Conception de systèmes tomographiques à imageurs multiples pour la dosimétrie à scintillation volumétrique

Cette thèse se cadre dans le développement de systèmes de dosimétrie à scintillation volumétrique. Étant donné la complexité des distributions de dose de radiation qu’impliquent les techniques modernes de traitement en radiothérapie externe, il est nécessaire d’avoir des outils cliniques offrant une mesure complète d’une distribution tridimensionnelle (3D) de dose administrée par un accélérateur linéaire médical. Des solutions prometteuses, ayant une capacité de mesure intéressante autant spatialement que temporellement, résident dans la dosimétrie à scintillation volumétrique. Dans le but de faciliter le développement de prototypes expérimentaux, cette thèse met de l’avant un processus de conception généralisé qui exploite les fonctionnalités de tracé réel de rayons d’un logiciel de conception optique. L’approche proposée permet une modélisation optique complète de systèmes tomographiques employant multiples caméras — de type standard ou plénoptique — pour imager la lumière fluorescente émise d’un volume scintillant sous l’effet de la radiation. La thèse fournit le bagage à la fois contextuel et théorique de la physique médicale et de l’ingénierie optique nécessaire à la compréhension et l’appréciation des travaux présentés, tout en cadrant la motivation de ceux-ci dans une réalité clinique, soit celle de la radiothérapie externe. Les contributions principales de la thèse se divisent en trois portions. D’abord, des travaux de simulation valident le processus généralisé de modélisation optique de systèmes à imageurs multiples et le calcul de leur modèle tomographique au moyen d’un logiciel de conception optique. Ensuite, à partir du processus de simulation de prototypes virtuels, une étude de faisabilité démontre la mise en application d’un prototype expérimental employant de multiples caméras plénoptiques pour des mesures volumétriques en dosimétrie à scintillation. Enfin, une analyse comparative entre l’emploi des caméras standards versus plénoptiques dans le contexte de tomographie à émission est présentée, puis des pistes à explorer sont discutées afin de mieux élucider la question de leur apport tomographique respectif. En somme, le processus de conception généralisé mis de l’avant dans cette thèse découple et simplifie les étapes de développement expérimental, offrant une flexibilité accrue de design de futurs outils cliniques pour la dosimétrie 3D. Ce travail ouvre la voie au développement d’une nouvelle génération de systèmes de dosimétrie à scintillation volumétrique.This thesis simplifies and generalizes the developmental workflow of volumetric scintillation dosimetry systems. Due to the high complexity of radiation dose distributions delivered to patients by means of modern radiotherapy treatment techniques, it is essential to have clinical tools capable of measuring the full three-dimensional (3D) dose distributions delivered by medical linear accelerators. Imaging-based volumetric scintillation dosimetry offers promising solutions with potential for both high spatial and temporal resolution. To ease the development of experimental prototypes, this thesis puts forth a generalized design workflow based on the real ray tracing capabilities of optical design software. The proposed method allows for a complete and precise optical modeling of tomographic systems composed of multiple cameras, either standard or plenoptic, used to image the fluorescent light emission induced by radiation in translucent scintillator volumes. This thesis provides the reader with both the contextual and theoretical background in medical physics and optical engineering to understand fully and to appreciate the work carried out in the context of external beam radiation therapy. The main contributions of the thesis are three-fold. First, a simulative study serves to validate the generalized workflow for optical and tomographic modeling of multiple imager-based scintillation dosimetry systems using optical design software. Subsequently, a feasibility study demonstrates the simulation-to-experimental implementation of a tomographic-based prototype using multiple plenoptic camera images of a plastic scintillator volume for volumetric dose measurements. Finally, a comparative analysis between the use of sandard versus plenoptic cameras in the context of emission computed tomography is carried out, leading to the discussion of potential future work needed to better define and quantify the tomographic contribution of each respective type of imaging system. Concretely, the generalized design workflow based on the innovative use of optical design software elaborated within the pages of this thesis both simplifies and decouples the phases of prototype development, offering increased flexibility in designing future clinical tools for 3D dosimetry. This work thus paves the way for developing next-generation measurement systems in volumetric scintillation dosimetry and other tomography-based imaging applications

