Dissertationen zum Thema „Medipix3“

X-ray imaging was a very important detection that is used in many useful functionalities as in healthcare. It would be powerful and facilitate several functions if we could come up with a method that gives a better resolution when doing X-ray imaging. This will be done by using two different detectors, a commercial detector (Innocare) and a direct converting detector (Medipix3). These two detectors have different properties. In addition to these detectors, a radiation source that radiates with X-rays has been used and a line mask that provides the ability to read the resolution of the image. The use of these detectors will provide answers to the investigation of limitations that its detectors have.
Röntgenavbildning är en viktig upptäckt som används i bland annat vården. Det skulle underlätta flera användningsområden om en metod kunde tas fram som ger bättre upplösning på bilder vid avbildning. I detta arbete undersöks vad som kan bidra till att få den bästa upplösningen vid röntgenavbildning. I undersökningen kommer det att användas två olika detektorer, en kommersiell detektor (Innocare) och en direktkonverterande detektor (Medipix3). Dessa två detektorer har olika egenskaper. Utöver dessa detektorer har en strålkälla som strålar med röntgenstrålar använts och en linjemask som ger möjligheten att läsa av bildens upplösning. Användningen av dessa detektorer kommer att ge svar på utredningen av begränsningar som dess detektorer har.

In this thesis a recently developed energy resolving x-ray detector (Medipix) is used to investigate potential medical applications of spectral x-ray imaging. Computed Tomography (CT) is one of the most important medical imaging modalities. Recent developments in CT techniques include dual-energy CT, where images are taken with two different x-ray spectra by either using two x-ray tubes operated at different voltages, or modulating the operating voltage of a single tube. These techniques provide spectral information in the CT dataset but are limited to what can be achieved by manipulating the x-ray source, since the detectors used in current CT systems are unable to provide spectral information about the detected x-rays. A preliminary investigation of the use of the Medipix detectors for two different medical applications is presented. The first, applications is imaging of blood vessels for diagnosis of vascular diseases, and the second, characterising and measuring the energy dependence of x-ray attenuation in fat and liver tissue using the Medipix2 detector. This second investigation is part of work towards (eventually) quantifying the fat content of liver tissue in vivo, which is important for the early diagnosis of fatty liver disease. While an early attempt to identify iron fluorescence x-rays in a Monte-Carlo simulation of blood vessel x-ray image was not successful, the potential for improving image contrast using the changes in x-ray attenuation at the iodine k-edge iodine have been investigated in a series of further simulations and appears to be feasible. The potential use of spectral imaging to differentiate and quantify tissues without the need for added contrast material has been investigated by using a Medipix2 detector to measure the energy dependence of x-ray absorption in fat and liver tissue. The results of this experimental work show significant differences in x-ray attenuation between these two tissues that suggest this form of spectral imaging may be useful in practice.

Computed Tomography is one of the most important image modalities in medical imaging nowadays. Recent developments have led to a new acquisition technique called 'dual-energy', where images are taken with different x-ray spectra. This enables for the first time spectral information in the CT dataset. Our approach was to use an energy resolving detector (Medipix) and investigate its potential in the medical imaging domain. Images are taken in different energy bins. For acquisition of the data, a CT scanner called 'Medipix All Resolution System' (MARS) scanner was constructed. It was upgraded to achieve better image quality as well as faster scan time and a stable operation. In medical imaging, it is important to achieve a high contrast and a good detail recognition at a low dose. Therefore, it is common practice to use contrast agents to highlight certain regions of the body like e.g. the vascular system. But with a broad spectrum acquisition, it is often impossible to distinguish highly absorbing body elements like bones from the contrast agent. We target this problem by a contrast agent study using different energy bins. This so called spectral contrast agent study has been conducted with small animals using the MARS scanner. The data has been processed to create an optimal CT reconstruction. The image enhancement techniques consist of corrections for noisy pixels, intensity fluctuations and eliminating streaks in the sinograms to reduce ring artifacts. In order to evaluate the data, we used two methods of material identification. The material reconstruction method works on projection data and uses a maximum-likelihood estimation to reconstruct images of base materials. The second method, the principal component analysis (PCA), identifies the relevant information from the spectral dataset in a few derived variables that account for most of the variance in the dataset. This resulted in images with enhanced contrast and removed redundancies. It is possible to combine these images in one colour image where anatomical structures are shown in good detail and certain materials show up in different colors. Based on this new information from spectral data, we could show that it is possible to distinguish the spinal bone from contrast agent.

