Monte Carlo is an important and well established research tool used in emission tomography. While used extensively in research applications, these techniques are not typically implemented clinically due to their low detection efficiency and long acquisition times. In order to make this computational tool faster, the variance reduction technique known as convolution-based forced detection (CFD) has been implemented into the SIMIND MC code (CFD-SIMIND) by our group. Briefly, at each site of interaction within the object, photons are forced to travel in a direction perpendicular to the detector and are then convolved with a distance dependent blurring kernel specific to that collimator and photon energy. A similar CFD method has already been implemented as an option in the SIMIND Monte Carlo program. The study presented in Chapter 2 performs a comparison between a well established, non-VRT Monte Carlo program, GATE, with our accelerated CFD-SIMIND. The intent of this work is to establish if CFD-SIMIND can either replace or be used in conjunction with GATE in order to gain significant reduction in simulation times for low and medium energy isotopes. A number of simulation studies were performed using point sources in air and water, along with the 3D XCAT phantom and a rectangular sheet source for Tc-99m with low and medium energy collimator and In-111 with medium energy collimator. A comparison in the projection domain was then performed in terms of spatial resolution, sensitivity, image profiles and energy spectra. The study has shown percent differences of between 3−5% in sensitivity between CFD-SIMIND and GATE with mean universal image quality index value of 0.994 ± 0.009 and spatial resolution within 0.2 mm of each other. CFD-SIMIND offers a significant reduction in simulation time by a factor of 5−6 orders of magnitude compared to GATE. This acceleration time is useful for many applications. This study also provides an objective tool that can help to determine if CFD-SIMIND can be used in place of GATE in order to achieve images of sufficient quality within a reduced time and at much lower computational cost. Simultaneous multi-isotope SPECT imaging has a number of applications in cardiac, brain and cancer imaging. The major concern however, is the significant crosstalk contamination due to photon scatter between the different isotopes. The second study (Chapter 3) focuses on a method of downscatter compensation between two isotopes iii in simultaneous dual isotope SPECT acquisition applied to cancer imaging using Tc-99m and In-111. We have developed an iterative image reconstruction technique that simulates the photon down-scatter from one isotope into the acquisition window of a second isotope. Our approach uses CFD-SIMIND for the forward projection step in an iterative reconstruction algorithm. The MC estimated scatter contamination of a radionuclide contained in a given projection view is then used to compensate for the photon contamination in the acquisition window of other nuclide. We use a modified ordered subset-expectation maximization (OS-EM) algorithm named simultaneous ordered subset-expectation maximization (Sim-OSEM), to perform this step. In this study, we have undertaken a number of simulation tests and phantom studies to verify this approach. The proposed reconstruction technique was also evaluated by reconstruction of experimentally acquired phantom data. Reconstruction using Sim- OSEM showed very promising results in terms of contrast recovery and uniformity of object background compared to alternative reconstruction methods implementing alternative scatter correction schemes (i.e., triple energy window or separately acquired projection data). In this study the evaluation is based on the quality of reconstructed images and activity estimated using Sim-OSEM. In order to quantitate the possible improvement in spatial resolution and signal to noise ratio (SNR) observed in this study, further simulation and experimental studies are required. It is perceived that in simultaneous dual-isotope breast SPECT studies using 123I-labelled Z-MIVE and Tc-99m sestamibi, I-123 labelled Z-MIVE not only detects the presence of estrogen receptor (ER) but, also thought to complement Tc-99m sestamibi in differentiating between benign and malignant breast lesions for patients with breast cancer (Chapter 4). The major concern in simultaneous Tc-99m/I-123 SPECT is the significant crosstalk contamination between the different isotopes used. The current study focuses on a method of crosstalk (downscatter and spillover) compensation between two isotopes with data acquired using Thallium activated Sodium Iodide (NaI(Tl)) detector (Energy resolution 9.8% at 140 keV ) and Cadmium Zinc Telluride (CZT) detector (Energy resolution 5% 140 keV ) respectively. The study uses Sim- OSEM for crosstalk compensation between the isotopes. We have undertaken a number of simulation studies using our modeled breast phantom to verify this approach. Reconstruction using Sim-OSEM showed very promising results in terms of crosstalk and scatter compensation and uniformity of background. In our case images obtained using Sim-OSEM were comparable or even better than the images reconstructed from separately acquired projection data using analytical attenuation based reconstruction. This may be due to better small angle scatter compensation in case of Sim-OSEM as CFD-SIMIND based MC forward projector was used. Compensation