal significantly affects the light distribution of the optical photons reaching the photodetectors, worsening the performance of this kind of encapsulated detector.

Compton Cameras are electronically collimated photon imagers suitable for sub-MeV to few MeV gamma-ray detection. Such features are desirable to enable in vivo range verification in hadron therapy, through the detection of secondary Prompt Gammas. A major concern with this technique is the poor image quality obtained when the incoming gamma-ray energy is unknown. Compton Cameras with more than two detector planes (multi-layer Compton Cameras) have been proposed as a solution, given that these devices incorporate more signal sequences of interactions to the conventional two interaction events. In particular, three interaction events convey more spectral information as they allow inferring directly the incident gamma-ray energy. A three-layer Compton Telescope based on continuous Lanthanum (III) Bromide crystals coupled to Silicon Photomultipliers is being developed at the IRIS group of IFIC-Valencia. In a previous work we proposed a spectral reconstruction algorithm for two interaction events based on an analytical model for the formation of the signal. To fully exploit the capabilities of our prototype, we present here an extension of the model for three interaction events. Analytical expressions of the sensitivity and the System Matrix are derived and validated against Monte Carlo simulations. Implemented in a List Mode Maximum Likelihood Expectation Maximization algorithm, the proposed model allows us to obtain four-dimensional (energy and position) images by using exclusively three interaction events. We are able to recover the correct spectrum and spatial distribution of gamma-ray sources when ideal data are employed. However, the uncertainties associated to experimental measurements result in a degradation when real data from complex structures are employed. Incorrect estimation of the incident gamma-ray interaction positions, and missing deposited energy associated with escaping secondaries, have been identified as the causes of such degradation by means of a detailed Monte Carlo study. As expected, our current experimental resolution and efficiency to three interaction events prevents us from correctly recovering complex structures of radioactive sources. However, given the better spectral information conveyed by three interaction events, we expect an improvement of the image quality of conventional Compton imaging when including such events. In this regard, future development includes the incorporation of the model assessed in this work to the two interaction events model in order to allow using simultaneously two and three interaction events in the image reconstruction.

Sensitivity and spatial resolution are the main parameters to maximize in the performance of a PET scanner. For this purpose, detectors consisting of a combination of continuous crystals optically coupled to segmented photodetectors have been employed. With the use of continuous crystals the sensitivity is increased with respect to the pixelated crystals. In addition, spatial resolution is no longer limited to the crystal size. The main drawback is the difficulty in determining the interaction position. In this work, we present the characterization of the performance of a full ring based on cuboid continuous crystals coupled to SiPMs. To this end, we have employed the simulations developed in a previous work for our experimental detector head. Sensitivity could be further enhanced by using tapered crystals. This enhancement is obtained by increasing the solid angle coverage, reducing the wedge-shaped gaps between contiguous detectors. The performance of the scanners based on both crystal geometries was characterized following NEMA NU 4-2008 standardized protocol in order to compare them. An average sensitivity gain over the entire axial field of view of 13.63% has been obtained with tapered geometry while similar performance of the spatial resolution has been proven with both scanners. The activity at which NECR and true peak occur is smaller and the peak value is greater for tapered crystals than for cuboid crystals. Moreover, a higher degree of homogeneity was obtained in the sensitivity map due to the tighter packing of the crystals, which reduces the gaps and results in a better recovery of homogeneous regions than for the cuboid configuration. Some of the results obtained, such as spatial resolution, depend on the interaction position estimation and may vary if other method is employed.

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.

A three-layer Compton Telescope, capable of measuring two and three-interaction events, is being developed at IRIS group (Image Reconstruction, Instrumentation and Simulations for medical imaging applications) of IFIC-Valencia. It is based on continuous LaBr 3 crystals coupled to SiPMs; consequently, a large number of projections can be obtained (high granularity). In order to achieve an accurate reconstruction process, precise estimations of the System Matrix and the Sensitivity Matrix are needed. Given that Monte-Carlo calculations of the aforementioned quantities are computationally expensive due to the high granularity of the imaging device, an analytical model would be advisable. The group has already proposed and assessed a model for two-interaction events. In the present work we propose a similar model for three-interaction events. Whereas three-interaction events allow to obtain the energy of incident gamma-rays without full-absorption requirement, efficiency loss and richer diversity of signal/background events must be handled. In this work we present a preliminary validation of the proposed model, by comparing its performance and predictions with Monte-Carlo simulations. Images of simulated point-like monochromatic sources, as well as a preliminary background study, are also reported.