The goal of this research was to demonstrate the potential of spectral CT for the discrimination of vulnerable atherosclerotic plaques. It was proposed that spectral CT has the potential to identify the presence of specific markers for vulnerable plaques: iron deposits and lipid core. A spectral micro-CT system incorporating the latest Medipix spectroscopic photon- counting detectors was commissioned for this purpose. Using spectroscopic methods developed with this system, it was possible to distinguish the presence of iron deposits and lipid core within ex vivo atherosclerotic plaques. Atherosclerosis or hardening of arteries is a systemic disease of the vessel wall that occurs in the aorta, carotid, coronary and peripheral arteries. It is characterised by the deposition of calcified plaques on the innermost layer of the artery wall. Vulnerable plaques are unstable, prone to rupture and put the person at risk of cardiovascular events and strokes. Factors that may lead to plaque instability are lipid content and iron deposits. This preclinical study is a precursor to the development of a clinical technique that will enable vulnerable atherosclerotic plaques to be identified in vivo prior to treatment or removal. Following a preliminary study on atherosclerotic plaques with a prototype system, the MARS-CT3 spectral micro-CT system incorporating Medipix3 was developed and commissioned for further plaque studies. The spectral CT data sets acquired by this system were assessed visually for morphology and analysed for material composition using a linear algebra method. The results were correlated with photography and histology (the histology is the current gold standard). The presence of iron and lipid can be differentiated from the background soft-tissue using a linear algebra method. However the quantification of iron in the presence of calcium is not currently possible without additional data or constraints. Nevertheless the presence of iron deposits within the plaques can be distinguished in the high resolution MARS-CT images and has been correlated with photographic and histological evidence. Thus, using the high spatial resolution spectral data from MARS-CT, the discrimination of lipid core and iron deposits within ex vivo advanced human atherosclerotic plaques is feasible. This may provide the basis for the development of a clinical technique that will identify vulnerable plaques in vivo by high resolution spectral CT.

The research in this thesis was undertaken because information on the relationship between scan parameters and image quality for the MARS spectral CT was lacking. However, the MARS spectral CT is expected to extend into clinical use in the future, so it is absolutely crucial that we know how the quality of the images that it produces is effected by different can parameters. This will allow us to make further improvements to the machine, and ultimately help clinicians to visualise important information in patients which are not revealed by other imaging modalities. This thesis provides information on how the image quality is affected by different scan parameters on the MARS spectral CT using a Medipix3 silicon quad detector. In particular, it explores how different numbers of projections, exposure time products (mAs), and peak tube voltages (kVp) with different threshold energies (kV) effect the image noise, image resolution and image uniformity, respectively. This provides a set of guidelines for future work using the MARS scanner to obtain images of optimal quality. This thesis also determines that the new image reconstruction software mART developed by Niels de Ruiter, is a suitable replacement for the reconstruction software OctopusCT that is currently being used by the MARS team. Using mART reduces the scan times and dose delivered by the MARS spectral CT.

This thesis is about the development of the MARS camera: a standalone portable digital x-ray camera with spectral sensitivity. It is built for use in the MARS Spectral system from the Medipix2 and Medipix3 imaging chips. Photon counting detectors and Spectral CT are introduced, and Medipix is identified as a powerful new imaging device. The goals and strategy for the MARS camera are discussed. The Medipix chip physical, electronic and functional aspects, and experience gained, are described. The camera hardware, firmware and supporting PC software are presented. Reports of experimental work on the process of equalisation from noise, and of tests of charge summing mode, conclude the main body of the thesis. The camera has been actively used since late 2009 in pre-clinical research. A list of publications that derive from the use of the camera and the MARS Spectral scanner demonstrates the practical benefits already obtained from this work. Two of the publications are first-author, eight are co-authored, and a further four acknowledge use of the MARS camera as part of the MARS scanner. The work has been presented at three MARS group meetings, two departmental conferences, and at an internal Medipix3 collaboration meeting hosted by ESRF in Grenoble.

