Made available in DSpace on 2014-10-09T12:26:57Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:18Z (GMT). No. of bitstreams: 0 / Este estudo abordou os aspectos teóricos e experimentais relacionados ao desenvolvimento de um sistema de tomografia por coerência óptica (OCT) no domínio de Fourier sensível à polarização (PS-FD-OCT) e a sua utilização na determinação da Matriz de Mueller (MM-OCT). O estudo iniciou-se com uma revisão bibliográfica que abordou desde os primeiros estudos relacionados à técnica até o estado da arte, bem como o formalismo matemático da interferometria de baixa coerência no domínio espectral e polarização da luz. Realizaram-se estudos baseados em simulação numérica sobre três tipos diferentes de algoritmos, responsáveis pela recuperação do sinal de espalhamento, sendo eles: a Transformada de Fourier Direta; Interpolação e Zero-Filling. Ao fim dessa etapa concluiu-se que o algoritmo Zero-Filling 2N apresentou melhores características quando comparado aos outros algoritmos. Na parte experimental, primeiramente, diferentes arranjos OCT foram montados e medidas realizadas para verificação aspectos relacionados à teoria. Consecutivamente, utilizando uma amostra polimérica, realizaram-se imagens de birrefringência, que permitiram a determinação da birrefringência da amostra quantitativamente. Finalmente, imagens em diferentes estados de polarização foram realizadas, através delas determinou-se as imagens referentes aos elementos das Matrizes de Mueller, que foram analisadas individualmente. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

The underlying opportunity for this study is that process materials may show considerable change in their electrical properties in response to an injected signal over a wide frequency range. The use of this concept to demonstrate the construction of tomographic images for a range of frequency bands is described. These can then provide a deeper understanding and interpretation of a process under investigation. The thesis presents an in-depth review of the characteristics of the various wideband signals that could be used for simultaneous spectral measurements. This includes an objective selection process that demonstrates that a Chirp signal form offers key advantages. It then addresses the details of the developed method and algorithms for WElT systems that deploy a Chirp wideband excitation signal and a further aspect of the method, based on the time-frequency analysis, particularly wavelet transform, which is used to reveal spectral data sets. The method has been verified by simulation studies which are described. To provide measurements over a required frequency range a linear chirp is deployed as the excitation signal and corresponding peripheral measurements are synthesised using a 2D model. The measurements are then analysed using a wavelet transform algorithm to reveal spectral datasets which are exemplified in the thesis. The thesis then examines the feasibility of the presented method through various experimental trials; an overview of the implementation of the electronic system is included. This provides a single-channel EIT chirp excitation implementation, in essence simulating a real-time parallel data collection system. through the use of pseudo-static tests on foodstuff materials. The experimental data were then analysed and tomographic images reconstructed using the frequency banded data. These included results illustrate the promise of this composite approach in exploiting sensitivity to variations over a wide frequency range. They indicate that the described method can augment an EIT sensing procedure to support spectroscopic analysis of the process materials.

We consider the inverse source problem arising in thermo-and photoacoustic tomography. It consists in reconstructing the initial pressure from the boundary measurements of the acoustic wave. Our goal is to extend versatile time reversal techniques to the case when the boundary of the domain is perfectly reflecting, effectively turning the domain into a reverberant cavity. Standard time reversal works only if the solution of the direct problem decays in time, which does not happen in the setup we consider. We thus propose a novel time reversal technique with a nonstandard boundary condition. The error induced by this time reversal technique satisfies the wave equation with a dissipative boundary condition and, therefore, decays in time. For larger measurement times, this method yields a close approximation; for smaller times, the first approximation can be iteratively refined, resulting in a convergent Neumann series for the approximation.

