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The small animal fast insert for mRi (SAFIR) positron emission tomography insert was proposed for quantitative dynamic acquisition inside a preclinical 7T magnetic resonance imaging scanner to study kinetics of short-lived tracers. For this purpose, the SAFIR readout should be capable of handling high count rates and achieving excellent timing performance. We evaluated one of the available application specific integrated circuits (ASICs) for SiPM readout, namely SiPM timing chip (STiC) version 3.1. In this paper, we show the performances of the SAFIR PET detector with the STiC ASIC readout. The SAFIR PET detector consists of an $8 \times 8$ array of lutetium yttrium oxyorthosilicate $2.1\,\,{\mathrm {mm}} \times 2.1\,\,{\mathrm {mm}} \times 12$ mm crystals coupled, with optical grease, to an $8 \times 8$ array of SiPMs with a $2.0\,\,{\mathrm {mm}} \times 2.0$ mm photo-sensitive area. Signals from the individual SiPM channels were digitized by the STiC ASIC. Hit’s arrival time and time-over-threshold (TOT) were recorded into time stamps with 50.2-ps least significant bit. We obtained an average energy resolution of 18.5% full width at half maximum (FWHM) at 511-keV photopeak after TOT nonlinearity correction and an average coincidence resolving time resolution of 244-ps FWHM with time walk correction that satisfy our requirements specification on the detector performance.

SAFIR is a high-rate PET insert for a preclinical 7 T MRI scanner, aiming at studying fast biological processes in small animals, using tracer activities up to 500 MBq. The full detector comprises 15360 channels, each composed of a LYSO crystal and a $S$ iPM photo-sensor. A dedicated readout system, operating inside the MRI magnet of 200 mm inner diameter acquires 40 kHz of hits per channel. PETA6 ASICs digitize amplitude and time of the hits. FPGAs read, process and transfer, via optical links, the digital data. We present the readout system in detail, together with first test results.

In this work, the fabrication and characterization of a microdosimeter based on a synthetic single crystal diamond is reported. The microdosimeter is realized by means of both standard photolithography and selective chemical vapor deposition techniques to accurately define its micrometric sensitive volume. Experimental measurements were carried out at the Ruđer Boskovic Institute microbeam facility using different particles such as proton, helium, lithium, carbon and oxygen. Ion beam induced charge (IBIC) technique was performed to characterize the microdosimeter response in terms of its charge collection properties. The experimental data were also analyzed by means of Geant4 Monte Carlo simulations. Diamond based microdosimeter shows a well-defined active volume. Homogeneity of the response was estimated at about 7% and linked to structural defects of the diamond surface as deduced by AFM inspection. The detector response shows a good linear behavior for ions of different kind and energy, indicating that the detector is suitable for measuring a wide range of particles and LET i.e. 100 ÷ 3000 keV/μm. Finally, microdosimetric capabilities of the diamond based microdosimeter were preliminarily tested in low LET radiation fields (i.e. protons beam).

Ion-beam therapy provides a high dose conformity and increased radiobiological effectiveness with respect to conventional radiation-therapy. Strict constraints on the maximum uncertainty on the biological weighted dose and consequently on the biological weighting factor require the determination of the radiation quality, defined as the types and energy spectra of the radiation at a specific point. However the experimental determination of radiation quality, in particular for an internal target, is not simple and the features of ion interactions and treatment delivery require dedicated and optimized detectors. Recently chemical vapor deposition (CVD) diamond detectors have been suggested as ion-beam therapy microdosimeters. Diamond detectors can be manufactured with small cross sections and thin shapes, ideal to cope with the high fluence rate. However the sensitive volume of solid state detectors significantly deviates from conventional microdosimeters, with a diameter that can be up to 1000 times the height. This difference requires a redefinition of the concept of sensitive thickness and a deep study of the secondary to primary radiation, of the wall effects and of the impact of the orientation of the detector with respect to the radiation field. The present work intends to study through Monte Carlo simulations the impact of the detector geometry on the determination of radiation quality quantities, in particular on the relative contribution of primary and secondary radiation. The dependence of microdosimetric quantities such as the unrestricted linear energy L and the lineal energy y are investigated for different detector cross sections, by varying the particle type (carbon ions and protons) and its energy.

Particle therapy is a highly conformal radiotherapy technique which reduces the dose deposited to the surrounding normal tissues. In order to fully exploit its advantages, treatment monitoring is necessary to minimize uncertainties related to the dose delivery. Up to now, the only clinically feasible technique for the monitoring of therapeutic irradiation with particle beams is Positron Emission Tomography (PET). In this work we have compared a Resistive Plate Chamber (RPC)-based PET scanner with a scintillation-crystal-based PET scanner for this application. In general, the main advantages of the RPC-PET system are its excellent timing resolution, low cost, and the possibility of building large area systems. We simulated a partial-ring scanner based on an RPC prototype under construction within the Fondazione per Adroterapia Oncologica (TERA). For comparison with the crystal-based PET scanner we have chosen the geometry of a commercially available PET scanner, the Philips Gemini TF. The coincidence time resolution used in the simulations takes into account the current achievable values as well as expected improvements of both technologies. Several scenarios (including patient data) have been simulated to evaluate the performance of different scanners. Initial results have shown that the low sensitivity of the RPC hampers its application to hadron-beam monitoring, which has an intrinsically low positron yield compared to diagnostic PET. In addition, for in-beam PET there is a further data loss due to the partial ring configuration. In order to improve the performance of the RPC-based scanner, an improved version of the RPC detector (modifying the thickness of the gas and glass layers), providing a larger sensitivity, has been simulated and compared with an axially extended version of the crystal-based device. The improved version of the RPC shows better performance than the prototype, but the extended version of the crystal-based PET outperforms all other options.

