Browsing by Subject "Tomography"
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Item A high-speed system for three-dimensional X-ray and neutron computed tomography(Texas Tech University, 1999-05) Davis, Anthony WayneComputed tomography for nondestmctive evaluation applications has been limited by system cost, resolution, and time requirements for three-dimensional data sets. FlashCT (Flat panel Amorphous Silicon High resolution Computed Tomography) is a system developed at Los Alamos National Laboratory to address these three problems. Developed around a flat panel amorphous silicon detector array, FlashCT is suitable for high energy x-ray and neutron computed tomography at 127 micron resolution. For objects smaller than 8 inches in any dimension, the system is capable of generating 360 views to create a high resolution three-dimensional tomographic dataset in less than 40 minutes, many times faster than with conventional linear detector array systems. Overall system size is small, allowing rapid transportation to a variety of radiographic sources. During system development, issues including integration time adjustment, exposure monitoring, and detector flaw correction were addressed to provide high quality output images suitable for later reconstmction into two- or three-dimensional density maps. System control software was developed in Lab VIEW for Windows NT to allow multithreading of data acquisition, data correction, and staging motor control. The system control software simplifies data collection and allows fiilly automated control of the data acquisition process, leading toward remote or unattended operation. The custom data processing software provides a simple graphical interface to control the calibration, filtering, and reconstmction of the acquired data.Item Advanced Array Imaging for Breast and Prostate Sonography(2010-05-14) Vaidyanathan, Ravi Shankar; Lewis, Matthew Allen[Abstract from Thesis “Introduction.”] In conventional medical ultrasound such as B-mode imaging, the amplitude of the backscattered ultrasound pulse is used to image tissues along a fixed beam direction1. This imaging technique works best in static organs, and it is difficult to image moving organs like the heart. The M-mode imaging technique is better for cardiac applications. For better image resolution, ultrasound tomography systems were developed in which ultrasound data were acquired by transducers placed in a circle around the object2. This task of deriving the structure of the object from scattered radiation is known as the inverse scattering problem. The inverse scattering problem is known by several names like reflectivity tomography3 and diffraction tomography5, 6, 7 etc. Scattering refers to the effects on wave propagation due to an inhomogeneous medium. Since the inhomogenieties are unknown, the goal is to determine their properties – the spatial variation in density, compressibility, geometrical distribution etc. With the scattered wave field, determining the scatterer is called the inverse problem. As for the geometry of the scattering theory, the scatterer is assumed to be present in a homogeneous reference medium with known properties. Following the notations used in Lehman8, the acoustic pressure, p, in this medium satisfies the Helmholtz equation (2 + k2) p(r) = 0 where the pressure field is given by p(r,t)=p0+p1(r,t) The ambient pressure, p0 is constant. Since the scatterer is present in the reference homogeneous medium, the pressure field can be written as p0(r) = pinc(r) + psc(r) where pinc refers to the incident field and psc is the scattered field. In an ideal situation the incident pressure field is taken as a plane wave pinc(r) = p0 eikz where k is the complex wave number which is given by k=(/c) (1- iM) where M is the compressional viscosity. Now, we are in a position to introduce the integral representations of the scattered field. In the region exterior to the scatterer, the pressure field is given by (2 + k2) p0(r) = 0 Introducing the Green’s function G(r – r’) = eik|r-r’|/|r-r’| that will satisfy the inhomogeneous impulse equation (2 + k2) G(r – r’) = -4(r-r’) Using one of the most frequently used approximations, the Rayleigh-Born approximation we can modify equation (7). At large distance the Green‟s function can be approximated by G(r – r’) ~ eikr/r e-ikr.r’ which holds true for k0r‟2/r <<1. A Fourier diffraction theorem based reconstruction technique using the Born approximation is derived in Radial Reflection Diffraction Tomography (RRDT) 8. Though my work is concerned with time-domain reconstruction techniques, I will discuss some existing frequency domain reconstruction techniques.Item Development of a neutron radiography and computed tomography system at a university research reactor(2006-05) Haas, Derek Anderson, 1981-; Biegalski, Steven R.Neutron radiography is a non-destructive analysis tool that complements X-ray transmission radiography. The use of neutrons provides the ability to image the interior of an object that has a metal core of steel or lead that would shield the interior from X-ray inspection. Neutron tomography is the use of a set of images of a single sample taken at various angles to produce a three dimensional rendition of the sample that greatly increases the effectiveness of neutron radiography as a non-destructive testing tool. A neutron radiography and tomography system has been built at the 1.1 MW TRIGA Mark II nuclear research reactor at The University of Texas at Austin in the Nuclear Engineering Teaching Lab. The Texas Neutron Imaging Facility is located on beam port five of the reactor and is housed in a shielding cave made of concrete to minimize radiation dose to users. The system itself integrates a sample positioning system and neutron sensitive camera through the use of a control code written in National Instruments Labview