In this contribution, large SiPM arrays (8 x 8 elements of 6 x 6 mm(2) each) are processed with an ASIC-based readout and coupled to a monolithic LYSO crystal to explore their potential use for TOF-PET applications. The aim of this work is to study the integration of this technology in the development of clinical PET systems reaching sub-300 ps coincidence resolving time (CRT). The SiPM and readout electronics have been evaluated first, using a small size 1.6 mm (6 mm height) crystal array (32 x 32 elements). All pixels were well resolved and they exhibited an energy resolution of about 20% (using Time-over-Threshold methods) for the 511 keV photons. Several parameters have been scanned to achieve the optimum readout system performance, obtaining a CRT as good as 330 +/- 5 ps FWHM. When using a black-painted monolithic block, the spatial resolution was measured to be on average 2.6 +/- 0.5 mm, without correcting for the source size. Energy resolution appears to be slightly above 20%. CRT measurements with the monolithic crystal detector were also carried out. Preliminary results as well as calibration methods specifically designed to improve timing performance, are being analyzed in the present manuscript. (C) 2017 Elsevier B.V. All rights reserved.

Organ dedicated PET devices provide improved imaging performance when compared to whole body systems. The present study summarizes the test carried out to study a new detector block designed for an organ dedicated PET system. This block includes three novel components namely the scintillator geometry and a retroreflector layer coupled to the entrance face, the photosensor and the readout electronics. We used arrays of 12 x 12 SiPM photosensors with 3 x 3 mm(2) active area each and a pitch of 4.2 mm. We are proposing a new readout electronics that permits to reduce the 12 row and columns signals to only 8 without significant detector performance degradation. This approach also allows for resolving radioactive sources in the whole volume of the proposed crystal, significantly reducing the edge effect that typically rejects these events. An overall spatial resolution of about 1.8 mm FWHM is obtained for the whole scintillation volume, with an average energy resolution of 13% FWHM and a photon depth of interaction resolution (FWHM) of 3.7 mm. (C) 2018 Elsevier B.V. All rights reserved.

A direct ray tracing algorithm has been proposed under the development of a dedicated brain PET insert scanner, called MindView. The presented method takes advantage of the quasi- continuum nature of the impact coordinates provided by detectors composed by of monolithic scintillation crystals (LYSO material). This method requires a two-step filtering process: first, a Butterworth filter, multiplied by a ramp filter, is applied on the reconstructed image to wipe out the low oversampled frequencies after performing a backprojection, and second, an adaptive median filter is applied to the image for the noise to be reduced. Our algorithm, has been evaluated with simulation and real data, and compared with some of the gold-standard algorithms, in terms of image spatial resolution, such as the Filtered Backprojection 3D Reprojected (FBP3DRP) or the List Mode Ordered Subset (LMOS). Data obtained with a Derenzo-like phantom was used showing the 1.6 mm rods and a mean peak-to-Svalley ratio of 2.4 with the proposed algorithm, while in the FBP and LMOS depicted 1.3 and 1.4, respectively.

We report on benchmark tests of a 3 TW/50 fs, table-top laser system specifically developed for proton acceleration with an intrinsic pump rate up to 100 Hz. In two series of single-shot measurements differing in pulse energy and contrast the successful operation of the diode pumped laser is demonstrated. Protons have been accelerated up to 1.6 MeV in interactions of laser pulses focused on aluminium and mylar foils between 0.8 and 25 μm thickness. Their spectral distributions and maximum energies are consistent with former experiments under similar conditions. These results show the suitability of our system and provide a reference for studies of laser targets at high repetition rate and possible applications.