This thesis is about simulation of semiconductor X-ray and particledetectors. The simulation of a novel coating for solid state neutrondetectors is discussed as well as the implementation of a simulationframework for hybrid pixel detectors.Today’s most common thermal neutron detectors are proportionalcounters, that use 3He gas in large tubes or multi wire arrays. Globalnuclear disarmament and the increase in use for homeland securityapplications has created a shortage of the gas which poses a problemfor neutron spallation sources that require higher resolution and largersensors. In this thesis a novel material and clean room compatible pro-cess for neutron conversion are discussed. Simulations and fabricationhave been executed and analysed in measurements. It has been proventhat such a device can be fabricated and detect thermal neutrons.Spectral imaging hybrid pixel detectors like the Medipix chipare the most advanced imaging systems currently available. Thesechips are highly sophisticated with several hundreds of transistors perpixel to enable features like multiple thresholds for noise free photoncounting measurements, spectral imaging as well as time of arrivalmeasurements. To analyse and understand the behaviour of differentsensor materials bonded to the chip and to improve development offuture generations of the chip simulations are necessary. Generally, allparts of the detector system are simulated independently. However, itis favourable to have a simulation framework that is able to combineMonte Carlo particle transport, charge transport in the sensor as wellas analogue and digital response of the pixel read-out electronics. Thisthesis aims to develop such a system that has been developed withGeant4 and analytical semiconductor and electronics models. Further-more, it has been verified with data from measurements with severalMedipix and Timepix sensors as well as TCAD simulations.Results show that such a framework is feasible even for imagingsimulations. It shows great promise to be able to be extended withfuture pixel detector designs and semiconductor materials as well asneutron converters to aim for next generation imaging devices.

The goal of this research is to contribute to the development of MARS spectral CT and to demonstrate the feasibility of molecular imaging using the technology. MARS is a newly developed micro CT scanner, incorporating the latest spectroscopic Medipix photon counting detector. I show that the scanner can identify both drug markers and stenosis of atherosclerosis labelled with non-toxic nanoparticles. I also show that spectral computed tomography using Medipix x-ray detectors can give quantitative measurements of concentrations of gold nanoparticles in phantoms, mice and excised atheroma. The characterisation of the Medipix2 assemblies with Si and CdTe x-ray sensors using poly-energetic x-ray sources has been performed. I measure the inhomogeneities within the sensors; individual pixel sensitivity response; and their saturation effects at higher photon fluxes. The effects of charge sharing on the performance of Medipix2 have been assessed, showing that it compromises energy resolution much more than spatial resolution. I have commissioned several MARS scanners incorporating several different Medipix2 and Medipix3 cameras. After the characterization of x-ray detectors and the geometrical assessment of MARS-CT, spectral CT data has been acquired, using x-ray energies that are appropriate for human imaging. The outcome shows that MARS scanner has the ability to discriminate among low atomic number materials, and from various concentrations of heavy atoms. This new imaging modality, used with functionalized gold nanoparticles, gives a new tool to assess plaque vulnerability. I demonstrated this by using gold nanoparticles, attached to antibodies, which targeted to thrombotic events in excised plaque. Likewise, the imaging modality can be used to track drugs labelled with any heavy atoms to assess how much drug gets into a target organ. Thus the methodology could be used to accelerate development of new drug treatments for cancers and inflammatory diseases.