Compton Cameras emerged as an alternative for real-time dose monitoring techniques for Particle Therapy (PT), based on the detection of prompt-gammas. As a consequence of the Compton scattering process, the gamma origin point can be restricted onto the surface of a cone (Compton cone). Through image reconstruction techniques, the distribution of the gamma emitters can be estimated, using cone-surfaces backprojections of the Compton cones through the image space, along with more sophisticated statistical methods to improve the image quality. To calculate the Compton cone required for image reconstruction, either two interactions, the last being photoelectric absorption, or three scatter interactions are needed. Because of the high energy of the photons in PT the first option might not be adequate, as the photon is not absorbed in general. However, the second option is less efficient. That is the reason to resort to spectral reconstructions, where the incoming γ energy is considered as a variable in the reconstruction inverse problem. Jointly with prompt gamma, secondary neutrons and scattered photons, not strongly correlated with the dose map, can also reach the imaging detector and produce false events. These events deteriorate the image quality. Also, high intensity beams can produce particle accumulation in the camera, which lead to an increase of random coincidences, meaning events which gather measurements from different incoming particles. The noise scenario is expected to be different if double or triple events are used, and consequently, the reconstructed images can be affected differently by spurious data. The aim of the present work is to study the effect of false events in the reconstructed image, evaluating their impact in the determination of the beam particle ranges. A simulation study that includes misidentified events (neutrons and random coincidences) in the final image of a Compton Telescope for PT monitoring is presented. The complete chain of detection, from the beam particle entering a phantom to the event classification, is simulated using FLUKA. The range determination is later estimated from the reconstructed image obtained from a two and three-event algorithm based on Maximum Likelihood Expectation Maximization. The neutron background and random coincidences due to a therapeutic-like time structure are analyzed for mono-energetic proton beams. The time structure of the beam is included in the simulations, which will affect the rate of particles entering the detector.

The development of novel detection devices and systems such as the AX-positron emission tomography (PET) demonstrator often introduce or increase the measurement of atypical coincidence events such as inter-crystal scattering (ICS). In more standard systems, ICS events often go undetected and the small measured fraction may be ignored. As the measured quantity of such events in the data increases, so too does the importance of considering them during image reconstruction. Generally, treatment of ICS events will attempt to determine which of the possible candidate lines of response (LoRs) correctly determine the annihilation photon trajectory. However, methods of assessment often have low success rates or are computationally demanding. In this investigation alternative approaches are considered. Experimental data was taken using the AX-PET prototype and a NEMA phantom. Three methods of ICS treatment were assessed--each of which considered all possible candidate LoRs during image reconstruction. Maximum likelihood expectation maximization was used in conjunction with both standard (line-like) and novel (V-like in this investigation) detection responses modeled within the system matrix. The investigation assumed that no information other than interaction locations was available to distinguish between candidates, yet the methods assessed all provided means by which such information could be included. In all cases it was shown that the signal to noise ratio is increased using ICS events. However, only one method, which used full modeling of the ICS response in the system matrix--the V-like model--provided enhancement in all figures of merit assessed in this investigation. Finally, the optimal method of ICS incorporation was demonstrated using data from two small animals measured using the AX-PET demonstrator.

Axial PET is a novel PET concept based on axially oriented crystals. The axial orientation of the scintillating elements mitigates the uncertainty in the determination of the depth of interaction and provides a scalable and exible design than can be adapted to di erent imaging scenarios. Two Axial PET modules have been fully assembled and characterized at CERN with both point-like and extended sources. The capability of Axial PET to break the trade-o between sensitivity and spatial resolution was successfully demonstrated.

AX-PET stands for a new geometrical concept for a high resolution and high sensitivity PET scanner, based on an axial arrangement of long scintillating crystals in the tomograph, for a parallax free PET detector. Two identical AX-PET modules – consisting of matrices of LYSO crystals interleaved with WLS strips – have been built. They form the AX-PET Demonstrator, which has been extensively characterized and successfully used for the reconstruction of images of several phantoms. In this paper we report on the current status of the project, with emphasis on the most relevant results achieved both in terms of detector characterization and image reconstruction. We also discuss the recent preliminary results obtained with the digital SiPM from Philips (dSiPM), which are currently being tested as a possible alternative photodetector for AX-PET. With their very good intrinsic time resolution, dSiPM could add Time of Flight capability to the AX-PET concept.