The suitability of monolithic Lutetium Fine Silicate (LFS) continuous scintillation crystals coupled to 64-channel SiPM-TSV matrices for small animal PET has been evaluated. Three crystals of size 25.8 × 25 . 8 × 15 mm 3 were tested: LYSO white coating, LFS white coating (LFS-W) and LFS specular coating (LFS-S). Electronically collimated measurements were carried out to compare the performance of LYSO and LFS crystals. Mean energy resolutions of 14%, 17% and 19% were measured for LYSO, LFS-S and LFS-W respectively. Similar intrinsic spatial resolutions were found for all three crystals in the central region. Different versions of the light model used to estimate the interaction position of the photons in the detector have been studied. Further measurements with two detector heads of each crystal type in coincidence have also been carried out with a moveable 22Na source. A ML-MLEM image reconstruction code is used to reconstruct the data. Final position resolutions for all three configurations show similar values. Taking into account that LYSO and LFS present similar performance levels (and considering that LFS has better timing resolution than LYSO) we conclude that LFS is a valid substitute for LYSO.

The IRIS group of IFIC-Valencia has developed and fully characterized a first version of MACACO (Medical Applications CompAct COmpton camera), a three-layer Compton telescope based on LaBr 3 crystals and SiPMs for treatment monitoring in hadron therapy. Results obtained in laboratory and beam tests have demonstrated the feasibility of the proposed technology. The limitations of this first version of MACACO have been identified, being energy resolution one of the most critical parameters in the system performance. For this reason, an improved version of the detectors employing a new SiPM array has been assembled. Characterization tests have been performed. Also, coincidence tests have been carried out with two new detectors operated in time coincidence. Point-like sources and an array of 37 point-like sources have been imaged. Tests with the new detectors show a general improvement of the device performance.

Compton imaging devices have been proposed and studied for a wide range of applications. We have developed a Compton camera prototype which can be operated with two or three detector layers based on monolithic lanthanum bromide ([Formula: see text]) crystals coupled to silicon photomultipliers (SiPMs), to be used for proton range verification in hadron therapy. In this work, we present the results obtained with our prototype in laboratory tests with radioactive sources and in simulation studies. Images of a [Formula: see text]Na and an [Formula: see text]Y radioactive sources have been successfully reconstructed. The full width half maximum of the reconstructed images is below 4 mm for a [Formula: see text]Na source at a distance of 5 cm.

A first prototype of a Compton camera based on continuous scintillator crystals coupled to silicon photomultiplier (SiPM) arrays has been successfully developed and operated. The prototype is made of two detector planes. The first detector is made of a continuous 16×18×5 mm 3 LaBr 3 crystal coupled to a 16-elements SiPM array. The elements have a size of 3×3 mm 3 in a 4.5×4.05 mm 2 pitch. The second detector, selected by availability, consists of a continuous 16×18×5 mm 3 LYSO crystal coupled to a similar SiPM array. The SPIROC1 ASIC is employed in the readout electronics. Data have been taken with a 22 Na source placed at different positions and images have been reconstructed with the simulated one-pass list-mode (SOPL) algorithm. Detector development for the construction of a second prototype with three detector planes is underway. LaBr 3 crystals of 32×36 mm 2 size and 5/10 mm thickness have been acquired and tested with a PMT. The resolution obtained is 3.5% FWHM at 511 keV. Each crystal will be coupled to four MPPC arrays. Different options are being tested for the prototype readout.