Ce document résume mes activités entre 1995 et 2010. Dans un premier temps, le développement d'un détecteur à pixels de silicium pour l'amélioration du Forward Proton Spectrometer de l'expérience H1 à DESY est présenté. Ensuite, les développements des circuits de lecture du détecteur à pixels de l'expérience ATLAS au CERN sont détaillés, en particulier le développement de bancs de test de ces circuits ainsi que la simulaton de leur comportement dans un environnement réaliste. Puis, l'infrastructure de la digitization des détecteurs à silicium (Pixels et SCT) est présentée. Une deuxième partie décrit la production de l'électronique frontale de lecture du calorimètre hadronique à tuiles scintillantes d'ATLAS (TileCal), en particulier le développement du système de test MobiDICK. Les logiciels de contrôle du système de calibration du TileCal par laser sont aussi détaillés. Pour finir, mes activités d'enseignement, d'encadrement et de diffusion des connaissances (en particulier le Cosmophone) sont mentionnées.

This licentiate thesis concerns the development and characterisation of X-ray imaging detector systems. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”.

With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the picture achieved. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the picture. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the picture.

The problem involving charge sharing which limits “colour” X-ray imaging is discussed in this thesis. Image quality, detector effectiveness and “colour correctness” are studied on pixellated detectors from the MEDIPIX collaboration. Characterisation measurements and simulations are compared to be able to understand the physical processes that take place in the detector. Simulations can show pointers for the future development of photon counting X-ray systems. Charge sharing can be suppressed by introducing 3D-detector structures or by developing readout systems which can correct the crosstalk between pixels.

This thesis concerns the development and characterisation of X-ray imaging systems based on single photon processing. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”. With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the image. This thesis presents characterisation and simulations to provide a detailed understanding of the physical processes concerning charge sharing in detectors from the MEDIPIX collaboration. Charge summing schemes utilising pixel to pixel communications are proposed. Charge sharing can also be suppressed by introducing 3D-detector structures. In the next generation of the MEDIPIX system, Medipix3, charge summing will be implemented. This system, equipped with a 3D-silicon detector, or a thin planar high-Z detector of good quality, has the potential to become a commercial product for medical imaging. This would be beneficial to the public health within the entire European Union.
Denna avhandling berör utveckling och karaktärisering av fotonräknande röntgensystem. ”Färgröntgen” öppnar nya perspektiv för medicinsk röntgendiagnostik och även för materialröntgen inom industrin. Skillnaden i absorption av olika ”färger” kan användas för att särskilja olika material i ett objekt. Färginformationen kan till exempel användas i sjukvården för att identifiera benskörhet. Färgen på röntgenfotonen kan identifieras om detektorsystemet kan detektera varje foton individuellt. Sådana detektorsystem kallas ”fotonräknande” system. Med modern teknik är det möjligt att konstruera fotonräknande detektorsystem som kan urskilja detaljer ner till en upplösning på circa 50 µm. Med så små pixlar kommer ett problem att uppstå. I en halvledardetektor ger varje absorberad foton upphov till ett laddningsmoln som bidrar till den erhållna bilden. För höga fotonenergier är storleken på laddningsmolnet jämförbar med 50 µm och molnet kan därför fördelas över flera pixlar i bilden. Laddningsdelning är ett centralt problem delvis på grund av att bildens upplösning försämras, men framför allt för att färginformationen i bilden förstörs. Denna avhandling presenterar karaktärisering och simulering för att ge en mer detaljerad förståelse för fysikaliska processer som bidrar till laddningsdelning i detektorer från MEDIPIX-projekter. Designstrategier för summering av laddning genom kommunikation från pixel till pixel föreslås. Laddningsdelning kan också begränsas genom att introducera detektorkonstruktioner i 3D-struktur. I nästa generation av MEDIPIX-systemet, Medipix3, kommer summering av laddning att vara implementerat. Detta system, utrustat med en 3D-detektor i kisel, eller en tunn plan detektor av högabsorberande material med god kvalitet, har potentialen att kunna kommersialiseras för medicinska röntgensystem. Detta skulle bidra till bättre folkhälsa inom hela Europeiska Unionen.

In the present work we deal with x-ray microradiography, which studies imaging of objects by x-rays. Objectives of this work was to prepare a few demonstrational biological objects, scan series of transmissions pictures on the pixel detector Medipix 2 and analyse them qualitatively with help of SNR method.