AX-PET is a novel PET detector based on axially oriented crystals and orthogonal wavelength shifter (WLS) strips, both individually read out by silicon photo-multipliers. Its design decouples sensitivity and spatial resolution, by reducing the parallax error due to the layered arrangement of the crystals. Additionally the granularity of AX-PET enhances the capability to track photons within the detector yielding a large fraction of inter-crystal scatter events. These events, if properly processed, can be included in the reconstruction stage further increasing the sensitivity. Its unique features require dedicated Monte-Carlo simulations, enabling the development of the device, interpreting data and allowing the development of reconstruction codes. At the same time the non-conventional design of AX-PET poses several challenges to the simulation and modeling tasks, mostly related to the light transport and distribution within the crystals and WLS strips, as well as the electronics readout. In this work we present a hybrid simulation tool based on an analytical model and a Monte-Carlo based description of the AX-PET demonstrator. It was extensively validated against experimental data, providing excellent agreement.

The AX-PET collaboration has developed a novel concept for high resolution PET imaging to overcome some of the performance limitations of classical PET cameras, in particular the compromise between spatial resolution and sensitivity introduced by the parallax error. The detector consists of an arrangement of long LYSO scintillating crystals axially oriented around the field of view together with arrays of wave length shifter strips orthogonal to the crystals. This matrix allows a precise 3D measurement of the photon interaction point. This is valid both for photoelectric absorption at 511 keV and for Compton scattering down to deposited energies of about 100 keV. Crystals and WLS strips are individually read out using Geiger-mode Avalanche Photo Diodes (G-APDs). The sensitivity of such a detector can be adjusted by changing the number of layers and the resolution is defined by the crystal and strip dimensions. Two AX-PET modules were built and fully characterized in dedicated test set-ups at CERN, with point-like 22 Na sources. Their performance in terms of energy ( R energy ≈ 11.8 % (FWMH) at 511 keV) and spatial resolution was assessed ( σ axial ≈ 0.65 mm ), both individually and for the two modules in coincidence. Test campaigns at ETH Zurich and at the company AAA allowed the tomographic reconstructions of more complex phantoms validating the 3D reconstruction algorithms. The concept of the AX-PET modules will be presented together with some characterization results. We describe a count rate model which allows to optimize the planing of the tomographic scans.

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.

Axial PET is a novel geometrical concept for Positron Emission Tomography (PET), based on layers of long scintillating crystals axially aligned with the bore axis. The axial coordinate is obtained from arrays of wavelength shifting (WLS) plastic strips placed orthogonally to the crystals. This article describes the design, construction and performance evaluation of a demonstrator set-up which consists of two identical detector modules, used in coincidence. Each module comprises 48 LYSO crystals of 100 mm length and 156 WLS strips. Crystals and strips are readout by Geiger-mode Avalanche Photo Diodes (G-APDs). The signals from the two modules are processed by fully analog front-end electronics and recorded in coincidence by a VME-based data acquisition system. Measurements with point-like 22 Na sources, with the modules used both individually and in coincidence mode, allowed for a complete performance evaluation up to the focal plane reconstruction of point sources. The results obtained are in good agreement with expectations and proved the set-up to be ready for the next evaluation phase with PET phantoms filled with radiotracers.

The design and construction of a PET camera module with high sensitivity, full 3-D spatial reconstruction and very good energy resolution is presented. The basic principle consists of an axial arrangement of long scintillation crystals around the Field Of View (FOV), providing a measurement of the transverse coordinates of the interacting 511 keV gamma ray. On top of each layer of crystals, an array of Wave-Length Shifter (WLS) strips, which collect the light leaving the crystals sideways, is positioned orthogonal to the crystal direction. The signals in the WLS strips allow a precise measurement of the z (axial) co-ordinate of the 511 keV γ-ray gamma impact. The construction of two modules used for demonstration of the concept is described. First preliminary results on spatial and energy resolution from one full module will be shown.

We describe a PET device based on a novel method to extract the coordinates of the interaction point of the 511keV γ rays from 100 mm long and thin LYSO (Lutetium Yttrium OxyorthoSilicate) scintillator bars, positioned axially in the tomograph. The coordinate along the hit crystal is measured by using a hodoscope of Wave Length Shifting (WLS) plastic strips mounted perpendicularly to each plane of scintillators. As photodetectors, new Geiger mode Avalanche PhotoDetectors (G-APDs) with integrated electronics are being used to detect both the hit crystal in a block (x and y coordinates) and the interaction point in the crystal (z coordinate) through the light escaping from the crystal and transmitted to the WLS strips. In this way, the γ interaction point can be determined with a spatial resolution of few cubic millimeters down to a minimum deposited energy of about 50 keV, resulting in a volumetric precision very close to the limits imposed by the physics of the positron annihilation. The method allows to increase the detection efficiency without affecting the spatial resolution by adding scintillator planes in the radial direction. A demonstrator scanner, based on two matrices of 8 × 6 LYS crystals and 312 WLS strips, slotted in between the crystals, is under construction. Preliminary results from the feasibility studies of the various components will be presented.