Volume-resolved gas velocity and spray measurements in engine applications

The ability to visualize in-cylinder phenomena in a three-dimensional (3D) manner is critical to further understand the complex physical and chemical processes within internal-combustion (IC) engines. Recently, plenoptic imaging techniques have been introduced to engine studies because they enable 3D measurements using a promising and simple single-camera setup. The fundamental concept is to record both the origin and direction of each light ray into a single light-field image by inserting a micro-lens array in front of the photosensor. Therefore, a single image contains enough information to reconstruct the 3D volume. In this study, we present the implementation of a plenoptic technique that allows 3D measurements of fuel-spray structure, as well as three-dimensional, three-component (3D3C) particle tracking velocimetry (PTV) of engine in-cylinder air flow. Flow-spray interactions and the impact on the 3D geometry of fuel sprays were investigated with single-shot plenoptic imaging. Volume-illuminated fuel sprays from a multi-hole injector were examined in an optically accessible four-valve gasoline direct-injection engine. The impact of air flows during the intake and compression strokes on the shape of the fuel plumes could readily be observed for individual sprays without averaging. The air flow was measured in a free jet flow and a steady-state engine flow bench employing a 3D3C PTV algorithm that analyzed volume-resolved images taken with a plenoptic camera. Silicone seed oil droplets were added to the air flows and were illuminated by the volume-expanded beam of a double-pulsed laser. Mie scattering from the droplets was recorded by the plenoptic camera, which was operated in double-frame mode. Results from the 3D3C PTV measurements were compared to two-dimensional (2D) planar particle-image velocimetry (PIV) and demonstrate the capability of the 3D velocimetry approach, presently delivering averaged flow fields.This material is based upon work supported by the National Science Foundation under Grant No. CBET 1402707.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139882/1/Chen_Sick_AVL2016.pd

Compressed Sensing and Parallel Acquisition

Parallel acquisition systems arise in various applications in order to moderate problems caused by insufficient measurements in single-sensor systems. These systems allow simultaneous data acquisition in multiple sensors, thus alleviating such problems by providing more overall measurements. In this work we consider the combination of compressed sensing with parallel acquisition. We establish the theoretical improvements of such systems by providing recovery guarantees for which, subject to appropriate conditions, the number of measurements required per sensor decreases linearly with the total number of sensors. Throughout, we consider two different sampling scenarios -- distinct (corresponding to independent sampling in each sensor) and identical (corresponding to dependent sampling between sensors) -- and a general mathematical framework that allows for a wide range of sensing matrices (e.g., subgaussian random matrices, subsampled isometries, random convolutions and random Toeplitz matrices). We also consider not just the standard sparse signal model, but also the so-called sparse in levels signal model. This model includes both sparse and distributed signals and clustered sparse signals. As our results show, optimal recovery guarantees for both distinct and identical sampling are possible under much broader conditions on the so-called sensor profile matrices (which characterize environmental conditions between a source and the sensors) for the sparse in levels model than for the sparse model. To verify our recovery guarantees we provide numerical results showing phase transitions for a number of different multi-sensor environments.Comment: 43 pages, 4 figure

Boundary layer flashback of swirl flames

Flame flashback in the boundary layer of swirling flows is investigated experimentally in a model swirl combustor. The model combustor features a mixing tube with an axial swirler and an attached center body. The findings provide novel insight into the mechanism facilitating boundary layer flashback of swirl flames. Turbulent, lean-premixed flames of methane and hydrogen are studied at atmospheric pressure and bulk flow velocities up to 5 m/s. Hydrogen contents range from 0% to 95% and equivalence ratios range from 0.4 to 1. The focus in the present work is on the upstream flame propagation inside the mixing tube. Stereoscopic particle image velocimetry (PIV) is applied at kilohertz-rate to provide the time-resolved, three-component velocity field. The flame front is detected simultaneously based on the acquired Mie scattering images. Simultaneous high-speed chemiluminescence imaging provides the overall flame shape and global propagation direction. In addition to the planar measurements, a technique capable of detecting the instantaneous, time-resolved, 3D flame front topography is developed and applied successfully. Oil droplets, which vaporize in the preheat zone of the flame, serve as the marker for the flame front. The droplets are illuminated with a laser and imaged from four different views followed by a tomographic reconstruction to obtain the volumetric particle field. The velocity field in the unburnt gas is measured using tomographic PIV. The resulting data include the simultaneous 3D flame front and volumetric velocity field at 5 kHz. Flashback is found to occur in the form of large-scale, convex-shaped flame tongues, which swirl in the bulk flow direction as they propagate in the negative axial direction along the center body wall. Gas dilatation associated with the heat release imposes a blockage effect on the approach flow, which causes a 3D deflection of streamlines. As a result, a region of negative axial velocity forms along the leading side of the flame tongues, which facilitates flashback. These regions of negative axial velocity, already observed in previous studies, are shown to be the result of a predominantly swirling fluid motion as opposed to boundary layer separation or flow recirculation. The effect of hydrogen addition on flashback is investigated. Flashback occurs at significantly leaner conditions for hydrogen-rich flames, but the mechanism driving flashback is found to be independent of the hydrogen content for the conditions investigated in the present work. Quantitative differences in the flame-flow interaction between methane and hydrogen-rich flashbacks are discussed in detail.Aerospace Engineerin

