• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1658
  • 1354
  • 301
  • 188
  • 89
  • 75
  • 52
  • 44
  • 29
  • 23
  • 15
  • 15
  • 15
  • 15
  • 15
  • Tagged with
  • 4534
  • 1728
  • 1295
  • 895
  • 753
  • 622
  • 622
  • 555
  • 540
  • 463
  • 463
  • 445
  • 431
  • 403
  • 400
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Volumetric and planar electrical capacitance tomography

Ye, Zhuoyi January 2015 (has links)
Electrical capacitance tomography (ECT) is a low cost and fast imaging technique able to obtain cross sectional images of dielectric permittivity distribution. ECT has been successfully used in industrial process tomography mainly for 2D imaging. One of the key challenges in 3D ECT imaging is a large scale forward problem arising with a large number of elements in the meshed ECT sensor model. Notably a complete sensor model will provide the most appropriate solution to the forward problem. A complete sensor model requires modelling a shielded area behind electrodes, which leads to increase in density of the finite element mesh. In this thesis, an approximation error model (AEM) has been applied to the ECT modelling for the first time. In addition to 3D AEM modelling and to further evaluate the effectiveness of the proposed AEM algorithm, it was implemented to compensate for uncertainty in electrode size and mesh density in 2D ECT. The results achieved using AEM are promising. In terms of application area, this thesis focuses on fundamental development for possible use of ECT in non-destructive evaluation (NDE) application. In more traditional industrial process application the object is surrounded by a number of electrodes on its boundary. In NDE applications a planar array ECT and volumetric imaging is needed. This thesis presents a 3D planar array ECT sensor using a 3D reconstruction algorithm. The results are validated with a number of experimental tests. 3D planar array ECT imaging was further extended to image both dielectric and metallic samples. To quantify the limitations of planar array ECT, a 3D ECT sensor and 3D ECT software have been implemented and used to evaluate the performance of the 3D ECT imaging with missing sides, with planar array ECT being the most extreme case of missing sides. The underlying inverse problem was analysed using singular value decomposition of the sensitivity matrix for the first time. This thesis introduces the use of a resolution matrix to analyse the performance of a 3D ECT reconstruction algorithm. These analysis methods, which enabled an in depth analysis of imaging performance with missing sides, are able to quantify the performance of planar array ECT.

Ionospheric delay correction for single-frequency receivers

Allain, Damien J. January 2009 (has links)
The majority of navigation satellite receivers operate on a single frequency and experience an error due to the ionospheric delay. They compensate for the ionospheric delay using an ionospheric model which typically only corrects for 50% of the delay. An alternative approach is to map the ionosphere with a network of real-time measurements, with either a thin shell approximation or a full 3D map. Here, a time-dependent 3D tomographic imaging technique is used to map the free electron density over the full-height of the ionosphere during solar maximum. The navigation solutions computed using corrections based upon models and thin-shell and full-height maps are compared in this project. The models and maps are used to calculate the excess propagation delay on the L1 frequency experienced by GPS receivers at selected locations across Europe and North America. The excess delay is applied to correct the pseudo-range single frequency observations at each location and the improvements to the resulting positioning are calculated. It is shown that the thin-shell and full-height maps perform almost as well as a dual-frequency carrier-smoothed benchmark and for most receivers better than the unfiltered dual-frequency benchmark. It is also shown that the unfiltered dual-frequency method is not reliable, which is of concern as it is a proposed upgrade to current positioning systems. The improvements in positioning accuracy vary from day to day depending on ionospheric conditions but can be up to 25m during mid-day at solar maximum conditions at European mid-latitudes. The full-height corrections perform well under all geomagnetic conditions and are considerably better than thin-shell corrections under extreme storm conditions. The transmission of the navigation correction requires a forecast, an image compression and a system of distribution across a local region. The feasibility of this is demonstrated for regions of land and near-land coastal regions across Europe.

Fast algorithm on tomography.

January 1997 (has links)
by Chun-pong Cheung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references (leaves 48-51). / Chapter Chapter 1 --- INTRODUCTION --- p.1 / Chapter §.1.1 --- Toeplitz and Circulant Matrix --- p.1 / Chapter §1.2 --- Conjugate Gradient Method --- p.5 / Chapter §1.3 --- Outline of the Thesis --- p.9 / Chapter Chapter 2 --- INVERSE PROBLEM --- p.11 / Chapter §2.1 --- Inverse Problem --- p.11 / Chapter §2.2 --- Tikhonov Regularization --- p.12 / Chapter Chapter 3 --- FAST ALGORITHM ON THERMAL TOMOGRAPHY --- p.14 / Chapter §3.1 --- Introduction --- p.15 / Chapter §3.2 --- Linearization --- p.15 / Chapter §3.3 --- Regularization by the Identity Operator --- p.17 / Chapter §3.4 --- Regularization by the Laplacian Operator --- p.18 / Chapter §3.5 --- Preconditioning with the Laplacian --- p.21 / Chapter Chapter 4 --- COMPUTERIZED TOMOGRAPHY SCAN --- p.27 / Chapter §4.1 --- Projection Problem --- p.27 / Chapter §4.2 --- Radon Transform --- p.29 / Chapter §4.3 --- Reformulation of Projection Problem --- p.31 / Chapter §4.4 --- Numerical Experiments --- p.35 / Chapter §4.5 --- Sirhplification of Formula --- p.41 / References --- p.48

Electrical impedance tomography at low frequencies.