Clinical and organ-dedicated PET systems typically require a high efficiency imposing the use of thick scintillators, normally through crystal arrays. To provide depth of interaction (DOI) information, two or more layers are sometimes mounted in the staggered or phoswich approach. In this paper, we are proposing an alternative using thick and large monolithic crystals. We have tested two surface treatments for a 50 mm x 50 mm x 20 mm LYSO block. We provide data in this paper as close as 5 mm to the lateral walls. We left those walls black painted and the exit face coupled to the photosensor (12 x 12 SiPM array) polished. The entrance face was: 1) black painted or 2) coupled to a retroreflector (RR) layer. These configurations keep a good DOI linearity and, on average, reached 4 mm DOI resolution, measured as the full width at half of the maximum. Approaches using RR layers return constant and good energy resolutions nearing 12%, compared to a range of 15%-16% in the case of totally black painted blocks. The best result concerning the detector spatial resolution was obtained when one of the smallest RR was used (120 um corner cube size), being 1.7 mm at the entrance crystal layer and 0.7 mm in the layer closest to the photosensor. These values worsen at least 30% for the black treatment case.

In PET systems with a geometry different from a closed ring, the Time of Flight (TOF) of the annihilation photons appears to be relevant. Such a system capability helps to compensate the missing angular information that generates artifacts on the reconstructed images. In this work, two ASIC designs are evaluated for their use in PET systems having accurate coincidence resolving times (CRT) while preserving both energy and spatial resolutions. One of the ASIC options is the TOFPET1 from PETsys and the other the so-called PETIROC2 from Weeroc. Detection blocks based on scintillation LYSO crystal arrays have been used to first optimize the experiments. Parameters such as spatial, energy and time resolution have been obtained. Promising CRT results were found nearing 350 ps FWHM for a pair of photosensor channels (PETsys) and roughly 800 ps for all 16 against 16 channels (Weeroc). In terms of spatial resolution, 2 mm pixels size are well resolved with energy values approaching 15% have preliminary been measured for both ASICs. With the TOFPET1 ASIC, also tests with monolithic crystals were carried out. Those showed the system to provide depth of interaction (DOI) information. A DOI resolution (FWHM) of about 3 mm was preliminary estimated in a 25×25×10 mm 3 LYSO scintillator. A calibration process for time alignment of the signal paths was carried out to provide accurate CRT results. The time walk error between channels was also compensated. Several methods for the timestamp averaging have been tried, resulting on preliminary CRT values in the 1 ns FWHM range.

The Multimodal Imaging of Neurological Disorders (MINDView) project aims to develop a dedicated brain Positron Emission Tomography (PET) scanner with sufficient resolution and sensitivity to visualize neurotransmitter pathways and their disruptions in mental disorders for diagnosis and follow-up treatment. The PET system should be compact and fully compatible with a Magnetic Resonance Imaging (MRI) device in order to allow its operation as a PET brain insert in a hybrid imaging setup with most MRI scanners. The proposed design will enable the currently-installed MRI base to be easily upgraded to PET/MRI systems.The current design for the PET insert consists of a 3-ring configuration with 20 modules per ring and an axial field of view of similar to 15 cm and a geometrical aperture of similar to 33 cm in diameter. When coupled to the new head Radio Frequency (RF) coil, the inner usable diameter of the complete PET-RF coil insert is reduced to 26 cm. Two scintillator configurations have been tested, namely a 3-layer staggered array of LYSO with 1.5 mm pixel size, with 35 x 35 elements (6 mm thickness each) and a black-painted monolithic LYSO block also covering about 50 x 50 mm(2) active area with 20 mm thickness.Laboratory test results associated with the current MINDView PET module concept are presented in terms of key parameters' optimization, such as spatial and energy resolution, sensitivity and Depth of Interaction (DOI) capability. It was possible to resolve all pixel elements from the three scintillator layers with energy resolutions as good as 10%. The monolithic scintillator showed average detector resolutions varying from 3.5 mm in the entrance layer to better than 1.5 mm near the photosensor, with average energy resolutions of about 17%. (C) 2016 Elsevier B.V. All rights reserved.

