Browsing by Subject "reconstruction"
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Item A Preliminary Reconstruction of the Yassiada Sixteenth-Century Ottoman Wreck(2010-07-14) Labbe, Matthew A.While excavating a late fourth-century Roman merchantman off the coast of Yassiada, Turkey in 1967, archaeologists discovered another, more recent wreck lying across the stern of the Roman wreck. The artifact assemblage, dendrochronology, and carbon-14 dating indicated that the wreck was of Ottoman origin and dated to the late sixteenth-century. In 1982 and 1983, archaeologists under the auspices of the Institute of Nautical Archaeology at Texas A&M University returned to the site to fully excavate the vessel and raise its timbers for detailed study and conservation at the Bodrum Museum of Underwater Archaeology in Turkey. The purpose of this thesis is to analyze the remains of the hull by building upon previous preliminary reconstruction efforts to determine the ship's intended form and function. To accomplish this task, 1:10 scale drawings of the timbers were used to construct a half breadth model of the ship. By matching the nail holes on the recovered planking to the preserved remains of the ship's framing, it was possible to assess the hull's contours through transfer to a lines drawing. The resulting drawings show a moderately sized vessel with a wide flat bottom. In order to place the reconstruction into perspective, archaeological remains of similar shipwrecks and period iconography were consulted in order to suggest the ship's type and function. Four shipwrecks were found that have similar construction features to those on the Ottoman wreck. Three of the wrecks had the same unusual knuckle joints used in securing futtocks to frames that the Ottoman wreck has, shedding light on design and construction philosophy of ships in the eastern Mediterranean. The preliminary analysis of period iconography in conjunction with the remains of similar shipwrecks indicated that the vessel was a cargo carrier that may have ties to the Ottoman navy. Four types of ships from the same general period, the felluca, polacre, and shebek were found to have similar design features to the Ottoman wreck, but the closest iconographic parallel was the saique, which was a two-masted cargo carrier found in the Black Sea and the west coast of Turkey between the sixteenth and eighteenth centuries.Item Evolution of the Galapagos Rise and the Bauer Microplate: implications for the Nazca plate(Texas A&M University, 2006-04-12) Wright, Jennifer Catherine McGuireAnalysis of the satellite altimetry based predicted bathymetry, magnetic anomalies, and earthquake seismic data relating to the geophysical structure on the Nazca plate indicates that the Galapagos Rise system served as the transitional spreading system between Pacific-Farallon spreading and the current East Pacific Rise (EPR) system. First order age/depth relationships for this area indicate that the Galapagos Rise, the most prominent extinct spreading system within the Nazca plate, accommodated most of the Pacific-Nazca plate separation from ~23 million years ago (Ma) to ~8 Ma. After this time, spreading was dominantly along the EPR, with probable ultra slow spreading along the Galapagos Rise continuing until very recent times (0-5 Ma). Magnetic lineations and depth trends across the Bauer Basin suggest that it was captured between the failing Galapagos Rise and the currently active EPR. Anomalously shallow ridge crests along the Galapagos Rise indicate that magmatic activity may have occurred until very recent time (0-5 Ma). Tightly curved (concave southward) fracture zones offsetting Galapagos Rise ridge segments indicate a pole of rotation at the present day position of ~22.5 S and ~99.5 W. The curvature of the fracture zones and the fan-shaped spreading pattern of seafloor structures produced at the Galapagos Rise indicate that the Galapagos Rise initiated parallel to the Menda??a fracture zone. Consistent with the rotation of the Nazca plate after the fragmentation of the Farallon plate, the Galapagos Rise rotated counterclockwise during its active phase. The Galapagos Rise initiated in the vicinity of Gallego fracture zones and propagated southward. Failure of the Galapagos Rise occurred as spreading jumped westward in stages to the East Pacific Rise.Item Imaging and Computational Methods for Exploring Sub-cellular Anatomy(2010-01-16) Mayerich, DavidThe ability to create large-scale high-resolution models of biological tissue provides an excellent opportunity for expanding our understanding of tissue structure and function. This is particularly important for brain tissue, where the majority of function occurs at the cellular and sub-cellular level. However, reconstructing tissue at sub-cellular resolution is a complex problem that requires new methods for imaging and data analysis. In this dissertation, I describe a prototype microscopy technique that can image large volumes of tissue at sub-cellular resolution. This method, known as Knife-Edge Scanning Microscopy (KESM), has an extremely high data rate and can capture large tissue samples in a reasonable time frame. We can therefore image complete systems of cells, such as whole small animal organs, in a matter of days. I then describe algorithms that I have developed to cope with large and complex data sets. These include methods for improving image quality, tracing filament networks, and constructing high-resolution anatomical models. These methods are highly parallel and designed to allow users to segment and visualize structures that are unique to high-throughput microscopy data. The resulting models of large-scale tissue structure provide much more detail than those created using standard imaging and segmentation techniques.Item Photoacoustic and thermoacoustic tomography: system development for biomedical applications(Texas A&M University, 2006-04-12) Ku, GengPhotoacoustic tomography (PAT), as well as thermoacoustic tomography (TAT), utilize electromagnetic radiation in its visible, near infrared, microwave, and radiofrequency forms, respectively, to induce acoustic waves in biological tissues for imaging purposes. Combining the advantages of both the high image contrast that results from electromagnetic absorption and the high resolution of ultrasound imaging, these new imaging modalities could be the next successful imaging techniques in biomedical applications. Basic research on PAT and TAT, and the relevant physics, is presented in Chapter I. In Chapter II, we investigate the imaging mechanisms of TAT in terms of signal generation, propagation