Detecting elementary particles and observing accompanying events in particle colliders is one of the most important field of current research in experimental physics. TimePix and its successor TimePix3 are types of the currently used detectors which are placed beside other in ATLAS experiment conducted by Eu- ropean Organization for Nuclear Research. Such detectors can produce huge amount of data about passing particles at high rate. The goal of the thesis is to develop methods for detecting and classification of elementary particles observed by detector network ATLAS-TPX3. Suitable methods for clustering and/or classification based on semi-labelled data should be identified or new one should be developed. The proposed methods will be implemented and their performance on real data will be evaluated. The results will also include an implementation of framework for preprocessing low level data from detector network ATLAS-TPX3 in real-time and creating outputs that are suitable for subsequent physics investigation (e.g. ROOT framework files) includ- ing the proposed or future methods for particle classification. 5

Medical imaging is a technique that allows us to visualize non surgically the internal structure of the human body in order to diagnose or treat medical conditions. It permits also monitoring of physical processes or functions of different organs inside the body. The medical imaging encompasses wide range of techniques based on different physical prin- ciples, including techniques using ionizing radiation. The quality of the images depends significantly on the quality of the used imaging detectors. There are many types of the detectors, from old analog devices (e.g. films) to fully digital detectors such as flat panels, that are the most widely used today. The newer technology is being developed and the techniques such as photon counting explored. However, the state of the art technology is the single photon counting, where the experimental detectors such as Medipix are able to count and process each individual photon. This works studies the properties, features and applications of the newest detector from the Medipix family Timepix3 in different imaging modalities. Firstly, a design of a new hardware readout interface for Timepix3 is presented together with data acquisition software and new analysis and calibration algorithms. Then, different applications of Timepix3 detector were explored: very...

Compton cameras localize γ-ray sources in 3D space by observing evidence of Compton scattering with detectors sensitive to ionizing radiation. This thesis proposes a software system for operating a novel Compton camera device comprised of Timepix3 detectors and Katherine readouts. To communicate with readouts using UDP-based protocol, a dedicated hardware library was developed. The presented software can successfully control the acquisition of multiple Timepix3 detectors and simultaneously process their measurements in a real-time setting. To recognize instances of Compton scattering among observed interactions, a chain of algorithms is applied with explicit consideration for a possibly high volume of measured information. Unlike alternate approaches, the presented work uses a recently published charge drift time model to improve its spatial resolution. In order to achieve localization of γ-ray sources, the software performs conical back projection into a discretized cuboid volume. Results of randomized evaluation with simulated data indicate that the presented implementation is correct and constitutes a viable method of γ-ray source localization in 3D space. Experimental verification with a prototype model is in progress.