Noor, Johan Andoyo Effendi, Physics, Faculty of Science, UNSW January 2007 (has links)
Most EIT machine operates at high frequencies above 10 kHz. Biological systems demonstrate dispersions of electrical impedance characteristics at very low frequencies below 2 kHz due to the presence of membrane surrounding the cells and diffusion polarisation effects. A study was made on the feasibility of the use of low frequencies in a range of 1.12 Hz to 4.55 kHz in EIT. One high frequency of 77.712 kHz similar to that normally used in common EIT was also used as a comparison. The impedance measurements employed a four-terminal method using the BULFIS, an ultra low frequency impedance spectrometer and used conducting and insulating material as the objects/phantoms. The results show that the conductance and capacitance of a metal object disperses at frequency range of 0.1 -10 kHz, which is consistent to the electrical properties of a double layer forming at the metal-electrolyte interface similar to the electrical properties of a membrane. The reconstructed images reveal that at low frequencies the conducting and the insulating bodies were indistinguishable. They appear differently at high frequencies above 4.55 kHz indicating that the use of multi frequency instrumentation in EIT covering the very low frequency range provides information that instrumentation restricted to frequencies above 10 kHz does not supply. While the internal structure of the double layers could not be delineated, the presence of the double layers could be readily detected by the behaviour of the images as the frequency was varied. This has potential for EIT because it might allow the detection of structures from the variation of the images with frequency. This variation with frequency does not occur at the higher frequencies more usually used for EIT.

Tomographic reconstruction of shock layer flows

Faletic, Rado, Rado.Faletic@anu.edu.au January 2005 (has links)
The tomographic reconstruction of hypersonic flows faces two challenges. Firstly, techniques used in the past, such as the Direct Fourier Method (DFM) or various backprojection techniques, have only been able to reconstruct areas of the flow which are upstream of any opaque objects, such as a model. Secondly, shock waves create sharp discontinuities in flow properties, which can be difficult to reconstruct both in position and in magnitude. This thesis will present a reconstruction method, utilising geometric ray-tracing and a sparse matrix iterative solver, which is capable of overcoming both of these challenges. It will be shown, through testing with phantom objects described in imaging and tomographic literature, that the results are comparable to those produced by the DFM technique. Finally, the method will be used to reconstruct three dimensional density fields from interferometric shock tunnel images, with good resolution.

Theory and application methods of time domain reflectometry/time domain transmission computed tomography (TDR/TDT CT)

Li, Jian. January 2007 (has links)
Thesis ()--University of Delaware, 2007. / Principal faculty advisor: Robert G. Hunsperger, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Seismic constraints on structure beneath hotspots : earthquake tomography & finite frequency tomography approaches /

Yang, Ting. January 2006 (has links)
Thesis (Ph. D.)--University of Rhode Island, 2006. / Typescript. Includes bibliographical references (leaves 131-150).

Time-Domain Polarization-Sensitive Optical Coherence Tomography in Soft Biological Tissue

Todorovic, Milos 15 May 2009 (has links)
A new, high-speed, fiber-based Mueller-matrix optical coherence tomography system with continuous source-polarization modulation is presented. For in vivo experimentation, the sample arm optics are integrated into a hand-held probe. The system’s parameters were verified through imaging standard optical elements. A unique feature of polarization-sensitive Mueller optical coherence tomography is that by measuring Jones or Mueller matrices, it can reveal the complete polarization properties of biological samples, even in the presence of diattenuation. Presented here for the first time are mapped local polarization properties of biological samples obtained by using polar decomposition in combination with least-squares fitting to differentiate measured integrated Jones matrices with respect to depth. In addition, a new concept of dual attenuation coefficients to characterize diattenuation per unit infinitesimal length in tissues is introduced. The algorithm was experimentally verified using measurements of a section of porcine tendon and the septum of a rat heart. The application of the system for burn imaging and healing monitoring was demonstrated on porcine skin because of its similarity to the human skin. The results showed a clear localization of the thermally damaged region. The local birefringence of the intravital porcine skin was mapped by using a differentiation algorithm. The burn areas in the OCT images agree well with the histology, thus demonstrating the system’s potential for burn-depth determination and post-injury healing monitoring. Another major application of the fiber-based Mueller-matrix optical coherence tomography system with continuous source-polarization modulation covered here is in vivo imaging of early stages of skin cancer. The OCT images of SENCAR mice skin affected by the tumorigenesis show the structural changes in skin resulting from precancerous papilloma formations that are consistent with histology, which proves the system’s potential for early skin cancer detection.

Scattering correction and image restoration in neutron radiography and computed tomography

Abdelrahman, Magdy Shehata. January 2000 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Development and analysis of image reconstruction algorithms in diffraction tomography /

Anastasio, Mark Anthony. January 2001 (has links)
Thesis (Ph. D.)--University of Chicago, Department of Radiology, June 2001. / Includes bibliographical references. Also available on the Internet.

Page generated in 0.0449 seconds