The interest in using continuous monolithic crystals in positron emission tomography (PET) has grown in the last years. Coupled to silicon photomultipliers (SiPMs), the detector can combine high sensitivity and high resolution, the two main factors to be maximized in a positron emission tomograph. In this work, the position determination capability of a detector comprised of a [Formula: see text] mm(3) LYSO crystal coupled to an [Formula: see text]-pixel array of SiPMs is evaluated. The 3D interaction position of γ-rays is estimated using an analytical model of the light distribution including reflections on the facets of the crystal. Monte Carlo simulations have been performed to evaluate different crystal reflectors and geometries. The method has been characterized and applied to different cases. Intrinsic resolution obtained with the position estimation method used in this work, applied to experimental data, achieves sub-millimetre resolution values. Average resolution over the detector surface for 5 mm thick crystal is ∼0.9 mm FWHM and ∼1.2 mm FWHM for 10 mm thick crystal. Depth of interaction resolution is close to 2 mm FWHM in both cases, while the FWTM is ∼5.3 mm for 5 mm thick crystal and ∼9.6 mm for 10 mm thick crystal.

International audience; A detector head composed of a continuous LaBr3 crystal coupled to a silicon photomultiplier array has been mounted and tested, for its use in a Compton telescope for dose monitoring in hadron therapy. The LaBr3 crystal has 16 mm×18 mm×5 mm size, and it is surrounded with reflecting material in five faces. The SiPM array has 16 (4 ×4) elements of 3 mm×3 mm size. The SPIROC1 ASIC has been employed as readout electronics. The detector shows a linear behavior up to 1275 keV. The energy resolution obtained at 511 keV is 7% FWHM, and it varies as one over the square root of the energy up to the energies tested. The variations among the detector channels are within 12%. A preliminary measurement of the timing resolution gives 7 ns FWHM. The spatial resolution obtained with the center of gravity method is 1.2 mm FWHM. The tests performed confirm the correct functioning of the detector.

The characterization of a PET detector head based on continuous LYSO crystals and silicon photomultiplier (SiPM) arrays as photodetectors has been carried out for its use in the development of a small animal PET prototype. The detector heads are composed of a continuous crystal and a SiPM matrix with 64 pixels in a common substrate, fabricated specifically for this project. Three crystals of 12 mm × 12 mm × 5 mm size with different types of painting have been tested: white, black and black on the sides but white on the back of the crystal. The best energy resolution, obtained with the white crystal, is 16% FWHM. The detector response is linear up to 1275 keV. Tests with different position determination algorithms have been carried out with the three crystals. The spatial resolution obtained with the center of gravity algorithm is around 0.9 mm FWHM for the three crystals. As expected, the use of this algorithm results in the displacement of the reconstructed position toward the center of the crystal, more pronounced in the case of the white crystal. A maximum likelihood algorithm has been tested that can reconstruct correctly the interaction position of the photons also in the case of the white crystal.

Given the strong variations in the sensitivity of Compton cameras for the detection of events originating from different points in the field of view (FoV), sensitivity correction is often necessary in Compton image reconstruction. Several approaches for the calculation of the sensitivity matrix have been proposed in the literature. While most of these models are easily implemented and can be useful in many cases, they usually assume high angular coverage over the scattered photon, which is not the case for our prototype. In this work, we have derived an analytical model that allows us to calculate a detailed sensitivity matrix, which has been compared to other sensitivity models in the literature. Specifically, the proposed model describes the probability of measuring a useful event in a two-plane Compton camera, including the most relevant physical processes involved. The model has been used to obtain an expression for the system and sensitivity matrices for iterative image reconstruction. These matrices have been validated taking Monte Carlo simulations as a reference. In order to study the impact of the sensitivity, images reconstructed with our sensitivity model and with other models have been compared. Images have been reconstructed from several simulated sources, including point-like sources and extended distributions of activity, and also from experimental data measured with 22Na sources. Results show that our sensitivity model is the best suited for our prototype. Although other models in the literature perform successfully in many scenarios, they are not applicable in all the geometrical configurations of interest for our system. In general, our model allows to effectively recover the intensity of point-like sources at different positions in the FoV and to reconstruct regions of homogeneous activity with minimal variance. Moreover, it can be employed for all Compton camera configurations, including those with low angular coverage over the scatterer.

Hadron therapy offers the possibility of delivering a large amount of radiation dose to tumors with minimal absorption by the surrounding healthy tissue. In order to fully exploit the advantages of this technique, the use of real-time beam monitoring devices becomes mandatory. Compton imaging devices can be employed to map the distribution of prompt gamma emission during the treatment and thus assess its correct delivery. The Compton telescope prototype developed at IFIC-Valencia for this purpose is made of three layers of LaBr3 crystals coupled to silicon photomultipliers. The system has been tested in a 4.44 MeV gamma field at the 3 MV Tandetron accelerator at HZDR, Dresden. Images of the target with the system in three different positions separated by 10 mm were successfully reconstructed. This indicates the ability of MACACO for imaging the prompt gamma rays emitted at such energies.