A new small animal PET based on SiPM and monolithic LYSO crystals has been developed. Eight detector modules form the PET ring, each mounting an array of 12 × 12 SiPMs coupled to a readout providing the summed signals of the pixels on each of the 12 rows and 12 columns of the SiPM array. This design makes it possible to accurately determine the centroid of the scintillation light distribution with about 1.6 mm full width at half maximum (FWHM) resolution without correction for the 1 mm source size, and the photon depth of interaction (DOI) with nearly 2 mm FWHM. This single ring PET system has a homogeneous spatial resolution across the entire 80 mm transaxial field of view (FOV) of about 1 mm FWHM. The noise equivalent count rate (NECR) peak is estimated to occur at around 39.2 MBq with a rate of approximately 82.7 kcps for the mouse-like phantom and 22 kcps at 48.1 MBq for the rat-like phantom. Following the NEMA protocol, the peak absolute sensitivity in the center of the FOV is 2.8% for a 30% peak energy window. A pilot test injecting NaF to a mouse of 20 grams is also presented. Finally, the PET ring has been tested in front of a high field 15.2 T Magnetic Resonance (MR). No significant variation on energy and spatial resolution across the FOV has been observed due to the presence of the magnetic field.

Magnetic Resonance Imaging (MRI) is a widely used technique to obtain images in different applications based on the nuclear magnetic renonance (NMR) phenomenon. Gradient coils are the responsible components for encoding the volume of interest (VOI). Linearity, inductance and resistance are taken in account to perform the gradient coil design. In this work, EM and thermal gradient coil properties are studied and two cooling system are presented to cool them. Finally, the gradient coils are tested in a biplanar permanent magnet system and a 2D phantom image is obtained.

2641 Objectives Design of a high performance PET subsystem using continuous crystals which can be combined with a MRI system or other imaging modalities. This poses significant design constraints to achieve sensitivity and count rate targets like a large axial FOV, a built-in cooling and fast electronics. Methods A black painted continuous crystal and a SensL 12×12 SiPMs array are enclosed in a detector housing with integrated air cooling. 8 modules are combined to form a detector ring with a 117 mm inner diameter. Three axial arranged rings achieve a 148 mm FOV. Nickel-free PCBs read out the 12 rows and 12 columns of each module. For sampling the signals are transferred to a 2×32 A/D custom board including on-board FPGA processing for 3D position decoding. Novel approaches are used to provide accurate photon impact position with 1 mm FWHM spatial resolution in the XY plane and 1 mm sigma DOI (app. 2 mm FWHM). Several systems have been already mounted and tested in different conditions, such as alone and in front of high field MR (15.2 T). Results The FWHM of the PSF of a Na source shows uniform volumetric values of about 0.9-1.0 mm3 (MLEM reconstruction) across the entire image FOV (148×80 mm: axial×transaxial). The Derenzo-like phantom has also been tested showing the system capabilities to resolve rods with sizes ranging from 2.4 to 0.75 mm. The NEMA NU4-2008 protocol has been applied to evaluate the performance of the new SiPM based PET system in sensitivity and NECR. A peak maximum sensitivity of 9% was determined at the center of the FOV and with a 256-767 keV energy window. Using new DAQ electronics and computer processing capabilities of this SiPM prototype, the NECR peak is determined to occur at a rate of approximately 576 kcps and around 986 uCi with the mouse-like phantom. About 330 kcps at 1275 uCi is observed when using the rat-like phantom. Conclusions The new design for this small animal PET featuring the emerging SiPM based detectors significantly outperforms its PSPMT based predecessor in all tested performance parameters. This PET system exhibits two main improvements that make it especially suitable for small-animal preclinical studies. On one hand, a highly accurate DOI determination of photon impacts within the continuous crystals allowed us to correct for the parallax error even very close to the FOV edge. Thus, a nearly uniform spatial resolution is observed across the entire FOV, avoiding resolution dependent partial volume effects and thus providing more consistent quantification for small structures.