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One of the biggest challenges facing geoscientists is the ability to accurately predict failure within a rock mass. Conventionally, numerical modeling is performed to predict the response of the rock mass due to excavation. However, numerical modeling relies heavily on the estimated physical characteristics of the rock mass. Unless dense, costly sampling of the rock mass has been performed, the results of the modeling are not robust. Seismic tomography offers a unique advantage of monitoring the rock mass response over conventional numerical modeling because it is able to measure the true alteration in response to excavation (Westman, 2003). This paper utilizes a tomographic inversion scheme using the Fast Marching Method for raypath tracing and the Simultaneous Iterative Reconstruction Technique to solve the p-wave velocity model of an underground mine and surrounding rock mass. The inversion scheme presented is tested using a data set from Creighton Mine in Sudbury, Ontario, Canada and includes 9,270 distinct events over 62 days. A total of 53 geophones recorded 191,856 p-waves that are able to be used for inversion. Temporal monitoring of the seismic p-wave velocity in the vicinity of a known Mw 2.6 fault-slip event that occurred on March 14th is performed by creating tomograms of the axial plane at the depth of the event and of an oblique plane where a dense distribution of events occurred including the March 14th event. Tomograms are produced on a weekly basis leading up to the event and also on a daily basis three days before the event. The weekly tomograms reveal a decrease in p-wave velocity in the vicinity of the Mw 2.6 event as time approaches the event and then a significant increase 1,600 ft/sec larger than the background velocity the week of the event. The daily tomograms reveal a 1,200 ft/sec velocity increase in the same area from March 13th to March 14th, however, no trends in the daily or weekly tomograms prior to the date of the March 14th event suggest the known event is imminent. / Master of Science / One of the biggest challenges facing geoscientists is the ability to accurately predict failure within a rock mass. Conventionally, numerical modeling is performed to predict the response of the rock mass due to excavation. However, numerical modeling relies heavily on the estimated physical characteristics of the rock mass. Unless dense, costly sampling of the rock mass has been performed, the results of the modeling are not robust. Seismic tomography offers a unique advantage of monitoring the rock mass response over conventional numerical modeling because it is able to measure the true alteration in response to excavation (Westman, 2003). This paper utilizes a tomographic inversion scheme using the Fast Marching Method for raypath tracing and the Simultaneous Iterative Reconstruction Technique to solve the p-wave velocity model of an underground mine and surrounding rock mass. The inversion scheme presented is tested using a data set from Creighton Mine in Sudbury, Ontario, Canada and includes 9,270 distinct events over 62 days. A total of 53 geophones recorded 191,856 p-waves that are able to be used for inversion. Temporal monitoring of the seismic p-wave velocity in the vicinity of a known M_w 2.6 fault-slip event that occurred on March 14th is performed by creating tomograms of the axial plane at the depth of the event and of an oblique plane where a dense distribution of events occurred including the March 14th event. Tomograms are produced on a weekly basis leading up to the event and also on a daily basis three days before the event. The weekly tomograms reveal a decrease in p-wave velocity in the vicinity of the M_w 2.6 event as time approaches the event and then a significant increase 1,600 ft/sec larger than the background velocity the week of the event. The daily tomograms reveal a 1,200 ft/sec velocity increase in the same area from March 13th to March 14th, however, no trends in the daily or weekly tomograms prior to the date of the March 14th event suggest the known event is imminent.

