Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2008. / Committee Chair: Zhigang Peng; Committee Member: Andrew V. Newman; Committee Member: Leland T. Long

Thesis (Ph. D.)--University of California, San Diego, 1997. / Vita. Includes bibliographical references (leaves 230-237).

A primary purpose of elasticity imaging, commonly known as elastography, is to extract the viscoelastic properties of a medium (including soft tissue) from the displacement caused by a stress field. Dynamic elastography methods that use the acoustic radiation force of ultrasound have several advantages, such as, non-invasiveness, low cost, and ability to produce a highly localized force field. A method for remotely generating localized low frequency shear waves in soft tissue is investigated, by using the modulated radiation force resulting from two intersecting quasi-CW confocal ultrasound beams of slightly different frequencies. In contrast to most radiation force-based methods previously presented, such shear waves are narrowband rather than broadband. As they propagate within a viscoelastic medium, different frequency-dependent effects will not significantly affect their spectrum, thereby providing a means for measuring the shear attenuation and speed as a function of frequency. Furthermore, to improve the detection signal-to-noise-ratio (SNR), increased acoustic pressure conditions may be needed, causing higher harmonics to be generated due to nonlinear propagation effects. Shear-wave propagation at harmonic modulation frequencies does not appear to have been previously discussed in the elastography literature. The properties of the narrowband shear wave propagation in soft tissue are studied by using the Voigt viscoelastic model and Green’s functions. In particular, the manner in which the characteristics of the viscoelastic medium affect their evolution under both low-amplitude (linear) and high-amplitude (nonlinear) source excitation and conditions that conform to human safety standards. It is shown that an exact solution of the viscoelastic Green’s function is needed to properly represent the propagation in higher-viscosity media, such as soft tissue, at frequencies much beyond a few hundred hertz. Methods for estimating the shear modulus and viscosity in viscoelastic media are developed based on both the fundamental and harmonic shear components.

L'interaction onde-matière a toujours été un sujet d'étude en Physique, c’est le cas de la propagation des ondes élastiques dans le corps humain qu’a conduit à plusieurs modalités d'imagerie. En particulier, les techniques d'elastographie reposent sur l'utilisation des ondes de cisaillement pour obtenir une image élastique des tissus mous. Dans ce contexte, cette thèse présente une étude des différentes techniques d'élastographie, en prêtant particulier attention aux aspects plus fondamentaux comme à ces potentielles applications.Tout d'abord, cette thèse montre que l'élastographie impulsionnelle unidimensionnelle (1D) peut être utilisée pour évaluer l'élasticité des couches de tissue d'épaisseur inférieure à la longueur d'onde utilisée. A cet effet, des simulations et des expériences ont été réalisées avec différents fantômes formés par une couche mince immergée dans un milieu d'élasticité différente. La concordance entre expériences et simulations, ainsi que le valeur de l'élasticité obtenue par élastographie 1D et le valeur de l'élasticité intrinsèque de la couche permettent de valider cette technique. Au même temps ces résultats ont été comparés avec ceux obtenus par la technique de Supersonic Shear Imaging (SSI), où l'onde est guidée le long de la plaque. On ajustant la courbe de dispersion expérimentale obtenue par SSI avec un modèle de Lamb, l'élasticité intrinsèque de chaque plaque est estimée. Les résultats obtenus par élastographie 1D et SSI montrent un bon accord entre eux. Le principal avantage de l'élastographie 1D est qu’il n'est pas nécessaire d'utiliser un modèle pour estimer l'élasticité de la plaque. Deuxièmement, deux nouvelles modalités d'imagerie quantitative pour l'extraction de élasticité des tissus mou à partir d'un champ élastique complexe sont approfondies: l'Elastographie par Retournement Temporel et le filtre inverse passif. Le but de ces deux techniques est d'estimer localement l'élasticité des tissus, par la mesure de la taille de la tâche focale dans une expérience virtuelle de retournement temporel avec des ondes de cisaillement. A partir de l'étude du processus de retournement temporel dans les solides mous, la faisabilité de ces deux techniques est démontrée in vitro dans des échantillons "bi-couche" et in vivo dans le foie et les muscles, en utilisant le bruit physiologique naturel crée par l'activité cardiaque et musculaire. L'efficacité de l'élastographie par retournement temporel diminue dans le cas d'un champ diffus non isotrope. L'emploie du filtre inverse adaptée à une configuration de source de bruit, permet de rétablir l'isotropie du champ et d'améliorer la résolution pour la détection de petites inclusions. Le filtre inverse passif permet, de surcroît, de contrôler la fréquence qui domine le champ de retournement temporel. Ceci est exploité, dans la dernière partie du manuscrit, pour mener la première expérience de spectroscopie passive en volume. Deux situations sont envisagées: la dispersion due à la propagation d'ondes guidées dans des plaques minces et la dispersion des ondes due à la viscosité. / The interaction between wave and matter has long been studied in Physics. In particular, regarding medical applications, wave propagation through the human body resulted in several imaging modalities, each of which uses a specific type of wave linked to a given physical property. The elasticity of soft biological tissues is directly linked to its shear wave speed. Thus, in Elastography, shear waves are tracked for non-invasive assessment of the mechanical properties of soft tissues. In this context, this thesis proposes a study of different elastography techniques from a basic point of view, as well as from its potential applications. Firstly, in this manuscript, the use of 1D transient elastography for the quantitative elasticity assessment of thin layered soft tissues is proposed. Experiments on three phantoms with different elasticities and plate thicknesses were performed. Experimental shear wave speed estimations inside the plate were obtained and validated with finite difference simulation. In addition, the Supersonic Shear Imaging (SSI) technique was performed. For the SSI technique, the propagating wave inside the plate is guided as a Lamb wave. Experimental SSI dispersion curves were fitted using a generalized Lamb model to retrieve the plate bulk shear wave speed. Finally both techniques resulted in similar shear wave speed estimations. The main advantage of 1D transient elastography is that the bulk shear wave speed can be directly retrieved from a time of flight measurement without requiring a dispersion model. Secondly, throughout this thesis, two novel quantitative imaging modalities for extracting the soft tissue's elasticity from a complex reverberated diffuse elastic field are deepen: Time Reversal Elastography (TRE) and the passive inverse filter. The goal of both techniques is to locally estimate the tissue's elasticity, by measuring the focal spot size in a virtual time reversal experiment involving shear waves. By studying the Physics of a time reversal process in soft solids, the feasibility of both techniques as a quantitative imaging techniques is demonstrated in vitro in bi-layer phantoms and in vivo in the liver-belly muscle, by using the physiological noise due to heartbeats and muscular activity. The efficiency of TRE decreases in the presence of a non-isotropic diffuse field. The use of the inverse filter adapted to a passive source configuration, restores the isotropy of the field. As a consequence, the resolution of the elasticity images is improved, leading to a better detection of small inclusions. In addition, the passive inverse filter allows to control the frequency dominating the time reversed field. This is exploited in the last part of the manuscript to conduct the first passive wave spectroscopy experiment in the volume of a soft solid. Two situations are considered: dispersion due to guided wave propagation in thin plates and wave dispersion due to viscosity effects.