Advanced Flame Monitoring and Emission Prediction through Digital Imaging and Spectrometry

This thesis describes the design, implementation and experimental evaluation of a prototype instrumentation system for burner condition monitoring and NOx emissions prediction on fossil-fuel-fired furnaces. A review of methodologies and technologies for burner condition monitoring and NOx emissions prediction is given, together with the discussions of existing problems and technical requirements in their applications. A technical strategy, incorporating digital imaging, UV-visible spectrum analysis and soft computing techniques, is proposed. Based on these techniques, a prototype flame imaging system is developed. The system consists mainly of an optical and fibre probe protected by water-air cooling jacket, a digital camera, a miniature spectrometer and a mini-motherboard with associated application software. Detailed system design, implementation, calibration and evaluation are reported. A number of flame characteristic parameters are extracted from flame images and spectral signals. Luminous and geometric parameters, temperature and oscillation frequency are collected through imaging, while flame radical information is collected by the spectrometer. These parameters are then used to construct a neural network model for the burner condition monitoring and NOx emission prediction. Extensive experimental work was conducted on a 120 MWth gas-fired heat recovery boiler to evaluate the performance of the prototype system and developed algorithms. Further tests were carried out on a 40 MWth coal-fired combustion test facility to investigate the production of NOx emissions and the burner performance. The results obtained demonstrate that an Artificial Neural Network using the above inputs has produced relative errors of around 3%, and maximum relative errors of 8% under real industrial conditions, even when predicting flame data from test conditions not disclosed to the network during the training procedure. This demonstrates that this off the shelf hardware with machine learning can be used as an online prediction method for NOx

Material Recognition Meets 3D Reconstruction : Novel Tools for Efficient, Automatic Acquisition Systems

For decades, the accurate acquisition of geometry and reflectance properties has represented one of the major objectives in computer vision and computer graphics with many applications in industry, entertainment and cultural heritage. Reproducing even the finest details of surface geometry and surface reflectance has become a ubiquitous prerequisite in visual prototyping, advertisement or digital preservation of objects. However, today's acquisition methods are typically designed for only a rather small range of material types. Furthermore, there is still a lack of accurate reconstruction methods for objects with a more complex surface reflectance behavior beyond diffuse reflectance. In addition to accurate acquisition techniques, the demand for creating large quantities of digital contents also pushes the focus towards fully automatic and highly efficient solutions that allow for masses of objects to be acquired as fast as possible. This thesis is dedicated to the investigation of basic components that allow an efficient, automatic acquisition process. We argue that such an efficient, automatic acquisition can be realized when material recognition "meets" 3D reconstruction and we will demonstrate that reliably recognizing the materials of the considered object allows a more efficient geometry acquisition. Therefore, the main objectives of this thesis are given by the development of novel, robust geometry acquisition techniques for surface materials beyond diffuse surface reflectance, and the development of novel, robust techniques for material recognition. In the context of 3D geometry acquisition, we introduce an improvement of structured light systems, which are capable of robustly acquiring objects ranging from diffuse surface reflectance to even specular surface reflectance with a sufficient diffuse component. We demonstrate that the resolution of the reconstruction can be increased significantly for multi-camera, multi-projector structured light systems by using overlappings of patterns that have been projected under different projector poses. As the reconstructions obtained by applying such triangulation-based techniques still contain high-frequency noise due to inaccurately localized correspondences established for images acquired under different viewpoints, we furthermore introduce a novel geometry acquisition technique that complements the structured light system with additional photometric normals and results in significantly more accurate reconstructions. In addition, we also present a novel method to acquire the 3D shape of mirroring objects with complex surface geometry. The aforementioned investigations on 3D reconstruction are accompanied by the development of novel tools for reliable material recognition which can be used in an initial step to recognize the present surface materials and, hence, to efficiently select the subsequently applied appropriate acquisition techniques based on these classified materials. In the scope of this thesis, we therefore focus on material recognition for scenarios with controlled illumination as given in lab environments as well as scenarios with natural illumination that are given in photographs of typical daily life scenes. Finally, based on the techniques developed in this thesis, we provide novel concepts towards efficient, automatic acquisition systems