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

No description available.

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

Performance and stability of bilateral teleoperation control systems are adversely affected by variations in environment dynamics and time delay in communication channel. Prior relevant research in the literature has mainly yielded control algorithms that sacrifice performance in order to guarantee robust stability. In contrast, this thesis proposes methods to deal with these two main problems in order to maintain the stability without compromising performance. To handle changes in environment dynamics, a multiple model controller for teleoperation is introduced. It is assumed that the dynamics of the environment are governed by a model from a finite set of environment models at any given time with Markov chain switching between these models. The first-order generalized pseudo-Bayesian (GPB1) multi-model estimation technique is used to identify the effective model at each time step given the sensory observations. The control action is a weighted sum of mode-based control laws that are designed for each mode of operation. The second major problem in teleoperation systems that this thesis deals with is communication channel time delay. The constant time-delay problem is solved using two different methods, i.e. discrete-time and continuous-time predictive type Linear Quadratic Gaussian (LQG) controllers. The treatment of the problem in the discrete-time domain allows for the development of a finite dimension state-space model that explicitly encompasses the time delay. The robustness of the controller with respect to uncertainty in the system parameters is examined via Nyquist analysis. In continuous-time, a modified state transformation is proposed to obtain delay-free dynamics based on the original dynamics with delayed inputs and outputs. The application of the continuous-time LQG control synthesis to these reduced dynamics yields a control law that guarantees closed-loop stability and performance. Mode-based controllers are designed for each phase of operation, i.e. free motion/soft contact and contact with rigid environments. Performance objectives such as position tracking and tool impedance shaping for free motion/soft contact, as well as position and force tracking for contact with rigid environments are incorporated into the LQG control design framework. Simulation and experimental results are presented for each of the proposed controllers in various scenarios. These results demonstrate the effectiveness of the proposed methods in providing a stable transparent interface for teleoperation in free motion and in contact with rigid environments. / Thesis / Master of Applied Science (MASc)