Medipix2 (MPX) sont des détecteurs semi-conducteurs au silicium montés sur 256x256 pixels. Chaque pixel a une aire de 55x55μm2. L’aire active d’un détecteur MPX est d’environ 2 cm2. Avec deux modes de détection, un seuil et un temps d’exposition ajustables, leur utilisation peut être optimisée pour une analyse spécifique. Seize de ces détecteurs sont présentement installés dans l’expérience ATLAS (A Toroidal LHC ApparatuS) au CERN (Organisation Européenne pour la Recherche Nucléaire). Ils mesurent en temps réel le champ de radiation dû aux collisions proton-proton, au point d’interaction IP1 (Point d’Interaction 1) du LHC (Grand Collisionneur d’Hadrons). Ces mesures ont divers buts comme par exemple la mesure du champ de neutrons dans la caverne d’ATLAS. Le réseau de détecteurs MPX est complètement indépendant du détecteur ATLAS. Le groupe ATLAS-Montréal s’est intéressé à l’analyse des données récoltées par ces détecteurs pour calculer une valeur de la luminosité du LHC au point de collision des faisceaux, autour duquel est construit le détecteur ATLAS. Cette valeur est déterminée indépendamment de la luminosité mesurée par les divers sous-détecteurs d’ATLAS dédiés spécifiquement à la mesure de la luminosité. Avec l’augmentation de la luminosité du LHC les détecteurs MPX les plus proches du point d’interaction détectent un grand nombre de particules dont les traces sont impossibles à distinguer sur les images ("frames") obtenues, à cause de leur recouvrement. Les paramètres de mesure de certains de ces détecteurs ont été optimisés pour des mesures de luminosité. Une méthode d’analyse des données permet de filtrer les pixels bruyants et de convertir les données des images, qui correspondent à des temps d’exposition propres aux détecteurs MPX, en valeur de luminosité pour chaque LumiBlock. Un LumiBlock est un intervalle de temps de mesure propre au détecteur ATLAS. On a validé les mesures de luminosité premièrement en comparant les résultats obtenus par différents détecteurs MPX, et ensuite en comparant les valeurs de luminosité relevées à celles obtenues par les sous-détecteurs d’ATLAS dédiés spécifiquement à la mesure de la luminosité.
Medipix2 (MPX) devices are silicon semiconductor detectors consisting of 256x256 pixels. Each pixel has 55x55μm2. The MPX sensitive area is ~2 cm2. With two modes of operation, an adjustable threshold, and adjustable exposure time, their use can be optimized for a specific analysis. Sixteen of these detectors are currently installed in the ATLAS detector at CERN (European Organization for Nuclear Research). They perform real time measurement of the radiation field due to proton-proton collisions occurring at the interaction point IP1 (Interaction Point 1) of the LHC (Large Hadron Collider). These measurements have different purposes, for instance the measurement of the neutron field in the ATLAS cavern. The network of MPX detectors is completely independent from the ATLAS detector. The ATLAS-Montreal group has taken interest in analysing data provided by these detectors to calculate a value for the luminosity of the LHC at the collision point of the beams around which is build the ATLAS detector that is completely independent from the value measured by ATLAS sub-detectors specifically dedicated to luminosity measurements. With the increase of the luminosity in LHC the MPX detectors near the interaction point record a large number of particles leaving tracks with a large number of overlaps in the resulting frames. These tracks are thus difficult to separate. The measurement parameters of some of these MPX detectors have been optimised for luminosity study. A method to analyse the recorded data was developed. It first filters the noisy pixels, then converts data from a frame format (corresponding to a given MPX exposure time), into luminosity in a LumiBlock format. A LumiBlock is a time measurement period used by ATLAS detector. To verify the results, comparisons between different MPXs have been made, as well as comparison between MPX results and the results of ATLAS sub-detectors specifically dedicated to luminosity measurements.