The current cardiac computed tomography (CT) technology is mainly limited by motion blurring and radiation dose. The conceptual multi-source interior CT scheme has provided a potential solution to reduce motion artifacts and radiation exposure. This dissertation work conducted multi-facet investigations on a novel multi-source interior CT architecture (G. Cao, et. al, IEEE Access, 2014;2:1263-71) which employs distributed stationary multi-beam Carbon-nanotube (CNT) X-ray sources and simultaneously operates multiple source-detector chains to improve temporal resolution. The collimation based interior CT is integrated in each imaging chain, to suppress radiation dose. The central thesis statement is: Compared to conventional CT design, this distributed source array based multi-source interior CT architecture shall provide ultra-fast CT scan of region-of-interest (ROI) inside body with comparable image quality at lower radiation dose. Comprehensive studies were conducted to separately investigate three critical aspects of multi-source interior CT: interior CT mode, X-ray scattering, and scatter correction methods. First, a single CNT X-ray source based interior micro-CT was constructed to serve as a down-scaled experimental verification platform for interior CT mode. Interior CT mode demonstrated comparable contrast-noise-ratio (CNR) and image structural similarity to the standard global CT mode, while inducing a significant radiation dose reduction (< 83.9%). Second, the data acquisition of multi-source interior CT was demonstrated at clinical geometry, via numerical simulation and physical experiments. The simultaneously operated source-detector chains induced significant X-ray forward / cross scattering and thus caused severe CNR reduction (< 68.5%) and CT number error (< 1122 HU). To address the scatter artifacts, a stationary beam-stopper-array (BSA) based and a source-trigger-sequence (STS) based scatter correction methods were proposed to enable the online scatter measurement / correction with further radiation dose reduction (< 50%). Moreover, a deterministic physics model was also developed to iteratively remove the scatter-artifacts in the multi-source interior CT, without the need for modifications in imaging hardware or protocols. The three proposed scatter correction methods improved CNR (< 94.0%) and suppressed CT number error (< 48 HU). With the dedicated scatter correction methods, the multi-source interior CT could provide ROI-oriented imaging with acceptable image quality at significantly reduced radiation dose. / Ph. D. / Cardiac computed tomography (CT) technology enables a non-invasive imaging examination of patients’ cardiovascular system, and thus it has been widely applied in the fields of the diagnosis, treatment, and scientific research of cardiovascular diseases (CVD). The image quality of the current cardiac CT is frequently degraded by rapid cardiac motion and X-ray scattering, and the potential radiation harm has also raised public concern. The above limitations could be theoretically overcome by a recently proposed conceptual stationary multi-beam X-ray sources based interior CT (i.e. multi-source interior CT) system architecture. This dissertation conducted a comprehensive investigation on the actual image quality and radiation dose of this conceptual CT system. The experiments demonstrated that a significant radiation dose reduction could be achieved in multi-source interior CT. The image quality of multi-source interior CT could be maintained with the in-housedeveloped scatter correction methods.

Structural characterization is essential to understand the formation mechanisms of the nanostructures in thin absorber layers in third generation solar cells and amyloid protein aggregates. Since to the dimension of the precipitated structures is in nanometer scale, electron tomography technique in transmission electron microscopy (TEM) has been applied as a major tool to analyze the 3D order and distribution of precipitates using the electron tomography technique.  Silicon rich silicon carbide (SRSC) films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique and annealed in the nitrogen atmosphere for 1 hour at 1100 °C. The spectrum-imaging (SI) technique in Energy filtered TEM (EFTEM) imaging mode was used to develop electron tomography. From the reconstructed sub-volumes, the complex, three dimensional interfacial nanostructure between the precipitated NPs and their parental matrix was observed and explained in terms of thermodynamic concepts. Additionally, the feasibility of raw data 4D electron tomography has been demonstrated using the EFTEM SI dataset. The aggregation process of the human islet amyloid polypeptide (hIAPP) has a great impact on human health. In this thesis, a model system has been taken to study the ultrastructure of the hIAPP aggregates that are present in the fat body tissue surrounding the brain of Drosophila melanogaster. Electron tomography technique on TEM revealed clear crystalline structures in 3D. For the first time, the presence of a 5-fold twinned structure in biology was discovered. An intriguing finding is the filament like interconnection of hIAPP protein granules observed predominantly along the nearest neighbor directions. This suggests the existence of the directional binding forces between two nearest protein granules in addition to dipole-dipole interactions.

Charged particle accelerators often have strict requirements on the beam energy, and timing to calibrate, or control background processes. Longitudinal Phase Space Tomography is a technique developed in 1987 to visualize the time, and energy coordinates of a beam. With non-invasive detectors, the beam can be visualized at any point during operation of a synchrotron. With the progress of computing power over the last 27 years, it is now possible to compute tomographic reconstructions in real time accelerator operations for many bunches around the accelerator ring. This thesis describes a real-time, multi-bunch tomography system developed and implemented in Fermilab's Main Injector and Recycler Rings, and a study of bunch growth when crossing transition. Implications of these studies for high intensity operation of the Fermilab accelerators are presented. / text

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