A Compton telescope for dose monitoring in hadron therapy is under development at IFIC. The system consists of three layers of LaBr3 crystals coupled to silicon photomultiplier arrays. 22Na sources have been successfully imaged reconstructing the data with an ML-EM code. Calibration and temperature stabilization are necessary for the prototype operation at low coincidence rates. A spatial resolution of 7.8 mm FWHM has been obtained in the first imaging tests., This work was supported in part by the European Commissions 7th Framework Programme through the ENVISION (G. A. num 241851) project and the Marie Curie ITN ENTERVISION (G. A. num 264552), by the Spanish Ministerio de Economia y Competitividad (FPA2010-14891, FIS2011-14585-E, and FPA2014-53599-R), the UVEG (UV-INV-PRECOMP12-80755), and the Generalitat Valenciana (GV/2013/133 and GVISIC/2012/020). Group members are supported by Ramón y Cajal, UVEG Atraccio de Talent, and Generalitat Valenciana contracts.

A three layer Compton telescope for dose monitoring in hadron therapy has been developed at IFIC. It consists of three detector layers and each layer is made of a continuous LaBr3 crystal coupled to four silicon photomultiplier (SiPMs) arrays. A dedicated image reconstruction method has also been developed. The telescope has been characterized and tested with radioactive sources and in-beam. Simulations with GATE have been carried out in order to optimize the detector geometry and response. Ongoing work is mainly focused on performance optimization and on the simultaneous acquisition and processing of two- and three-layer interaction events.

Continuous scintillator crystals can offer several advantages over pixellated crystals. Apart from an increase of the active volume, spatial resolution is not limited to the crystal size and Depth of interaction (DoI) can be estimated from the light distribution. Continuous crystals coupled to finely segmented photodetectors can improve simultaneously spatial resolution and sensitivity, the main two factors to maximize in Positron Emission Tomography (PET). The main drawback is the difficulty in the position estimation within the crystal; thus, accurate position estimation algorithms are needed. However, if the standard cuboid crystals are used in the full ring geometry, there will be large wedge-shaped gaps between detector heads. For maximizing the sensitivity and avoiding data truncation, a scanner based on tapered continuous crystals is considered. In this work, a basic comparison between two scanners of the same dimensions based on tapered and cuboid crystals was performed. The study is focused on the quantification of the increase on sensitivity while first preliminary results were obtained in terms of image quality and spatial resolution, An increase of 14.08% on the average sensitivity was obtained for tapered crystals. Image reconstruction was implemented with Filtered Back-Projection (FBP) method using the ideal position interaction of the annihilation photon within the crystal given by the simulations. An improvement on the recovery of homogeneous regions was observed due to less data truncation using tapered crystal.

A three layer Compton telescope for dose monitoring in hadron therapy is under development at IFIC - Valencia. It will consist of three detector layers and each layer will be made of a continuous LaBr 3 crystal coupled to four Silicon Photomultiplier (SiPM) arrays. A prototype made of two detector layers has already been assembled and tested. The detector layers consist of a 32×36×5 mm3 and 32×36×10 mm3 LaBr 3 crystal, each coupled to four SiPM arrays. The front-end electronics of each detector is based on the VATA64HDR16 application specific integrated circuit (ASIC). The gamma rays coming from a 22Na source have been detected with the two detector layers working in time coincidence, allowing energy and position estimation.

A prototype of a high resolution small animal PET scanner with continuous LYSO crystals coupled to SiPM matrices has been developed. Images of point-like sources in different positions of the FOV have been obtained with a FWHM better than 1 mm. In the near future, a full-ring PET will be developed. Due to the increase in the number of detector heads, a new Data Acquisition (DAQ) system is needed. First tests with the VATA64HDR16 ASIC from Gamma Medica - IDEAS have been carried out. Also, the use of tapered crystals is being considered for reduction of the gaps between detectors and therefore increase the full-ring sensitivity.

