Elastography is a non-invasive medical imaging modality that is used as a diagnostic tool for the early detection of several pathological changes in soft tissues. Elastography techniques provide the local strain distributions experienced by soft tissues due to compression. The resulting strain images are called “elastograms”. In elastography, the local tissue strains are usually estimated as the gradient of local tissue displacement. The local tissue displacements are estimated from the time delays between gated pre- and post-compression echo signals. The quality of the resulting elastograms is highly dependent on the accuracy of these local displacement estimates. While several time delay estimation (TDE) techniques have been proposed for elastography applications, there is a lack of systematic study that statistically compares the performance of these techniques. This information could prove to be of great importance to improve currently employed elastographic clinical methods. This study investigates the performance of selected time delay estimators for elastography applications. Time delay estimators based on Generalized Cross Correlation (GCC), Sum of Squared Differences (SSD) and Sum of Absolute Differences (SAD) are proposed and implemented. Within the class of GCC algorithms, we further consider: an FFT-based cross correlation algorithm (GCC-FFT), a hybrid time-domain and frequency domain cross correlation algorithm with prior estimates (GCC-PE) and an algorithm based on the use of fractional Fourier transform to compute the cross correlation (GCC -FRFT) . Image quality factors of the elastograms obtained using the different TDE techniques are analyzed and the results are compared using standard statistical tools. The results of this research suggests that correlation based techniques outperform SSD and SAD techniques in terms of SNRe, CNRe, dynamic range and robustness. The sensitivity of GCC-FFT and SSD were statistically similar and statistically higher than those of all other methods. Within the class of GCC methods, there is no statistically significant difference between SNRe of GCC-FFT, GCC-PE and GCC –FRFT for most of the strain values considered in this study. However, in terms of CNRe, GCC-FFT and GCC-FRFT were significantly better than other TDE algorithms. Based on these results, it is concluded that correlation-based algorithms are the most effective in obtaining high quality elastograms.

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 16, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Roger Fales. Vita. Includes bibliographical references.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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No description available.

This thesis concerns delay-dependent robust stability analysis and stabilization for time-delay system with uncertainties. By choosing new Lyapunov-Krasovskii functional and using methods which can reduce conservativeness of stability condition in the literature, new delay-dependent sufficient stability conditions are obtained in terms of linear matrix inequality. It is shown that the new stability conditions can provide less conservative results than some existing ones. Furthermore, the new stability conditions are also used to design the state feedback controllers. Finally, numerical examples are given to show the derived results and compared with results in the literature.

Thesis (M.S.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 26-28). Also available on the Internet.

Thesis (M.S.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 26-28). Also available on the Internet.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Thesis (Ph.D.). / Written for the Dept. of Electrical and Computer Engineering. Title from title page of PDF (viewed 2008/02/12). Includes bibliographical references.