The X-HPD is a modern implementation of the hybrid concept first employed by the Philips SMART and the QUASAR tubes of the Lake Baikal experiment, aiming at improved performance and maximum simplicity. The glass envelope is essentially spherical and a spatial scintillator crystal is mounted in its center. Photoelectrons from the cathode are accelerated by a potential difference of 20–30 kV and deposit their kinetic energy in the scintillator (or Phosphor). The generated scintillation light is detected by a small and low cost photodetector, e.g. a conventional PMT. The spherical symmetry leads to a uniform collection efficiency and very small intrinsic time spread over the full viewing angle of 120 ∘ ( 3 π solid angle). We report about the design, fabrication and test results of a first X-HPD prototype of 208 mm diameter with a conical LYSO crystal anode. Monte Carlo studies with Geant4 led to a qualitative understanding of the light transport in the anode assembly.

The concept of hybrid photodetectors which combine a large area photocathode with a crystal scintillator anode in the centre of a hemispherical vacuum tube was demonstrated first with the Philips SMART tubes and, more recently, with the QUASAR tubes of the Lake Baikal experiment. Photoelectrons from the cathode are accelerated by a potential difference of 20–30 kV and deposit their kinetic energy in the scintillator (or Phosphor). The generated scintillation light is detected by a small and low cost photodetector, e.g. a conventional PMT. The X-HPD study is a variation of this concept aiming at improved performance and maximum simplicity. The glass envelope is essentially spherical and a spatial scintillator crystal is mounted in its centre. The spherical symmetry leads to a uniform collection efficiency and very small intrinsic time spread over the full viewing angle of 120° (3 π solid angle). We report about test results of a the first X-HPD prototypes of 208 mm diameter with cylindrical and conical LYSO crystal anodes.

Positron emission tomography (PET) is currently the only in situ and non invasive beam monitoring in light ions tumor therapy. The optimization of an in-beam PET scanner requires a deep knowledge of the β + -activity distribution and also of the background consisting of heavy fragments exiting the patient. The capability of the Geant4 toolkit to reproduce available experimental data of carbon ion interactions has been studied in the present work.

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.

Hadron-therapy (HT) delivers high dose to the tumor, significantly sparing surrounding healthy tissues. Positron Emission Tomography (PET) can be used to image short-lived positron-emitters, produced by the hadron beam in the patient. PET is nowadays the most assessed imaging technique to monitor HT treatments. The number of positron-emitters produced is much lower than in conventional clinical scenarios, dedicated scanners and acquisition protocols are then required. Sensitivity becomes a crucial parameter since the duration of the acquisition is limited by the beam delivery during treatment (in-beam PET) as well as by the physiological wash-out of the radioisotopes after treatment (in-room PET). AX-PET, a novel PET concept with axially oriented crystals, provides a higher sensitivity almost fully decoupled from its 3D spatial resolution. The flexible design can overcome many of the geometrical constraints imposed by HT monitoring. This work presents a preliminary feasibility study of the application of AX-PET to the HT scenario in in-room modality.

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.

Hadron-therapy (HT) aims to treat tumors by maximizing the dose released to the target and sparing the dose to normal tissues. For a successful outcome it is very important to determine where the maximum dose is deposited; therefore range verification is necessary for treatment optimization and patient safety. Secondary positron emitting isotopes and prompt gamma radiation are produced after the hadron beam passage. This secondary radiation coming from tissue activation could be used for quality control of treatment, provided that it can be detected and employed to reconstruct the beam path using imaging techniques. This is one of the main goals of the ENVISION project: to evaluate and develop on-line monitoring devices for HT, like PET for detecting the annihilation photons from positrons and Compton Cameras (CCs) for prompt gamma radiation detection. In both technologies high sensitivity is required to increase the signal-to-noise ratio of the reconstructed image for a given therapeutic dose. This simulation study focuses on the sensitivity optimization of such devices, PET and CC, taking into account its use and constraints for on-line HT monitoring. Several configurations of both technologies have been investigated using sources generated from hadron beams. In the case of PET data, the time-of-flight (TOF) information has been included too. For individual hadron beams acquired after 5 minutes, differences in range of 3 mm are detected for all the PET configurations, except for the partial-ring of 60 cm diameter. In addition, patient data from a carbon ion treatment at GSI have been simulated and reconstructed. The system with higher sensitivity and angular sampling recovers more accurately areas with no activity (nasal cavity). In the case of the CC data, the quality of the reconstructed image when using 2-interaction events is notably improved when the detector layers are placed covering a larger solid angle.