Silicon photomultipliers (SiPM) are becoming more and more popular in different fields of physics. The use of SiPM arrays requires a data acquisition (DAQ) system capable of reading multiple channels for each event. A DAQ system for the control and the readout of 64-channel detectors was designed and fabricated. Significant improvements in performance and new functionalities have been done. The performance of our DAQ system has been tested in a real application: a Compton telescope prototype. Tests, experimental results and future work are described.

A Compton telescope prototype for dose monitoring in hadron therapy is under development. It will consist of three detector layers and each layer will be made of a continuous LaBr3 crystal coupled to four Silicon Photomultiplier (SiPM) arrays. As a first test, a small prototype made of only two detectors layers has been assembled. The camera is based on continuous LaBr3 and LYSO crystals coupled to a SiPM arrays for gamma-ray detection and readout. The setup consists of a 16×18×5 mm3 LaBr3 crystal coupled to a SiPM array as the first layer of the telescope, and a 16×18×5 mm3 LYSO crystal coupled to another SiPM array as the second layer. The SPIROCI ASIC is employed in the readout electronics. The first coincidence tests have been successfully carried out, using a 22Na source placed at different positions, and allow the reconstruction of a point-like source, encouraging the development and the improvement of the prototype to a full imaging system.

International audience; A prototype of a high resolution small animal PET scanner with continuous LYSO crystals coupled to SiPMs matrices has been developed. First tomographic images of point-like sources in different positions have been reconstructed, demonstrating the feasibility of the system. In order to improve the scanner performance, work in three different approaches is being performed: improved data analysis for noise reduction, improvement of the detector performance, and simulations to have a better understanding of the detector response and to predict its performance in different conditions. Innovative detector geometries such as the use of tapered crystals are being considered for a further enhancement of the detector sensitivity and reduction of the gaps between the detectors. Also, the placement of the photodetector in the inner part of the crystal is expected to improve the performance. The possible benefits of these innovations are being investigated. Simulations bring a powerful tool to test different geometries and their impact in the detector behavior in an easy way. They also help to understand complex physical processes that take place inside the scintillator crystal or in the crystal-detector interface. A GEANT4 simulation of the prototype, including the optical properties of the detectors, has been carried out. The results have been compared to real data in terms of light distribution in the crystal, number of SiPMs elements illuminated, energy resolution and reconstructed interaction position. Preliminary results show good agreement between simulated and real data. Besides, the simulation is being adapted to new crystal geometries and system configurations to study potential improvements.

The use of Silicon Photomultiplier (SiPM) arrays requires the use of multichannel data acquisition (DAQ) systems. For this reason a dedicated DAQ system has been developed for the read-out of several SiPM-based detector layers. The frontend of a system is based on the 64-channel ASIC V ATA64HDR16 from Gamma Medica - Ideas. As first application, the DAQ system will be employed in the construction of Compton telescope for dose monitoring in hadron therapy. However the designed system is suitable for any other devices that need to treat large number of SiPM channels. Tests are presented with SiPM matrix of 16 (4×4) elements each coupled to LYSO and LaBr3 continuous crystals. The complete characterization of the DAQ system and the obtained results are presented.

Image reconstruction for Compton camera data can be problematic due to the common trade-off between physically realistic models and speed of computation. In this investigation a novel method of system matrix calculation - Simulated One-Pass Listmode (SOPL) - is extended to incorporate Compton camera data. The method reduces the Cone Surface Response for the Compton camera to an ensemble of Siddon-rays and is conducted in two stages. As part of the ENVISION project for monitoring in hadron therapy, a continuous-crystal Lanthanum Bromide Compton camera has been developed and experimental data acquired. Continuous detection geometries are particularly susceptible to variation in both spatial and spectral resolution over the detection volume and so accurate yet flexible models of detection are particularly important. The SOPL-Compton method was applied via the Maximum Likelihood - Expectation Maximization algorithm to experimental data taken using the prototype device. In this investigation, detection modeling using SOPL-Compton in a two interaction Compton camera is validated and the incorporation of a shift-invariant image-space model confirmed as a useful modification to reduce computational expense. Finally experimental data taken using the prototype LaBr3 Compton camera provide confirmation of the SOPL-Compton approach to system modeling. Results indicate a fast, flexible and accurate algorithm that can easily be extended to alternate and novel detection geometries.