Les détecteurs ATLAS-MPX sont des détecteurs Medipix2-USB recouverts de convertisseurs de fluorure de lithium et de polyéthylène pour augmenter l’efficacité de détection des neutrons lents et des neutrons rapides respectivement. Un réseau de quinze détecteurs ATLAS-MPX a été mis en opération dans le détecteur ATLAS au LHC du CERN. Deux détecteurs ATLAS-MPX de référence ont été exposés à des sources de neutrons rapides 252 Cf et 241 AmBe ainsi qu’aux neutrons rapides produits par la réaction 7Li(p, xn) pour l’étude de la réponse du détecteur à ces neutrons. Les neutrons rapides sont principalement détectés à partir des protons de recul des collisions élastiques entre les neutrons et l’hydrogène dans le polyéthylène. Des réactions nucléaires entre les neutrons et le silicium produisent des particules-α. Une étude de l’efficacité de reconnaissance des traces des protons et des particules-α dans le détecteur Medipix2-USB a été faite en fonction de l’énergie cinétique incidente et de l’angle d’incidence. L’efficacité de détection des neutrons rapides a été évaluée à deux seuils d’énergie (8 keV et 230 keV) dans les détecteurs ATLAS-MPX. L’efficacité de détection des neutrons rapides dans la région du détecteur couverte avec le polyéthylène augmente en fonction de l’énergie des neutrons : (0.0346 ± 0.0004) %, (0.0862 ± 0.0018) % et (0.1044 ± 0.0026) % pour des neutrons rapides de 2.13 MeV, 4.08 MeV et 27 MeV respectivement. L’étude pour déterminer l’énergie des neutrons permet donc d’estimer le flux des neutrons quand le détecteur ATLAS-MPX est dans un champ de radiation inconnu comme c’est le cas dans le détecteur ATLAS au LHC.
ATLAS-MPX detectors are Medipix2-USB detectors covered with lithium fluoride and polyethylene converters in order to increase the detection efficiency of slow neutrons and fast neutrons respectively. A network of fifteen ATLAS-MPX detectors has been put in operation in the ATLAS detector at CERN-LHC. Two reference detectors ATLAS-MPX were exposed to two fast neutrons sources 252 Cf and 241 AmBe as well as fast neutrons produced by the reaction 7 Li( p, xn) for the study of the detector response to those neutrons. Fast neutrons are primarily detected by recoil protons from elastic collisions between neutrons and hydrogen in the polyethylene. In addition, α -particles are produced by nuclear reactions between neutrons and silicon. A study of the efficiency of proton and alpha particle track recognition of Medipix2-USB was done as a function of the initial kinetic energy and incidence angle. The detection efficiency of fast neutrons was evaluated for two energy thresholds (8 keV and 230 keV) of ATLAS-MPX detector. The fast neutron detection efficiency of the detector region covered in polyethylene increases with neutron energy: (0.0346 ± 0.0004) %, (0.0862 ± 0.0018) % and (0.1044 ± 0.0026) % for fast neutrons of 2.13 MeV, 4.08 MeV and 27 MeV respectively. The method for the measurement of neutron energy allows an estimate of the neutron flux when the ATLAS MPX detector is in an unknown radiation field as it is the case in the ATLAS detector at LHC.