of the image degradation effects (i.e. attenuation, scatter and collimator-detector response) is necessary for an accurate quantification in SPECT imaging. We have previously proposed an accelerated Monte Carlo (MC) based iterative SPECT reconstruction algorithm that accurately corrects for attenuation and scatter once provided with attenuation information (Chapters 3 and 4). This algorithm uses SIMIND MC program accelerated through the implementation of a variance reduction technique known as, convolution forced detection (CFD), (CFD-SIMIND). With ever increasing number of hybrid SPECT/CT systems, CT-based attenuation correction is becoming a standard clinical protocol. This co-registered CT image with SPECT data can also be used to incorporate anatomical information as a prior into a maximum a-posteriori (MAP) SPECT image reconstruction algorithm. The study presented in Chapter 5 proposes a MAP reconstruction algorithm that includes CFD-SIMIND as a forward projector and a CT-image as an anatomical prior (CFD-AMAP) for simultaneous compensation of scatter and attenuation and, enhancement of spatial resolution during reconstruction. We have performed a number of simulation and experimental studies to elaborate the advantages of CFD-AMAP. These studies show an accurate quantification (within ±5% and ±8% for simulation and experimental studies respectively) accompanied by a significant reduction in coefficient of variation (CoV ). This reduction of CoV results in an improved boundary delineation and the Gibbs artifact compensation. However, this compensation comes at the cost of loss of an overall contrast in the reconstructed images due to a more uniform distribution of estimated activity over the regions of interest (ROI’s). Further studies with more complex phantoms and real patient data, task-based ROC studies, improvement in CFD-SIMIND in terms of speed and use of better Bayesian image reconstruction algorithms are needed to elaborate on the strengths and weaknesses of this proposed MC based forward projector and to pave the way for CFD-SIMIND based image reconstruction algorithms from research to clinic. / Thesis / Doctor of Philosophy (PhD)

Cette thèse présente tout d'abord un méthode d’exploration mono-robot, ensuite une stratégie coopérative décentralisée d'exploration pour un groupe de robots équipés de range finders. Une carte 2-D de la zone explorée est construite sous la forme de figure en pixels et est étendue par les robots en utilisant un planner local qui décide automatiquement entre l'information gagnée et le coût d'exploration. La carte est construite en utilisant la méthode des moindres carrés pour réduire les erreurs des données des capteurs. En divisant la tâche globale en sous-tâches, un contrôleur intelligent permet de réduire la complexité. Cependant, la fusion de différents comportements avec des objectifs différents peut entraîner des contradictions et modifier ainsi la stabilité du système. Par conséquent, la question de mécanisme de coordination de comportements est essentielle pour réaliser un mouvement sécurisé sans collisions. Une méthode intégrée par la coordination des comportements et de commande par fusion est proposée dans le présent travail. Une nouvelle approche basée sur cinq comportements de base pour la navigation de robots mobiles est discutée. Player/Stage est un projet de logiciel open-source pour la recherche sur la robotique. Ses composants comprennent le serveur de réseau et les simulateurs de robot pour plusieurs types de plates-formes de robots. Nous utilisons principalement simulation sous Player/Stage pour tester nos algorithmes en mono-agents/multi-agents , en cartographie et en navigation. Les résultats obtenus montrent que les solutions proposées sont efficaces et peuvent être utilisées dans des robots réels. / In this Ph. D., we will present firstly a single robot exploration method, then a decentralized cooperative exploration strategy for a team of mobile robots equipped with a range finders. A two dimensional map of the explored area is built in the form of a pixel figure. This is expanded by the robots by using a randomized local planner that authomatically realizes a decision between information gain and navigation cost. In our work, the map is reconstructed using a least-mean square method to reduce the errors of the sensor data. In dividing the overall task into subtasks, the intelligent controller allows reducing the robots task complexity. But the fusion of different behaviors with different objectives may cause contradiction in the procedure and alter the stability of the system. Therefore, the issue of behavior coordination mechanisms is crucial in order to realize the non-collision safety-ensured movements. A method integrated by behavior coordination and command fusion is proposed. A new approach with five basic behaviors for mobile robot navigation is discussed.Player/ Stage is an open-source software project for research in robotics and sensor systems. Its components include the Player network server and the Stage robot platform simulators providing a hardware abstraction layer to several popular robot platforms. Player is one of the most popular robot interfaces in research. We mainly use Player/Stage simulation to test our algorithms in mono-agent/multi-agent exploration, map reconstruction and robot navigation. Obtained results show that the proposed approaches are effective and can be applied in real robots.