Increased sensitivity is one of the major challenges in novel detector development. While pixelated crystals are standard, the use of continuous crystals can provide increased active volume and thus sensitivity. Accurate estimation of the interaction positions of photons in continuous crystals requires a sophisticated algorithm. Additionally, including depth of interaction (Dol) in the reconstruction process can mitigate parallax effects near the edges of field of view, therefore Dol estimation is highly desirable. In this work, the interaction position coordinates in 3D are estimated adapting an already existing analytical model to our system with no additional hardware, training data sets or Monte Carlo simulations. The algorithm is implemented in a two-detector-head system comprised of continuous crystals coupled to SiPM arrays. Acquired data is stored in list-mode where the continuous nature of the interaction position is preserved. For image reconstruction, since the crystal is continuous, pre-computation of the system matrix elements would require discretization of the detected measurements. To avoid data discretization the system matrix elements are calculated on-the-fly. Spatial information is later on retained in the reconstruction algorithm by using a Siddon-like algorithm where the ray end-points correspond to the estimated interaction positions in continuous space, introducing the interaction position estimation uncertainty in the calculation of the system matrix elements. Reconstruction using continuous data is compared to reconstruction using artificially discretized data. Results show that the spatial resolution measured from a reconstructed point source using ML-EM obtained with a continuous crystal is superior to the spatial resolution obtained with a pixelated crystal.

International audience; Sensitivity and spatial resolution are the two main factors to maximize in emission imaging. The improvement of one factor deteriorates the other with pixelated crystals. In this work we combine SiPM matrices with monolithic crystals, using an accurate γ-ray interaction position determination algorithm that provides depth of interaction. Continuous crystals provide higher sensitivity than pixelated crystals, while an accurate interaction position determination does not degrade the spatial resolution. Monte Carlo simulations and experimental data show good agreement both demonstrating sub-millimetre intrinsic spatial resolution. A system consisting in two rotating detectors in coincidence is currently under operation already producing tomographic images.

International audience; A high resolution PET detector head for small animal PET applications has been developed. The detector is composed of a 12 mm x 12 mm source continuous LYSO crystal coupled to a 64-channel monolithic SiPM matrix from FBK-irst. Crystal thicknesses of 5 mm and 10 mm have been tested, both yielding an intrinsic spatial resolution around 0.7 mm FWHM with a position determination algorithm that can also provide depth-of-interaction information. The detectors have been tested in a rotating system that makes it possible to acquire tomographic data and reconstruct images of 22Na sources. An image reconstruction method specifically adapted for continuous crystals has been employed. The Full Width at Half Maximum measured from a point source reconstructed with ML-EM was 0.7 mm with the 5 mm crystal and 0.8 mm with the 10 mm crystal.

OMEGA - Electronique; International audience; A small animal PET prototype with continuous LYSO crystals coupled to silicon photomultipliers has been developed and the first images have been reconstructed. The system consists of two detector heads attached to a rotating stage. Each detector head is composed of a 12mm x 12mm x 5mm crystal coupled to a monolithic silicon photomultiplier matrix. The matrix has 8x8 pixel elements of 1.4mm x 1.5mm size in a 1.5mm x 1.5mm pitch. Measurements have been carried out with two black crystals and two white crystals. Different position determination algorithms in continuous crystals are being tested to determine the interaction position in the detector heads. In addition, tests with pixellated crystal arrays of 64 pixels of 1.5mm x 1.5mm x 10mm size have been carried out in order to compare the imaging performance. Data have been taken with one and two Na-22 sources in different positions. Images have been reconstructed with filtered backprojection (FBP) and statistical iterative maximum likelihood methods.

The silicon photomultiplier (SiPM) is nowadays an attractive technology in various fields of physics including medical imaging applications. The use of detectors with a large sensitive area has created the necessity to handle detectors with many channels. This has led to the development of data acquisition system for the read-out in parallel of four SiPM matrices with 16 pixels each. The matrices are coupled to a scintillator crystal and the front-end of the system is based on the VATA64HDR16 application specific integrated circuit (ASIC) from Gamma Medica - Ideas. The first test and characterization results and the planned future development activities are described.