Real-time monitoring techniques are receiving increased attention due to their great potential for improving the precision of treatment delivery for hadron-therapy (HT). The study of prompt-γ(PG) focuses on the energetic photon radiation exiting the patient few nanoseconds after the beam irradiation. Its high intensity compared to other similar technologies like in-beam Positron Emission Tomography (PET) and good correlation with the original dose deposition profile are promising features. On the other hand, the use of neutral particles to monitor the dose comes together with a high noise background, mainly due to neutrons and scattered γ, which are uncorrelated with the original dose map. The search of a robust detector that fully exploits the information from PG is still a matter of research.

The usage of long, axially oriented scintillator crystals in a PET scanner has been shown by the AX-PET Demonstrator as a possible solution for a high resolution and high sensitivity PET detector. In the AX-PET implementation, arrays of wavelength shifting (WLS) strips, placed orthogonally behind every crystal layer, are used to define the axial coordinate. After extensive characterization measurements, the AX-PET Demonstrator has been successfully used for the reconstruction of several phantoms and a few rodents. Possible extensions of the AX-PET concept towards Time Of Flight capabilities have been investigated, using Philips digital SiPMs as alternative photodetector. Promising CRT values equal to 211 ps have been demonstrated, using long crystals with dual sided readout and mean timing method. Finally, we report about an alternative way to reconstruct the axial coordinate: exploiting the light sharing of 100 mm long crystals with special surface treatment resulted in axial resolutions of the order of 4 mm FWHM.

A Compton telescope for dose delivery monitoring in hadron therapy is under development at IFIC Valencia within the European project ENVISION. The telescope will consist of three detector planes, each one composed of a LaBr3 continuous scintillator crystal coupled to four silicon photomultiplier (SiPM) arrays. After the development of a first prototype which served to assess the principle, a second prototype with larger crystals has been assembled and is being tested. The current version of the prototype consists of two detector layers, each one composed of a 32.5 × 35 mm2 crystal coupled to four SiPM arrays. The VATA64HDR16 ASIC has been employed as front-end electronics. The readout system consists of a custom made data acquisition board. Tests with point-like sources have been carried out in the laboratory, assessing the correct functioning of the device. The system optimization is ongoing.

In the development of prototype systems for positron emission tomography a valid and robust image reconstruction algorithm is required. However, prototypes often employ novel detector and system geometries which may change rapidly under optimization. In addition, developing systems generally produce highly granular, or possibly continuous detection domains which require some level of on-the-fly calculation for retention of measurement precision. In this investigation a new method of on-the-fly system matrix calculation is proposed that provides advantages in application to such list-mode systems in terms of flexibility in system modeling. The new method is easily adaptable to complicated system geometries and available computational resources. Detection uncertainty models are used as random number generators to produce ensembles of possible photon trajectories at image reconstruction time for each datum in the measurement list. However, the result of this approach is that the system matrix elements change at each iteration in a non-repetitive manner. The resulting algorithm is considered the simulation of a one-pass list (SOPL) which is generated and the list traversed during image reconstruction. SOPL alters the system matrix in use at each iteration and so behavior within the maximum likelihood-expectation maximization algorithm was investigated. A two-pixel system and a small two dimensional imaging model are used to illustrate the process and quantify aspects of the algorithm. The two-dimensional imaging system showed that, while incurring a penalty in image resolution, in comparison to a non-random equal-computation counterpart, SOPL provides much enhanced noise properties. In addition, enhancement in system matrix quality is straightforward (by increasing the number of samples in the ensemble) so that the resolution penalty can be recovered when desired while retaining improvement in noise properties. Finally the approach is tested and validated against a standard (highly accurate) system matrix using experimental data from a prototype system--the AX-PET.

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.

In PET, as the spatial resolution of the measurement system is increased, multiple interactions of a single photon may be separately measured and such events are often removed from the data used in image reconstruction. While for some PET imaging tasks this effect is unimportant, for primate, brain and high-spatial-resolution imaging where sensitivity is important, such Inter Crystal Scattering (ICS) events may constitute a large fraction of the measured data. On-the-fly list-mode image reconstruction is generally required in order to use all the information provided by ICS. Simulated One-Pass List-mode image reconstruction is used in this investigation to study the inclusion of ICS information into the reconstructed image using data acquired with the AX-PET prototype. In almost all cases an increase in image quality was observed, particularly for larger features. A small resolution penalty was also observed.

The Axial PET (AX-PET) concept proposes a novel detection geometry for PET, based on layers of long scintillating crystals axially aligned with the bore axis. Arrays of wavelength shifting (WLS) strips are placed orthogonally and underneath the crystal layers; both crystals and strips are individually readout by G-APDs. The axial coordinate is obtained from the WLS signals by means of a Center-of-Gravity method combined with a cluster algorithm. This design allows spatial resolution and sensitivity to be decoupled and thus simultaneously optimized. In this work we present the latest results obtained with the 2-module AX-PET scanner prototype, which consists of 6 radial layers of 8 LYSO crystals each (crystal size: 3 × 3 × 100 mm3). The WLS arrays comprise 26 strips (3-mm wide) per layer. The estimated energy resolution from point-like measurements is 11.8% (FWHM at 511 keV). The intrinsic spatial resolution was measured for the two modules in coincidence at two different configurations using point-like sources, showing very little degradation when the modules were placed oblique to each other. The axial spatial resolution was 1.5 mm (FWHM) in all the studied cases. Tomographic data of extended phantoms filled with fluorine-18 have been acquired. Imaging a larger transaxial Field-of-View (when compared to the previous measurement campaign) was possible thanks to implementing secondary motion of one of the modules. We have also developed various reconstruction approaches which take into account the particular nature of AX-PET data, as well as a count rate model which allowed us to develop an acquisition protocol able to compensate for count losses. The reconstructed phantom images confirm the imaging capabilities of AX-PET, and the recent advancements in the DAQ let us expect significant improvements for future campaigns.