A medição das propriedades viscoelásticas de líquidos por ultrassom tem sido um tema de pesquisa importante desde meados do século XX, principalmente, pela possibilidade de realizar medições em tempo real da viscosidade. O método de medição baseia-se na determinação do coeficiente de reflexão complexo (magnitude e fase) de ondas de cisalhamento refletidas da interface definida entre um sólido e uma amostra líquida. Enquanto a medição da magnitude é relativamente simples e precisa, a medição da fase é muito difícil devido à forte dependência da temperatura, que gera uma incerteza muito grande. No entanto, na medição da viscosidade em regime newtoniano somente a magnitude é requerida. Ocorre que os resultados experimentais reportados até o momento na literatura não coincidem quantitativamente com a teoria. Neste trabalho, uma nova técnica que permite determinar com boa precisão a magnitude do coeficiente de reflexão e, por conseguinte, a viscosidade de líquidos em regime newtoniano foi desenvolvida. Foi mostrado que a maioria dos líquidos testados apresentam comportamento newtoniano à frequência de trabalho de 1MHz, com exceção de alguns óleos automotivos de alta viscosidade, que possuem claramente comportamento viscoelástico. Desvios menores que 12,5% nas medições da viscosidade dinâmica de substâncias com moléculas relativamente simples, como glicerina e glicose, mostram a viabilidade da técnica. Também é proposto um novo método que permite calibrar o dispositivo de medição para monitorar a viscosidade de um líquido ou grupo de líquidos. Além disso, uma aplicação na monitoração da contaminação da água por substâncias oleosas é apresentada. / The ultrasonic measurement of viscoelastic properties of liquids has been an important research topic since the mid-twentieth century mainly due to the possibility of real-time viscosity measurements. The measurement technique is based on the determination of the complex reflection coefficient (magnitude and phase) of shear waves reflected from the solid-liquid interface. While magnitude measurement is relatively simple and precise, phase measurement is a difficult task due to strong temperature dependence that induces large errors. However, when the measurement is carried out under Newtonian regimen only the magnitude is required. Nonetheless, experimental results reported in literature until now do not agree quantitatively with the theory. In this work, it was developed a novel technique that accurately and precisely determines the reflection coefficient magnitude and, consequently, the Newtonian viscosity. Most tested liquids showed Newtonian behavior at the working frequency of 1 MHz except for some automotive oils of high viscosity, which showed an evident viscoelastic behavior. Deviations up to 12.5% in the dynamic viscosity measurement of substances with relatively simple molecules, such as glycerin and corn syrup, show the technique suitability. A calibration method of the measurement cell allowing the indirect viscosity measurement of a liquid or group of liquids is proposed. Moreover, a new application to oil-in-water contamination monitoring is presented.