software. The code was written to increase the efficiency of the imaging process and to provide flexibility in the system. Precise sample positioning and timing of image acquisition provided by the code allows for the collection of data that can be used in computed tomography. The system has produced results in the form of radiographs and 3-D reconstructions of sample objects.Item Evaluation of X-ray imaging to investigate hydraulic performance of vapor-liquid contactors(2001) Schmit, Carolyn Elizabeth; Bonnecaze, R. T. (Roger T.); Eldridge, Robert BruceItem Hydraulic characterization of structured packing via x-ray computed tomography(2006) Green, Christian Wayne; Eldridge, R. Bruce; Allen, David T.Item Longitudinal phase space tomography of charged particle beams(2014-08) Evans, Nicholas John; Kopp, SachaCharged 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.Item Mapping the Rivera and Cocos subduction zone(2013-12) Suhardja, Sandy Kurniawan; Grand, Stephen P.The crust and upper mantle seismic structure beneath southwestern Mexico was investigated using several techniques including teleseismic tomography using 3D raytracing, a joint tomographic inversion of teleseismic and regional data that included relocation of regional seismicity, and a P to S converted wave study. The data used in these studies came from a broadband seismic deployment called MARS. The seismic deployment lasted 1.5 years from January 2006 to June 2007 and the stations covered much of Jalisco and Colima states as well as the western part of Michoacan states. At depth less than 50 km, P-wave receiver function images show a clear dipping slow velocity anomaly above a fast velocity layer. The slow anomaly convertor seen in receiver functions is directly above a fast dipping seismic anomaly seen in regional tomography results. The slow velocity with high Vp/Vs ratio is interpreted as a high pore fluid pressure zone within the upper layer of subducting oceanic crust. Regional seismicity was located using the double difference technique and then relocated in a tomography inversion. The seismicity is located very close to the slow dipping boundary to depths of 30-35 km and thus along the plate interface between the subducted and overlying plate. Deeper events are below the slow layer and thus are intraplate. Receiver function results also show a weaker continental Moho signal above the dipping slab that I interpret as a region of mantle serpentinization in the mantle wedge. Inland of the subduction zone, a clear Moho is observed with a maximum thickness of near 42 km although it thins to near 36 km depth towards the north approaching the Tepic-Zacoalco Rift. Using H-K analysis to examine Vp/Vs ratios in the crust, I find a band of very high Vp/Vs along the Jalisco Volcanic lineament as well as beneath the Michoacan-Guanajuato volcanic field. These observations suggest the continental crust is warm and possibly partially molten over broad areas associated with these two magmatic regions and not just locally beneath the volcanoes. I also found seismicity associated with the Jalisco Volcanic Lineament but it was trenchward of the volcanoes. This may indicate extension in this region is part of the explanation for this magmatic activity. At depths below 100 km, the tomography results show clear fast anomalies, about 0.3 km/s faster than the reference model, dipping to the northeast that I interpret as the subducting Rivera and Cocos plates. Tomography models show that the Rivera slab is dipping much steeper than the Cocos plate at depth. Below 150 km depth, the Rivera plate shows an almost vertical dip supporting the interpretation that the slab has steepened through time beneath Jalisco leading to a coastward migration of young volcanism with mixed geochemical signatures. The location of the young volcanism of the Jalisco Volcanic Lineament is just at the edge of the steeply dipping slab seen in the tomography. The magmatism is thus likely a nascent arc. The models also display evidence of a gap between the Rivera and Cocos plates that increases in width with depth marking the boundary between the two plates. The gap lies just to the west of Colima graben and allows asthenosphere to rise above the plates feeding Colima volcano. Another interesting finding from this study is a possibility of a slab tear along the western edge of the Cocos plate at a depth of about 50 km extending 60 km horizontally. The tear is coincident with a lack of seismicity in this region although there are events below and above the tear.Item Properties of Some Integral Transforms Arising in Tomography(2013-10-11) Moon, SunghwanThis dissertation deals with several types of imaging: radio tomography, single scattering optical tomography, photoacoustic tomography, and Compton camera imaging. Each of these tomographic techniques leads to a Radon-type transform: radio tomography brings about an elliptical Radon transform, single scattering optical tomography reduces to the V-line Radon transform, and photoacoustic tomography with line detectors boils down to a cylindrical Radon transform. We also introduce a different Radon-type transform arising in photo acoustic tomography with circular detectors, and study mathematically similar object, a toroidal Radon transform. We also consider the cone transform arising in Compton camera imaging as well as the windowed ray transform. We provide inversion formulas for all these transforms. When given some Radon- type transform, we are interested not only in inversion formulas, but also in range conditions, and stability. We thus address range conditions, a stability estimate for some of the Radon-type transforms above.Item Scattering correction and image restoration in neutron radiography and computed tomography(2000-08) Abdelrahman, Magdy Shehata; Abdurrahman, Naeem M.Neutron radiography and computed