A whole-body PET device is sometimes not suitable for brain studies because the achieved image resolution is typically not sufficient to investigate small size structures. Thus, a dedicated brain PET insert system with high performance would overcome such limitations. Moreover, these functional studies lack of anatomical information. It is shown elsewhere the convenience of simultaneously acquisition of PET and MR data. In this work we show the final design and first pilot evaluation tests of a novel brain PET insert. Each detector block is based on a monolithic scintillation crystal, an array of SiPMs and a readout allowing characterizing the scintillation light distribution in the X and Y detector axes. The scintillators have a parallelepiped geometry with dimensions of 50×50×20 mm 3 . Their lateral walls are black painted and with the entrance face coupled to a retroreflector optical layer. We have determined an average (XYZ) detector spatial resolution through the FWHM of 1.2 mm (whole scintillator volume). The DOI resolution was measured with lateral incidence experiments and found to be about 3.5 mm, also on average for all photons depth of interactions and crystal positions. Thanks to the retroreflector, the energy resolution improves when compared to a case with all surfaces black painted, resulting on an average value of 13%. The tomographic reconstruction of the data was evaluated using different algorithms, including analytical (FBP STIR-3D), iterative (MLEM and List Mode OS) and a novel method that provides images by directly tracing the measured LORs. The minimum pixel/voxel sizes that were tried are 0.8/0.4 mm, 1.0/0.5 mm and 0.16/0.16 mm, respectively. All methods made it possible to show the PET system capabilities to resolve 1.6 mm rods in a Derenzo-like phantom filled with 150 uCi and scanned for 20 minutes. Pilot tests of the PET insert inside a clinical 3T MR showed a good system performance for most of the sequences typically used for brain imaging.

Achieved spatial resolution of the PET systems is often limited by the parallax error due to the lack of information about the Depth of Interaction (DoI) inside the crystal of the incoming 511 keV annihilation photons. The smaller the diameter of the PET ring and the thicker the scintillator are, the more this error affects imaging performance. In this work, a DoI calculator suitable for monolithic scintillation crystals and based on the shape of the scintillation light distribution at the photodetector surface has been proposed. To test the estimator performance, a test PET module with a 50 x 50 x 20 mm monolithic LYSO crystal coupled to a 12 x 12 SiPM array has been employed. In addition, for calibration and validation of the method, Geant4 simulations have been also used. The key result of the application of the proposed DoI estimator is obtaining a continuous DoI estimation with an average DoI resolution of about 5 mm in the 20 mm-thick crystal. Benefiting from the DoI estimation capabilities of the method, it has been also possible to achieve additional important goals, first of all reducing the parallax error. First, because the scintillation light collection varies as a function of the 3D position of the interaction of the annihilation photon inside the crystal, a method to correct this response variation via a proper 3D look-up-table is proposed. This has led to an improvement of about 35% in energy resolution. Moreover, a DoI-dependent position algorithm has been proposed, allowing an improvement of both planar (X-Y) position linearity and planar spatial resolution. This algorithm is specifically developed for the rows/columns multi-channel readout logic, that reduces the number of independent channels from N x N to N + N, where N is the number of SiPM photodetection elements (12 in our case) in each row and column. This development was performed in the framework of the MindView PET/MRI brain imaging project.

In this work, a direct image reconstruction algorithm for PET is proposed. This method follows naturally from the concept of LOR, and easily admits depth of interaction and time of flight information. In addition, this algorithm does not require the computation of sinograms or a system matrix. Thus, it is not restricted in the formation of high pixel density images and allows the image formation in real-time (simultaneously to the data acquisition process). One of the key advantages of this algorithm is the preservation of high frequencies, producing accurate images that can be generated in high resolution, because of the lack of space restrictions imposed by the intermediate generation of sinograms, aside of the native support of real-time image reconstruction. In this work, a C++ implementation of the described algorithm has been developed. Image reconstructions of simulated PET events according to the MindView (a brain dedicated PET insert) scanner geometry and real data measured with the first one ring prototype system have been performed. Results show capability of correct image formation, either with simulated and real data.

In PET detectors based in monolithic scintillators the interaction coordinates of each photon impact, in particular the 511 keV energy, can be determined from the light distribution sampled at the photosensors pixels. However, some of these distributions can be produced by pure dark noise from the SiPM photosensor, multi-interaction events with long travelled distance, pile-up, etc. Early rejection of these events can improve the SNR of the detector block and, as a result, in the also in reconstructed PET image. Moreover, this early rejection can reduce the statistics necessary to reconstruct those images preserving the SNR. To perform the rejection of noisy distributions, a comparison between a theoretical light distribution and the measured one is made by means of the Pearson product-moment correlation coefficient. This coefficient provides a metric, with range [-1, 1], suitable for assessing the similarity between the two distributions and, thus, allowing the rejection of uncorrelated events. The theoretical model of the light distribution has been obtained using a model based on Montecarlo simulations of a detector block formed by a thick LYSO scintillator crystals (50×50×20 mm 3 ). We carried out an experiment based on two detector blocks with the described monolithic LYSO scintillators coupled to a custom photosensor SiPM array. After analyzing the FWHM of small size 22 Na sources, an improvement of the SNR from 2.2 to 4.1 was observed when the filter was applied. In another test with reconstructed images using data belonging to the one ring prototype of the MindView brain PET insert also showed an improvement in the SNR. Moreover, the filter reduced the data used for reconstruction in about 20% but obtaining, as mentioned above, an improved image quality.