Simulated One Pass List-mode is a means of image-reconstruction that distributes on-the-fly calculations of the full system matrix across multiple iterations during Maximum Likelihood-Expectation Maximization image reconstruction. On the fly list-mode processing of coincidence data provides a very flexible approach to image reconstruction that can be applied to many novel systems for Positron Emission Tomography. However, in addition to standard on-the-fly calculation and list-mode methods, Simulated One Pass List-mode provides for extra possibilities during image reconstruction. Two possibilities are investigated here: system modeling accuracy can be altered at each iteration and enhanced modeling can be conduced only for specific Regions of Interest. In this investigation simulated data from a simple two dimensional PET system is used to demonstrate these two advantages of Simulated One Pass List-mode image reconstruction for Positron Emission Tomography.

Axial-PET (AX-PET) concept aims at significant reduction in parallax error and simultaneous improvement in sensitivity and spatial resolution by employing a novel geometry based on stacks of axially oriented long crystals and orthogonally placed wavelength shifter strips. Current AX-PET demonstrator has two modules. Better sampling and larger field-of-view (FOV) are achieved by rotating one of the modules up to ±60°. By using a rotating source, projections can be taken from all possible angles. In this study, we investigate analytical reconstruction strategies for the list-mode AX-PET data. This process consists of pre-processing steps such as construction of 3D sinograms (histogramming) and dedicated geometrical corrections. In histogramming, we used 1, 0.8750, and 0.4375 mm radial and axial sampling with the angular sampling of 1°. The sinograms were constructed by using span9 axial compression scheme. The constructed 3D sinogram data were corrected in order to have homogeneously sampled sinograms and also for the inter-crystal gaps by using transradial bicubic interpolation method. For image reconstruction, we used 3D reprojection (3DRP) with Colsher's filter (as implemented in Software for Tomographic Image Reconstruction, STIR). The physical phantoms were scanned with the two-module AX-PET demonstrator. The quantitative evaluations were performed by calculating the Full Width at Half Maximum (FWHM) values of the fitted Gaussian curves to the manually drawn profiles across the phantom inserts. The preliminary reconstructed phantoms showed that we are able to resolve the NEMA insert down to 2 mm. Moreover, with the reduced parallax error, we are able to obtain more uniform spatial resolution in the transaxial field-of-view.

This is a preliminary study on the feasibility of using Resistive-Plate-Chambers (RPC) based Positron Emission Tomography (PET) for hadron-therapy monitoring. The imaging capabilities of the RPC gas detector are being investigated for PET. Their main advantages are excellent timing resolution, low cost and Depth Of Interaction information (DOI) due to their layered structure. Hadron-therapy (HT) aims at treating tumors by maximizing the dose released to the target and sparing healthy tissue. During irradiation with a hadron beam, fragments, positron emitting isotopes and gamma radiation are produced. This radiation coming from the tissue activation could be used for quality control of the treatment. Our work focuses on imaging the positron emitting isotopes using an RPC-based PET scanner. The low cost of RPC makes possible to enlarge the angular coverage of the scanner guaranteeing the higher sensitivity needed by the in-beam monitoring. In addition, the TOF capability and spatial resolution makes worth investigating it. A first protoype is currently under construction at CERN in the TERA group. For this study, Monte Carlo simulations by means of GATE were employed. Up on our knowledge, such gas detectors have not been simulated yet in GATE and the new version of GATE offers new tools for dosimetry. These are the main reasons of choosing GATE. A set of linear sources, simulating the delivery of a hadron beam, has proposed to evaluate the image quality for an ideal time resolution of the system and the TOF information is being considered in the reconstruction. For the full ring simulated system, no important differences were found between TOF and non-TOF images, only the contrast was always better for the TOF-images. However, further simulations should be performed adapting the system to the real situation (partial ring) for HT and lowering the detected events to a clinical number. Under these circumstances, we expect to be crucial the TOF capabilities of the RPC-PET.