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L'analyse des propriétés statistiques de la distribution de la matière dans le cosmos (Grandes Structures, LSS or Large Scale Structure) est l'une des principales sondes cosmologiques qui permettent l'étude du modèle standard cosmologique, en particulier les paramètres caractérisant la matière noire et l'énergie noire. Les Oscillations Acoustiques Baryoniques (BAO's) sont l'une des mesures qui peuvent être extraites de l'étude de la distribution de matière à grande échelle (LSS).L'observation de la distribution cosmique de la matière à partir de l'émission à 21 cm de l'hydrogène atomique neutre (HI) est une nouvelle méthode, complémentaire des relevés optiques pour cartographier la distribution de la matière dans le cosmos. La méthode de cartographie d'intensité (Intensity Mapping) a été proposée depuis moins d'une dizaine d'années comme une méthode efficace pour cartographier en trois dimensions l'émission radio à 21 cm. Elle n'implique en particulier pas la détection des objets individuels (galaxies), et peut donc être effectué avec des instruments plus modestes en taille que ceux comme SKA ou FAST qui sont conçus pour détecter les galaxies à 21 cm à des distances cosmologiques. Des interféromètres radio utilisant un ensemble de réflecteurs cylindriques ou paraboliques fixes, observant le ciel en mode transit sont adaptés à la cartographie d'intensité. Le mode d'observation spécifique de ce type de radio télescope en cartographie d'intensité est étudié dans le cadre de ce travail de thèse. On montre en particulier qu'une méthode spécifique de reconstruction des cartes du ciel à partir des visibilités peut être appliquée aux observations de ces interféromètres fonctionnant en mode transit. Cette méthode correspond à la décomposition en modes m des harmoniques sphériques et est très performante pour la reconstruction de grandes zones du ciel observées en mode transit. Un code de reconstruction fondé sur ce principe a été développé, ainsi que différents critères de comparaison des performances instrumentales, comme le lobe d'antenne synthétisé, le spectre de bruit sur les cartes reconstruites et la réponse globale de l'instrument dans le plan (l,m) des harmoniques sphériques. La méthode a été appliquée à différentes configurations des interféromètres composés de réflecteurs paraboliques ou cylindriques dans le cadre des projets PAON-4 et Tianlai. Outre l'optimisation des configurations des interféromètres Tianlai et PAON-4, le travail présenté inclut une première application de la méthode aux données PAON-4. / The analysis of the statistical properties of the distribution of matter in the cosmos (LSS or Large Scale Structure) is one of the main cosmological probes that allow the study of the cosmological standard model, in particular the parameters characterizing dark matter and dark energy. Baryonic Acoustic Oscillations (BAO's) are one of the measurements that can be extracted from the study of matter distribution in large-scale structure (LSS).The observation of the cosmic distribution of the matter from neutral atomic hydrogen (HI) 21 cm emission is a new method, complementary to the optical observation to map the distribution of matter in the cosmos. In the last decade, the Intensity Mapping method has been proposed as an effective method for mapping the 21cm radio emission in three dimensions. In particular, it does not require the detection of individual objects (galaxies), and can therefore be performed with instruments smaller in size than those such as SKA or FAST, which are designed to detect 21 cm galaxies at cosmological distances. A radio interferometer using a set of fixed cylindrical or parabolic reflectors observing the sky in transit mode are suitable instruments for intensity mapping surveys. The specific observational mode from this type of radio telescope by intensity mapping is studied in the context of this thesis. We show in particular that a specific sky maps reconstruction method from the visibilities can be applied to the observations of these interferometers operating in transit mode. This method corresponds to the m-modes decomposition of the spherical harmonics and is very efficient for the reconstruction of large sky areas observed in transit mode. A reconstruction code based on this principle has been developed, as well as different criteria for the comparison of instrumental performances, such as the synthesized antenna lobe, the noise spectrum of the reconstructed maps and the overall instrument response in the spherical harmonics (l,m) plane. The method has then been applied to different configurations of interferometers composed of parabolic or cylindrical reflectors in the PAON-4 and Tianlai projects. In addition to optimizing the Tianlai and PAON-4 interferometer configurations, the work presented here includes a first application of the method to the PAON-4 data.