We designed a novel PET insert based on monolithic LYSO crystals. From our first evaluation, we can conclude that sub-millimeter detector spatial resolution, combined with accurate photon DOI determination, make it possible to acquire high resolution reconstructed images. This enables us to combine simultaneously high resolution and sensitivity PET with high field preclinical MRI to extract simultaneously complex data capable to provide both anatomical and molecular information and to dynamically follow non-invasively animal models of different pathologies with no compromise in performance of each imaging modality. Each detector head (8 in total) mounts a 50×50×10 mm 3 LYSO block coupled to a high density array of SiPMs. The readout used on each detector returns the X and Y projections of the scintillation light distribution. The PET geometry defines an axial and transaxial FOV of 46 mm (1 ring system) and 80 mm, respectively. The PET is surrounded of carbon fiber to avoid RF field interferences into the PET electronics without generating eddy currents affecting the MRI integrity. Current results show a PET resolution nearing 750 um (using Derenzo-like phantoms) simultaneously working with the MRI. In-vivo pilot tests of the PET insert system at the KU-Leuven (Belgium) inside a 7 T Bruker MRI equipped with a BGA 20S-HP gradient coil have shown the capability of both PET and MRI to simultaneously work providing high resolution PET and MRI images.

Background: The interaction of highly intense laser pulses with solid targets covers a wide range of phenomena over several orders of magnitude in laser intensity. Time-of-flight measurements provide an accurate reconstruction of the ion energy spectra. We report on the observation of aluminium ions from a laser-plasma interaction by a scintillator-based detector with adjustable dynamic range. Methods: Data have been taken at a 30 fs, 200 TW pulsed Ti:Sapphire laser focused on aluminium foils with 1.8 and 12.5 μm thickness. A time-of-flight detector consisting in a plastic scintillator with fibre-optic coupling to a PMT has been mounted 50 cm behind the target. The PMT output pulses have been recorded on a fast oscilloscope. Results: After an initial peak caused by prompt X-rays and relativistic electrons, a second peak has been observed in the time-of-flight spectra after several microseconds. It corresponds to Al ions with energies of the order 1 keV. Quantitative spectra have been reconstructed. A PIC simulation of the plasma expansion gives an explanation for the acceleration of initially thermal ions to the observed energies. Discussion: At the given ion energies the detection mechanism, based on electrostatic charge transfer, is different to the usual ionisation process in scintillators. Additional tests have been performed to corroborate its working principle. At the nominal, focused laser intensity with femtosecond pulses, proton acceleration to MeV energies would be expected, but is excluded by our observations. This detailed study of the interaction kinematics indicates the formation of a cold plasma by a prepulse several picoseconds ahead of the peak intensity.

In Positron Emission Tomography (PET) detectors based on monolithic scintillators, the photon interaction position needs to be estimated from the light distribution (LD) on the photodetector pixels. Due to the finite size of the scintillator volume, the symmetry of the LD is truncated everywhere except for the crystal center. This effect produces a poor estimation of the interaction positions towards the edges, an especially critical situation when linear algorithms, such as Center of Gravity (CoG), are used. When all the crystal faces are painted black, except the one in contact with the photodetector, the LD can be assumed to behave as the inverse square law, providing a simple theoretical model. Using this LD model, the interaction coordinates can be determined by means of fitting each event to a theoretical distribution. In that sense, the use of neural networks (NNs) has been shown to be an effective alternative to more traditional fitting techniques as nonlinear least squares (LS). The multilayer perceptron is one type of NN which can model non-linear functions well and can be trained to accurately generalize when presented with new data. In this work we have shown the capability of NNs to approximate the LD and provide the interaction coordinates of γ-photons with two different photodetector setups. One experimental setup was based on analog Silicon Photomultipliers (SiPMs) and a charge division diode network, whereas the second setup was based on digital SiPMs (dSiPMs). In both experiments NNs minimized border effects. Average spatial resolutions of 1.9 ±0.2 mm and 1.7 ±0.2 mm for the entire crystal surface were obtained for the analog and dSiPMs approaches, respectively.