La tomodensitométrie par rayons-X (CT) est une modalité d’imagerie produisant une carte tridimensionnelle du coefficient d’atténuation des rayons-X d’un objet. En radiothérapie, le CT fournit de l’information anatomique et quantitative sur le patient afin de permettre la planification du traitement et le calcul de la dose de radiation à livrer. Le CT a plusieurs problèmes, notamment (1) une limitation au niveau de l’exactitude des paramètres physiques quantitatifs extraits du patient, et (2) une sensibilité aux biais causés par des artéfacts de durcissement du faisceau. Enfin, (3) dans le cas où le CT est fait en présence d’un agent de contraste pour améliorer la planification du traitement, il est nécessaire d’effectuer un deuxième CT sans agent de contraste à des fins de calcul de dose, ce qui augmente la dose au patient. Ces trois problèmes limitent l’efficacité du CT pour certaines modalités de traitement qui sont plus sensibles aux incertitudes comme la protonthérapie. Le CT spectral regroupe un ensemble de méthodes pour produire plusieurs cartes d’atténuation des rayons-X moyennées sur différentes plages énergétiques. L’information supplémentaire, pondérée en énergie qui est obtenue permet une meilleure caractérisation des matériaux analysés. Le potentiel de l’une de ces modalités spectrales, le CT bi-énergie (DECT), est déjà bien démontré en radiothérapie, alors qu’une approche en plein essor, le CT spectral à comptage de photons (SPCCT), promet davantage d’information spectrale à l’aide de détecteurs discriminateurs en énergie. Par contre, le SPCCT souffre d’un bruit plus important et d’un conditionnement réduit. Cette thèse investigue la question suivante : y a-t-il un bénéfice à utiliser plus d’information résolue en énergie, mais de qualité réduite pour la radiothérapie ? La question est étudiée dans le contexte des trois problèmes ci-haut. Tout d’abord, un estimateur maximum a posteriori (MAP) est introduit au niveau de la caractérisation des tissus post-reconstruction afin de débruiter les données du CT spectral. L’approche est validée expérimentalement sur un DECT. Le niveau de bruit du pouvoir d’arrêt des protons diminue en moyenne d’un facteur 3.2 à l’aide de l’estimateur MAP. Celui-ci permet également de conserver généralement le caractère quantitatif des paramètres physiques estimés, le pouvoir d’arrêt variant en moyenne de 0.9% par rapport à l’approche conventionnelle. Ensuite, l’estimateur MAP est adapté au contexte de l’imagerie avec agent de contraste. Les résultats numériques démontrent un bénéfice clair à utiliser le SPCCT pour l’imagerie virtuellement sans contraste par rapport au DECT, avec une réduction de l’erreur RMS sur le pouvoir d’arrêt des protons de 2.7 à 1.4%. Troisièmement, les outils développés ci-haut sont validés expérimentalement sur un micro-SPCCT de la compagnie MARS Bioimaging, dont le détecteur à comptage de photons est le Medipix 3, qui est utilisé pour le suivi de particules au CERN. De légers bénéfices au niveau de l’estimation des propriétés physiques à l’aide du SPCCT par rapport au DECT sont obtenus pour des matériaux substituts à des tissus humains. Finalement, une nouvelle paramétrisation du coefficient d’atténuation pour l’imagerie pré-reconstruction est proposée, dans le but ultime de corriger les artéfacts de durcissement du faisceau. La paramétrisation proposée élimine les biais au niveau de l’exactitude de la caractérisation des tissus humains par rapport aux paramétrisations existantes. Cependant, aucun avantage n’a été obtenu à l’aide du SPCCT par rapport au DECT, ce qui suggère qu’il est nécessaire d’incorporer l’estimation MAP dans l’imagerie pré-reconstruction via une approche de reconstruction itérative.
X-ray computed tomography (CT) is an imaging modality that produces a tridimensional map of the attenuation of X-rays by the scanned object. In radiation therapy, CT provides anatomical and quantitative information on the patient that is required for treatment planning. However, CT has some issues, notably (1) a limited accuracy in the estimation of quantitative physical parameters of the patient, and (2) a sensitivity to biases caused by beam hardening artifacts. Finally, (3) in the case where contrast-enhanced CT is performed to help treatment planning, a second scan with no contrast agent is required for dose calculation purposes, which increases the overall dose to the patient. Those 3 problems limit the efficiency of CT for some treatment modalities more sensitive to uncertainties, such as proton therapy. Spectral CT regroups a set of methods that allows the production of multiple X-ray attenuation maps evaluated over various energy windows. The additional energy-weighted information that is obtained allows better material characterization. The potential of one spectral CT modality, dual-energy CT (DECT), is already well demonstrated for radiation therapy, while an upcoming method, spectral photon counting CT (SPCCT), promises more spectral information with the help of energy discriminating detectors. Unfortunately, SPCCT suffers from increased noise and poor conditioning. This thesis thus investigates the following question: is there a benefit to using more, but lower quality energy-resolved information for radiotherapy? The question is studied in the context of the three problems discussed earlier. First, a maximum a posteriori (MAP) estimator is introduced for post-reconstruction tissue characterization for denoising purposes in spectral CT. The estimator is validated experimentally using a commercial DECT. The noise level on the proton stopping power is reduced, on average, by a factor of 3.2 with the MAP estimator. The estimator also generally con- serves the quantitative accuracy of estimated physical parameters. For instance, the stopping power varies on average by 0.9% with respect to the conventional approach. Then, the MAP estimation framework is adapted to the context of contrast-enhanced imaging. Numerical results show clear benefits when using SPCCT for virtual non-contrast imaging compared to DECT, with a reduction of the RMS error on the proton stopping power from 2.7 to 1.4%. Third, the developed tools are validated experimentally on a micro-SPCCT from MARS Bioimaging, which uses the Medipix 3 chip as a photon counting detector. Small benefits in the accuracy of physical parameters of tissue substitutes materials are obtained. Finally, a new parametrization of the attenuation coefficient for pre-reconstruction imaging is pro- posed, whose ultimate aim is to correct beam hardening artifacts. In a simulation study, the proposed parametrization eliminates all biases in the estimated physical parameters of human tissues, which is an improvement upon existing parametrizations. However, no ad- vantage has been obtained with SPCCT compared to DECT, which suggests the need to incorporate MAP estimation in the pre-reconstruction framework using an iterative reconstruction approach.