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. Title from PDF of title page (University of Missouri--Columbia, viewed on December 22, 2009). Thesis advisor: Dr. Brent L. Rosenblad. Includes bibliographical references.

A primary purpose of elasticity imaging, commonly known as elastography, is to extract the viscoelastic properties of a medium (including soft tissue) from the displacement caused by a stress field. Dynamic elastography methods that use the acoustic radiation force of ultrasound have several advantages, such as, non-invasiveness, low cost, and ability to produce a highly localized force field. A method for remotely generating localized low frequency shear waves in soft tissue is investigated, by using the modulated radiation force resulting from two intersecting quasi-CW confocal ultrasound beams of slightly different frequencies. In contrast to most radiation force-based methods previously presented, such shear waves are narrowband rather than broadband. As they propagate within a viscoelastic medium, different frequency-dependent effects will not significantly affect their spectrum, thereby providing a means for measuring the shear attenuation and speed as a function of frequency. Furthermore, to improve the detection signal-to-noise-ratio (SNR), increased acoustic pressure conditions may be needed, causing higher harmonics to be generated due to nonlinear propagation effects. Shear-wave propagation at harmonic modulation frequencies does not appear to have been previously discussed in the elastography literature. The properties of the narrowband shear wave propagation in soft tissue are studied by using the Voigt viscoelastic model and Green’s functions. In particular, the manner in which the characteristics of the viscoelastic medium affect their evolution under both low-amplitude (linear) and high-amplitude (nonlinear) source excitation and conditions that conform to human safety standards. It is shown that an exact solution of the viscoelastic Green’s function is needed to properly represent the propagation in higher-viscosity media, such as soft tissue, at frequencies much beyond a few hundred hertz. Methods for estimating the shear modulus and viscosity in viscoelastic media are developed based on both the fundamental and harmonic shear components.

Graduation date: 1998

This thesis consists of two major parts. In the first part, I monitor the temporal change of S-wave velocity in shallow soil layers using seismic data collected in an experiment at Panola Mt. Atlanta, GA, 2006. I use the cross correlation function to find the arrival time differences for different water levels, and then calculate the change of Rayleigh wave phase velocity according to different frequencies in the range 5 to 50 Hz. After that, I find a reference 1-D layered P and S-wave velocity model from the measured Rayleigh wave dispersion curve, and put 6 sets of Gaussian perturbations into the reference velocity structure to invert for the actual temporal change of velocity structure in the experiment. I find a clear increase of S-wave velocity in the water injection area, and the S-wave velocity gradually recovers to the initial value after we stop pumping water. In the second part, I analyze temporal changes in fault zone site response along the Karadere-Düzce branch of the North Anatolian Fault, starting 8 days before and ending 72 days after the 1999 Mw7.1 Düzce, Turkey, earthquake. The analysis involves comparisons of strong motion seismic records at station VO inside the Karadere fault and station FP about 300 m away from the fault. I compare all available seismic waveforms at these stations, including those generated by foreshocks, the mainshock, aftershocks and seismic noise, and cut them into 10 s windows with a 5 s overlap. Fourier amplitude spectra are computed for seismic data in each window, and the average amplitude spectra for the two horizontal components are used to obtain the spectral ratio for each on/off fault pair of seismic records. The spectral ratios are smoothed over every 10 points in the frequency domain (0.5 Hz). The results show a shift of the spectral peak to lower frequencies during the main shock. The peak frequency reduces from 4.3 Hz several days before the main shock to 2.9 Hz (67.4% of the pre mainshock value) right after the mainshock. It quickly recovers to 3.8 Hz (64% recovery of the dropped value) after a day, and then gradually recovers to 4.0 Hz (79% recovery of the dropped value) after 72 days. I also compare the results from all the seismic data including direct S-wave, S coda waves and seismic noise and from coda waves only and find that the results from coda waves which are generally less scattered than those from all the data, and show lower amplitude of spectra ratio with higher peak frequencies. The observations suggest a nonlinear behavior of the fault zone material under strong ground motion of nearby major earthquakes. Finally I attempt to link the two parts by identifying their implications for the nonlinear site effects.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.