A series of small size 22 Na point sources have been simulated using GATE, following the NEMA protocol, and applied to the geometry of a brain dedicated PET so-called MindView. The simulated scanner geometry is currently composed by one ring of 20 detector blocks with a distance of about 330 mm between opposite detectors. Each detector module includes a monolithic LYSO crystal of 50 × 50 × 20 mm 3 and a custom 12 × 12 SiPM array (TSV-type, SensL). The system defines an axial and trans-axial fields of view (FoV) of about 46 mm and 240 mm, respectively. The scintillation light inside the monolithic block was simulated in GEANT4 and is sampled following the readout scheme providing information on each SiPM array row and column (12 + 12). Out of this, three different approaches for the determination of the impact coordinates within the monolithic crystals have been considered. A first approach makes use of the center of gravity (CoG) calculation for the planar XY coordinates but the photon impact depth of interaction (DoI) is not considered. The other methods obtain the XY coordinates using the Raise to Power algorithm (RTP) before CoG calculation, with both discrete (second method) and continuous (third one) DoI information. The data is introduced on the STIR platform for reconstruction using a filtered back projection 3D re-projected algorithm (FBP3DRP). Spatial resolutions ranging in the intervals [2 mm, 3.4 mm], [2.3 mm, 3.3mm] and [2.2 mm, 2.3 mm] have been obtained for the radial, tangential and axial directions, respectively. The points sources were positioned at a radial offset of 100 mm radially out of the center.

The use of monolithic crystals to detect the incident gamma photons for PET imaging has the flexibility to virtually define the detector pixel size. These pixels determine the Lines Of Response (LORs) needed during the reconstruction process. Although there are very few groups using monolithic crystals for PET, the most common approach is to homogeneously define the dimensions for the virtual pixels forming a regular grid on the crystal. In this work we present a dimensional pixel gradient method to improve the detector response model. The typical spatial resolution degradation caused by the border effect, is mitigated here using larger pixel sizes towards the crystal edges. This new approach was implemented for the LMEM algorithm using the Tube Of Response (TOR) backprojector. This backprojector identifies the emission probabilities with an approximation of the intersection volume of the TOR within each voxels it passes through. This operator has been modified to calculate the emission probabilities with variable initial and final sizes of each TOR. This method has been tested in two PET prototypes, one of them uses SiPMs and the other uses PSPMTs as photo-detectors. To evaluate the effect of variable virtual pixel sizes in the crystal, the eccentricity and the spatial resolution (according to NEMA standard acquisitions) on reconstructed images of point sources were determined. These results were compared with the LMEM and the MLEM algorithms using homogeneous pixels. The results show similar volumetric resolutions but improvements in the eccentricity. An improvement in more than 50% in eccentricity for the most offcenter radially displaced sources is achieved when the SiPM based system was used. In case of the detector based on PSPMTs, the eccentricity improvement reaches 17%. Finally, this method has been further validated on in-vivo images namely mice injected with NaF using the SiPM-based system and FDG injected patients acquired with a breast dedicated PET based on PSPMTs.

We have developed a detector block composed by a monolithic LYSO scintillator coupled to a custom made 12×12 SiPMs array. The design is mainly focused to applications such as Positron Emission Tomography. The readout electronics is based on 3 identical and scalable Application Specific Integrated Circuits (ASIC). We have determined the main performance of the detector block namely spatial, energy, and time resolution but also the system capability to determine the photon depth of interaction, for different crystal surface treatments. Intrinsic detector spatial resolution values as good as 1.7mm FWHM and energies of 15% for black painted crystals were measured.