AX-PET is a novel PET detector based on long axially arranged crystals and orthogonal Wavelength shifter (WLS) strips, both individually readout by Geiger-mode Avalanche Photo Diodes (G-APD). Its design was conceived in order to reduce the parallax error and simultaneously improve spatial resolution and sensitivity. The sensitivity can be further enhanced by adding additional crystal layers as well as by including Inter-Crystal Scatter (ICS) events, identified and processed post-acquisition. Its unique features require dedicated Monte Carlo (MC) simulations and its non-conventional design makes modeling rather challenging. We developed an AX-PET model based on Geant4 and GATE packages. Simulations were extensively validated against experimental data obtained from both small scale laboratory and full module setups. The first simulations aimed at developing an analytical model of the WLS behavior which was afterwards coupled to GATE. Full AX-PET acquisitions were used to test the GATE simulations. The agreement between data and simulations was very good. AX-PET simulations are employed to test and optimize image reconstruction software and, at the same time, train ICS identification and reconstruction algorithms.

Hadron therapy, a subject of study by the ENVISION project, promises to provide enhanced accuracy in the treatment of cancer. The Bragg-peak, characteristic of the hadron-beam structure provides larger dose to the tumor while being able to spare surrounding tissue - even tissues in the beam-path, beyond the tumor-site. However, increased dose gradients require more precise treatment, as small beam misalignment can result in dose to healthy, often delicate, surrounding tissue. The requirement for accuracy necessitates imaging during therapy, yet the lack of a transmitted beam makes this difficult. The particulate beam interacts with the target material producing neutrons, positron emitting isotopes and a broad spectra of gamma radiation. Photons from positron-annihilation allow in-beam PET to provide on-line measurements of dose deposition during therapy. However, ib-PET suffers from low statistics and lost projections due to low sensitivity and detector constraints respectively. Instead, Compton imaging of gamma radiation is proposed to provide on-line monitoring for hadron therapy. Compton imaging suffers similarly from low statistics, especially, as is the case here, when incident energy is unknown. To surmount this problem, a method of Compton image reconstruction is proposed and tested using simulated data, which reconstructs incident energy along with the spatial variation in emission density. Through incident energy estimation, a larger range of measurements are available for image-reconstruction - greatly increasing the sensitivity of the system. It is shown in this preliminary study that, even with few statistics, a reasonable estimate of the beam path is calculable.

Standard Positron Emission Tomography (PET) cameras need to reach a compromise between spatial resolution and sensitivity. To overcome this limitation we developed a novel concept of PET. Our AX-PET demonstrator is made of LYSO crystals aligned along the z coordinate (patient's axis) and WLS strips orthogonally placed with respect to the crystals. This concept offers full 3D localization of the photon interaction inside the camera. Thus the spatial resolution and the sensitivity can be simultaneously improved and the reconstruction of Compton interactions inside the detector is also possible. Moreover, by means of G-APDs for reading out the photons, both from LYSO and WLS, the detector is insensitive to magnetic fields and it is then suitable to be used in a combined PET/MRI apparatus. A complete Monte Carlo simulation and dedicated reconstruction software have been developed. The two final modules, each composed of 48 crystals and 156 WLS strips, have been built and fully characterized in a dedicated test set-up. The results, obtained with a 22 Na point source (0.25 mm diameter), of the single module performances and a first estimation of the performances with the two module system are reported.

The goal of the AX-PET project is to build and test a demonstrator for a high resolution, high sensitivity PET scanner, based on a novel geometrical concept of long axially oriented crystals. The demonstrator comprises two PET modules used in coincidence. The two modules have been constructed and characterized (both individually and in coincidence) in dedicated test setups, with point-like sources. Good performance in terms of energy, spatial and timing resolution have been demonstrated. First measurements with extended phantoms filled with FDG-radiotracers have been recently performed.

AX-PET is a novel PET concept based on long crystals axially arranged and orthogonal Wavelength shifter (WLS) strips, both individually readout by Geiger-mode Avalanche Photo Diodes (G-APD). Its design was conceived in order to reduce the parallax error and simultaneously improve spatial resolution and sensitivity. The assessment of the AX-PET concept and potential was carried out through a set of measurements comprising individual module characterizations and scans in coincidence mode of point-like and extended sources. The estimated energy and spatial resolutions from point-like measurements are R FWHM =11.6% (at 511 keV) and 1.7–1.9 mm (FWHM) respectively as measured with point-like sources placed in different positions of the FOV. First results from scans of extended phantoms confirmed our expectations.

The AX-PET is a new geometrical concept for a high resolution 3D PET scanner, based on matrices of axially oriented LYSO crystals interleaved by stacks of WLS, both individually read out by G-APDs. A PET demonstrator, based on two detector modules used in coincidence, is currently under construction.

The AX-PET demonstrator is based on a new concept in PET detectors, with LYSO crystals aligned along the z coordinate (patient's axis) and WLS (Wave-length shifter) strips placed orthogonal to them. This kind of structure permits to avoid parallax errors due to different depths of interaction of the photons in the crystals, to register the three coordinates of the impinging photon and to reconstruct Compton events. In this way both the spatial resolution and the sensitivity can be highly improved. Moreover, as both the LYSO crystals and the strips are readout via Geiger-mode Avalanche Photo Diodes (G-APDs) the detector is insensitive to magnetic fields and is then suitable to be used in a combined PET/MRI apparatus. A complete Monte Carlo simulation and dedicated reconstruction software, suited to the particular geometry arrangement, have been developed. The two final modules, each composed by 48 crystals and 156 WLS strips have been built and fully characterized in a dedicated test set-up. The results on the performances of the system obtained with a 22Na point source (0.25 mm diameter) are reported.