tomography are nondestructive imaging techniques for the assessment of internal structure of objects. Neutron scattering in such objects can cause image degradation and complicate image interpretation. The removal of the scattering effect is one of the most challenging problems in neutron imaging. In this work, a new method for scattering correction is being developed. Experimental measurements and Monte Carlo simulations were used to investigate the effect of thermal and fast neutron scattering on neutron image degradation both qualitatively and quantitatively. Neutron scattering degrades the quality of radiographs and in cases of severe scattering, could blur sharp edges in neutron images. In addition, neutron scattering imposes an error in neutron radiography quantitative measurements. In this study, scattering correction was approached in two different ways. The first consisted of image restoration using the Slow Evolution from the Continuation Boundary (SECB) method. The SECB method was investigated and implemented to deblur neutron images for such cases when neutron scattering effect is severe enough to blur produced radiographs. The SECB method is a noniterative linear image deblurring method based on the slow evolution constraint, which is highly effective in suppressing noise amplification. The second approach for a scattering correction, which has been developed for the first time as part of this study, is based on the conjecture that there exists a correlation between the pattern of scattered neutrons as observed from a given side of the object as the object is irradiated from different sides. This suggests rotating the sample with some angle to clear the direct neutron view and obtain an image of pure scattering. The correlation between this side image and the scattering component of the forward image could be used to obtain an estimate of the forward scattering component. The estimated scattering component would then be subtracted from the degraded image to get a scattering-free image. Data manipulation of the scattering side-images was used to correlate the scattering side-image to the forward scattering component utilizing the scattering information outside of the neutron beam scope. Another approach was to implement artificial neural networks to capture the correlations between scattering side-images and the forward scattering components as obtained from numerical simulations for typical samples and utilize these networks to get an estimate of the forward scattering component for the object of interest.Item Seismic imaging and velocity model building with the linearized eikonal equation and upwind finite-differences(2014-05) Li, Siwei, 1987-; Fomel, Sergey B.Ray theory plays an important role in seismic imaging and velocity model building. Although rays are the high-frequency asymptotic solutions of the wave equation and therefore do not usually capture all details of the wave physics, they provide a convenient and effective tool for a wide range of geophysical applications. Especially, ray theory gives rise to traveltimes. Even though wave-based methods for imaging and model building had attracted significant attentions in recent years, traveltime-based methods are still indispensable and should be further developed for improved accuracy and efficiency. Moreover, there are possibilities for new ray theoretical methods that might address the difficulties faced by conventional traveltime-based approaches. My thesis consists of mainly four parts. In the first part, starting from the linearized eikonal equation, I derive and implement a set of linear operators by upwind finite differences. These operators are not only consistent with fast-marching eikonal solver that I use for traveltime computation but also computationally efficient. They are fundamental elements in the numerical implementations of my other works. Next, I investigate feasibility of using the double-square-root eikonal equation for near surface first-break traveltime tomography. Compared with traditional eikonal-based approach, where the gradient in its adjoint-state tomography neglects information along the shot dimension, my method handles all shots together. I show that the double-square-root eikonal equation can be solved efficiently by a causal discretization scheme. The associated adjoint-state tomography is then realized by linearization and upwind finite-differences. My implementation does not need adjoint state as an intermediate parameter for the gradient and therefore the overall cost for one linearization update is relatively inexpensive. Numerical examples demonstrate stable and fast convergence of the proposed method. Then, I develop a strategy for compressing traveltime tables in Kirchhoff depth migration. The method is based on differentiating the eikonal equation in the source position, which can be easily implemented along with the fast-marching method. The resulting eikonal-based traveltime source-derivative relies on solving a version of the linearized eikonal equation, which is carried out by the upwind finite-differences operator. The source-derivative enables an accurate Hermite interpolation. I also show how the method can be straightforwardly integrated in anti-aliasing and Kirchhoff redatuming. Finally, I revisit the classical problem of time-to-depth conversion. In the presence of lateral velocity variations, the conversion requires recovering geometrical spreading of the image rays. I recast the governing ill-posed problem in an optimization framework and solve it iteratively. Several upwind finite-differences linear operators are combined to implement the algorithm. The major advantage of my optimization-based time-to-depth conversion is its numerical stability. Synthetic and field data examples demonstrate practical applicability of the new approach.Item Spherical radon transforms