In hadron therapy particulate radiation is used to treat regions in close proximity to vital organs or tissue. Protons, or Carbon ions, provide enhanced treatment due to the characteristic Bragg peak structure of dose deposition, which constrains maximum dose deposition to a small volume of the patient. However, as there is no transmitted beam novel methods of beam monitoring must be developed to ensure accuracy in dose delivery. The particulate beam activates the target material giving rise to positrons, prompt gamma-rays and neutrons. While positron emission imaging is possible, Compton imaging has some distinct advantages in this role. The broad spectrum of prompt radiation means that incident photon energy is unknown. Unless incident photon energy can be determined, Compton imaging requires that either three or more interactions be measured or that the photon spectrum also be estimated during image reconstruction. However, the extension of the image space to include incident energy is inconvenient as the image space dimension is increased. Here pencil-beam therapy is considered, which allows the inclusion of beam information into the reconstruction algorithm. Coincident hodoscope information constrains the unknown spatial distribution of the emission field to a single dimension leaving one spatial and one spectral dimension. In this investigation hodoscope coincidence imaging using a Compton camera was explored using data simulated with GEANT4 and was shown to be a promising approach to the challenge of monitoring in hadron therapy.

In hadron therapy, a proton or carbon ion beam interacts with the tissue and prompt gamma rays are emitted from the excited nuclei that can be employed for dose monitoring. A Compton telescope makes it possible to locate the origin of these photons, allowing an indirect estimation of the delivered dose. The telescope prototype being developed at IFIC-Valencia within the ENVISION project will consist of three detector layers. Each layer will be composed of a continuous LaBr 3 crystal coupled to four Silicon Photomultiplier (SiPM) arrays. Coincidence tests have been carried out with a first layer of a 16 mm × 18 mm × 5 mm LaBr 3 crystal coupled to a 16 mm × 18 mm SiPM array and a second layer of a 12 mm × 12 mm × 5 mm LYSO crystal coupled to another 16 mm × 18 mm SiPM array. The test system is read out by two SPIROC1 boards working in time coincidence. First data have been taken and the results demonstrate the feasibility of operating the system.

Sensitivity represents one of the major limitations for high resolution systems used in emission imaging for nuclear medicine. Currently, detectors are based on pixelated scintillators to detect ionizing radiation. There is a recent growing interest in continuous scintillators due to their increased sensitivity by eliminating insensitive areas in the detector crystals and reduced cost. To use such crystal an accurate position estimation algorithm is required to determine the location where photons interact, as opposed to pixelated scintillators, where the position is given by the crystal where the interaction took place. Additionally, including the depth where the photon interacted (DoI) inside the scintillator in the reconstruction algorithm, can be used to mitigate parallax effects. In this work we investigate the feasibility of using an existing analytical position estimation method applied to two different detectors designed to be used in two different imaging modalities: Positron Emission Tomography (PET) and Compton imaging. An LYSO crystal coupled to a 8×8 SiPM array, as one of the detector heads for the PET prototype, and a LaBr 3 crystal coupled to a 4×4 SiPM array, as part of a Compton telescope comprised of several stacked detectors, is used for each application. Results show that submillimetric resolution is measured with both detectors in most of the studied positions, near the centre and close to the edges, in Monte Carlo simulations and experimentally. The measured FWHM for the PET detector is 0.6 mm while the FWHM measured for the detector used in the Compton camera is ∼0.7 mm. DoI measurements were taken only in simulations of the PET detector, where submillimetric bias was achieved and ∼1.6 mm FWHM was measured.

The 4DMPET project aims to develop PET detector modules compatible with operations inside a MRI scanner. PET/MRI hybrid imaging will improve the diagnostic capabilities in soft tissues like the human brain, providing functional and morphological information within the same device. The block detector, designed to provide a measurement of the Depth of Interaction and the Time Of Flight, will feature Silicon Photomultipliers (SiPM) coupled to LYSO scintillator slabs. The Monte Carlo simulations presented here are validated by the comparison with experimental data obtained with 4 × 4 and 8 × 8 SiPM matrices coupled to a LYSO scintillator. The proposed block layout is based on a 48 mm × 48 mm × 10 mm LYSO slab, read out by two 16 × 16 identical SiPM matrices on both sides. The preliminary results of the simulation of the proposed detector are presented, showing good performance in terms of energy, spatial and time resolution.