We describe the concept and first experimental tests of a novel 3D axial Positron Emission Tomography (PET) geometry. It allows for a new way of measuring the interaction point in the detector with very high precision. It is based on a matrix of long Lutetium-Yttrium OxyorthoSilicate (LYSO) crystals oriented in the axial direction, each coupled to one Geiger Mode Avalanche Photodiode (G-APD) array. To derive the axial coordinate, Wave Length Shifter (WLS) strips are mounted orthogonally and interleaved between the crystals. The light from the WLS strips is read by custom-made G-APDs. The weighted mean of the signals in the WLS strips has proven to give very precise axial resolution. The achievable resolution along the three axes is mainly driven by the dimensions of the LYSO crystals and WLS strips. This concept is inherently free of parallax errors. Furthermore, it will allow identification of Compton interactions in the detector and for reconstruction of a fraction of them, which is expected to enhance image quality and sensitivity. We present the results of proof-of-principle tests and qualification measurements of the various components prepared to build a larger scale demonstrator consisting of two matrices of 8×6 LYSO crystals and 312 WLS strips., We would like to thank T. Krähenbühl, ETH Zürich, who did excellent work in this project as part of his master thesis in physics. We are grateful to the numerous technicians in our labs who contributed with their competence and initiative to the progress of the project.

In PET imaging, improving sensitivity while maintaining very good spatial resolution is crucial. To achieve this goal, we propose a novel concept of PET scanner, with axially arranged crystals, providing a high sensitivity and a 3D reconstruction of the gamma interaction point. The trans-axial coordinate is given by the crystal hit, while the z coordinate is reconstructed by the weighted distribution of light escaping the crystal and entering into an array of Wave Length Shifting (WLS) strips interleaving the crystal layers. This novel configuration allows full identification of Compton interactions in the crystals that can be included in image reconstruction thus enhancing the sensitivity. We present preliminary results obtained by a small prototype consisting of 4×4 crystals with orthogonally interleaved WLS strips. Experimental data are compared to simulated data.

In this work, a detailed analysis of the properties of a novel microdosimeter based on a synthetic single crystal diamond is reported. Focused ion microbeams were used to investigate the device spectropscopic properties as well as the induced radiation damageeffects. A diamond based Schottky diode was fabricated by chemical vapor deposition with a very thin detecting region, about 400 nm thick (approximately 1.4 μm water equivalent thickness), corresponding to the typical size in microdosimetric measurements. A 200 × 200 μm2 square metallic contact was patterned on the diamond surface by standard photolithography to define the sensitive area. Experimental measurements were carried out at the Ruder Boškovic′ Institute microbeam facility using 4 MeV carbon and 5 MeV silicon ions. Ion beam induced charge maps were employed to characterize the microdosimeter response in terms of its charge collection properties. A stable response with no evidence of polarization or memory effects was observed up to the maximum investigated ion beam flux of about 1.7 × 109 ions·cm−2·s−1. A homogeneity of the response about 6% was found over the sensitive region with a well-defined confinement of the response within the active area. Tests of the radiation damageeffect were performed by selectively irradiating small areas of the device with different ion fluences, up to about 1012 ions/cm2. An exponential decrease of the charge collection efficiency was observed with a characteristic decay constant of about 4.8 MGy and 1 MGy for C and Si ions, respectively. The experimental data were analyzed by means of GEANT4 Monte Carlo simulations. A direct correlation between the diamond damaging effect and the Non Ionizing Energy Loss (NIEL) fraction was found. In particular, an exponential decay of the charge collection efficiency with an exponential decay as a function of NIEL is observed, with a characteristic constant of about 9.3 kGy-NIEL for both carbon and silicon ions.

Dynamical Dark Energy (DE) is a viable alternative to the cosmological constant. Yet, constructing tests to discriminate between Lambda and dynamical DE models is difficult because the differences are not large. In this paper we explore tests based on the galaxy mass function, the void probability function (VPF), and the number of galaxy clusters. At high z the number density of clusters shows large differences between DE models, but geometrical factors reduce the differences substantially. We find that detecting a model dependence in the cluster redshift distribution is a hard challenge. We show that the galaxy redshift distribution is potentially a more sensitive characteristics. We do so by populating dark matter halos in Nbody simulations with galaxies using well-tested Halo Occupation Distribution (HOD). We also estimate the Void Probability Function and find that, in samples with the same angular surface density of galaxies in different models, the VPF is almost model independent and cannot be used as a test for DE. Once again, geometry and cosmic evolution compensate each other. By comparing VPF's for samples with fixed galaxy mass limits, we find measurable differences., 12 pages, 11 figures, dependence on mass-luminosity relation discussed, minor changes to match the accepted version by MNRAS