and mathematical problems of thermoacoustic tomography(2009-06-02) Ambartsoumian, GaikThe spherical Radon transform (SRT) integrates a function over the set of all spheres with a given set of centers. Such transforms play an important role in some newly developing types of tomography as well as in several areas of mathematics including approximation theory, integral geometry, inverse problems for PDEs, etc. In Chapter I we give a brief description of thermoacoustic tomography (TAT or TCT) and introduce the SRT. In Chapter II we consider the injectivity problem for SRT. A major breakthrough in the 2D case was made several years ago by M. Agranovsky and E. T. Quinto. Their techniques involved microlocal analysis and known geometric properties of zeros of harmonic polynomials in the plane. Since then there has been an active search for alternative methods, which would be less restrictive in more general situations. We provide some new results obtained by PDE techniques that essentially involve only the finite speed of propagation and domain dependence for the wave equation. In Chapter III we consider the transform that integrates a function supported in the unit disk on the plane over circles centered at the boundary of this disk. As is common for transforms of the Radon type, its range has an in finite co-dimension in standard function spaces. Range descriptions for such transforms are known to be very important for computed tomography, for instance when dealing with incomplete data, error correction, and other issues. A complete range description for the circular Radon transform is obtained. In Chapter IV we investigate implementation of the recently discovered exact backprojection type inversion formulas for the case of spherical acquisition in 3D and approximate inversion formulas in 2D. A numerical simulation of the data acquisition with subsequent reconstructions is made for the Defrise phantom as well as for some other phantoms. Both full and partial scan situations are considered.Item Study of the photon propagation paradox(2012-05) West, Amy; Grave de Peralta, Luis; Bernussi, Ayrton A.Surface Plasmon Polaritions(SPP) are electromagnetic waves that travel in the interface of a metal and dielectric and are coupled to collective oscillation of free electrons. SPP tomography is an imaging technique that permits the study of the interference features formed at the metal-air interface of a sample due to the superposition of SPP beams, with high fidelity. We present an experimental study on the formation of SPP standing waves. We demonstrate that SPP tomography in a quantum eraser arrangement permits the detection of photons passing through the dark fringes of the observed interference pattern. We present a comprehensive explanation of how photons could pass undetected across the regions occupied by the dark fringes.Item Tomographic imaging techniques using broadband polarized light for tissue diagnostics(2005) Kim, Eunha; Milner, Thomas E.Item Volumetric segmentation via three-dimensional active shape models(Texas Tech University, 2002-05) Dickens, Molly M.A volumetric image segmentation algorithm has been developed and implemented by extending a 2D algorithm based on Active Shape Models. The new technique allows segmentation of 3D objects that are embedded within volumetric image data. The extension from 2D involved four components: landmarking, shape modeling, gray-level modeling, and segmentation. Algorithms and software tools have been implemented to allow a user to efficiently landmark a 3D object training set. Additional tools were built that subsequently generate models of 3D object shape and gray-level appearance based on this training data. An object segmentation strategy was implemented that optimizes these models to segment a previously unseen instance of the object. The nature of volumetric images required the development of tools to visualize the features of the models within the volumes. Results of this new 3D segmentation algorithm have been generated for synthetic as well as x-ray CT volumetric image data.Item X-ray microtomographic image analysis for identification of cotton contaminants(Texas Tech University, 2002-08) Pai, Ajay STechnologies currently used for cotton contaminant assessment suffer from some fundamental limitations. These limitations severely restrict the ability of existing technologies to accurately detect and classify contaminants in cotton. Such inaccuracies result in the misassessment of the cotton quality, and have a serious impact on its economic value. The fundamental limitations of existing methods include the inability to detect contaminants under the surface of cotton, the inability to accurately measure shapes and sizes, sample preparation requirements, and poor spatial resolution. These limitations may be easily overcome by the use of x-ray tomographic imaging, which allows for highly accurate imaging of the internal features of an object in a non-destructive fashion. This thesis describes in detail the design of a GUI based interactive cotton contaminant analysis tool. Through the use of an x-ray microtomographic scanner and image processing algorithms, it is shown that x-ray tomographic imaging can provide very accurate information regarding shape, size, and density of cotton contaminants. This information has been analyzed using a fuzzy-logic-based classification scheme to create a highly accurate contaminant analysis tool. Despite its obvious advantages, x-ray imaging does have some drawbacks, principle among which pertains to the time taken to perform the procedure. These drawbacks, along with possible solutions have also been discussed in this thesis. It is our firm belief, however, that if realized in real-time, this procedure will have a definite impact on the cotton cleaning process, and indeed on the economic value of cotton.