and detection. We present a theoretical analysis as well as simulations of such imaging characteristics as contrast and resolution, accompanied by experimental results from phantom and tissue samples. In Chapter III, we discuss the further application of TAT to the imaging of biological tissues. The microwave absorption difference in normal and cancerous breast tissues, as well as its influence on thermoacoustic wave generation and the resulting transducer response, is investigated over a wide range of electromagnetic frequencies and depths of tumor locations. In Chapter IV, we describe the mechanism of PAT and the algorithm used for image reconstruction. Because of the broad bandwidth of the laser-induced ultrasonic waves and the limited bandwidth of the single transducer, multiple ultrasonic transducers, each with a different central frequency, are employed for simultaneous detection. Chapter V further demonstrates PAT??s ability to image vascular structures in biological tissue based on blood??s strong light absorption capability. The photoacoustic images of rat brain tumors in this study clearly reveal the angiogenesis that is associated with tumors. In Chapter VI, we report on further developing PAT to image deeply embedded optical heterogeneity in biological tissues. The improved imaging ability is attributed to better penetration by NIR light, the use of the optical contrast agent ICG (indocyanine green) and a new detection scheme of a circular scanning configuration. Deep penetrating PAT, which is based on a tissue??s intrinsic contrast using laser light of 532 nm green light and 1.06 ??m near infrared light, is also presented.Item Photoacoustic computed tomography in biological tissues: algorithms and breast imaging(Texas A&M University, 2004-11-15) Xu, MinghuaPhotoacoustic computed tomography (PAT) has great potential for application in the biomedical field. It best combines the high contrast of electromagnetic absorption and the high resolution of ultrasonic waves in biological tissues. In Chapter II, we present time-domain reconstruction algorithms for PAT. First, a formal reconstruction formula for arbitrary measurement geometry is presented. Then, we derive a universal and exact back-projection formula for three commonly used measurement geometries, including spherical, planar and cylindrical surfaces. We also find this back-projection formula can be extended to arbitrary measurement surfaces under certain conditions. A method to implement the back-projection algorithm is also given. Finally, numerical simulations are performed to demonstrate the performance of the back-projection formula. In Chapter III, we present a theoretical analysis of the spatial resolution of PAT for the first time. The three common geometries as well as other general cases are investigated. The point-spread functions (PSF's) related to the bandwidth and the sensing aperture of the detector are derived. Both the full-width-at-half-maximum of the PSF and the Rayleigh criterion are used to define the spatial resolution. In Chapter IV, we first present a theoretical analysis of spatial sampling in the PA measurement for three common geometries. Then, based on the sampling theorem, we propose an optimal sampling strategy for the PA measurement. Optimal spatial sampling periods for different geometries are derived. The aliasing effects on the PAT images are also discussed. Finally, we conduct numerical simulations to test the proposed optimal sampling strategy and also to demonstrate how the aliasing related to spatially discrete sampling affects the PAT image. In Chapter V, we first describe a prototype of the RF-induced PAT imaging system that we have built. Then, we present experiments of phantom samples as well as a preliminary study of breast imaging for cancer detection.Item Reconstruction in tomography with diffracting sources(Texas A&M University, 2005-02-17) Xu, YuanIn this dissertation, we first derive exact reconstuction algorithms for thermoacoustic tomography (TAT) and broadband diffraction tomography (a linearized inverse scattering problem) using derived time-reversal formulas. Then we focus on some important practical problems of TAT including the limited-view problem, the effects of acoustic heterogeneity, and fast reconstruction algorithms. In Chapter II, we propose time-reversal methods and apply them to tomography with diffrating sources. We first develop time-domain methods to time-reverse a transient scalar wave using only the field measured on an arbitrary closed surface enclosing the initial sources. Under certain conditions, a time-reversed field can be obtained with the delay-and-sum algorithm (backprojection to spheres) used in synthetic aperture imaging.Consequently, the physicalmeaningandthe validconditions of this widely used algorithm are revealed quantitatively for the first time from basic physics. Then exact reconstruction for TAT and broadband diffraction tomography is proposed by time-reversing the measured field back to the time when each source or secondary source is excited. The theoretical conclusions are supported by a numerical simulation ofthree-dimensional diffraction tomography.The extension ofour time-reversal methods to the case using Green function in a heterogeneous medium is also discussed. In Chapter III, the limited-view problem is studied for TAT. We define a "detection region," within which all points have sufficient detection views. It is explained analytically and shown numerically that the boundaries of any object inside this region can be recovered stably.Otherwise some sharp details become blurred.One can identify in advance the parts of the boundaries that will be affected if the detection view is insufficient. Computations are conducted for both numerically simulated and experimental data. The reconstructions confirm our theoretical predictions. In Chapter IV, the effects of wavefront distortions induced by acoustic heterogeneities in breast TAT are studied. Amplitude distortions are shown to be insignificant for different scales of acoustic heterogeneities. After that we consider the effects of phase distortions (errors in time-of-flight) in our numerical studies. The numerical results on the spreads of point sources and boundaries caused by the phase distortions are in good agreement with the proposed formula. We also demonstrate that the blurring of images can be compensated for by using the distribution of acoustic velocityin the tissues in the reconstructions. In Chapter V, we discuss exact and fast Fourier-domain reconstruction algorithms for TAT in planar and circular configurations.