Uterine fibroids, endometriosis, and adenomyosis are gynecological disorders causing significant symptoms and have clinical struggles around diagnosis and treatment planning. The tissue changes associated with these disorders may support assessment via ultrasound shear wave elastography (SWE), an imaging method that estimates tissue stiffness. This dissertation examined the feasibility of gynecological SWE for the in vivo assessment of uterine tissues. Specifically, this thesis evaluated: 1) the validity and reliability of endoprobe SWE stiffness measured in tissue mimicking materials (TMMs), 2) the validity of indentation testing in TMMs, 3) the impact of indentation parameters on the agreement between indentation stiffness and stiffness parameters measured by SWE, and 4) the reliability of and confounders on in vivo SWE of the uterus. First, to establish the reliability and validity of gynecological endoprobe SWE stiffness measures, 31 homogenous TMM phantoms were manufactured with a range of elastic moduli. SWE stiffness was measured at 1cm, 3cm, and 5cm depths using a linear, curvilinear, and endo- SWE probes. Quasi-static ramp compression stiffness was the validity benchmark. Phantom moduli ranged from 17.10 kPa to 88.12 kPa. Endoprobe SWE stiffness reliability was excellent at all depths, however endoprobe SWE validity was poorer at 1cm depths than that of both the linear and curvilinear probe. The validity of endoprobe SWE stiffness was lower at the 3cm depth compared to the 1cm depth, and outcomes were not valid at the 5cm depth. Second, to establish the validity of indentation testing as a benchmark standard for SWE applications, indentation testing was conducted in TMM phantoms under a series of testing parameters with quasi-static ramp compression used as the benchmark. The best agreement between indentation and ramp compression outcomes was observed at low strain ranges with the lowest testing rate (0.01Hz). Higher strains required different material model and geometry parameters to improve validity. Regardless, indentation and ramp compression outcomes were highly correlated. It is recommended that researchers match testing parameters to in vivo loading conditions, while using this work to support material model and sample geometry selection. Third, to identify the optimal indentation testing parameters to maximize agreement between SWE and indentation stiffness, several loading strain, loading rate, sample geometry, and indentation models were evaluated in TMM phantoms. Findings showed strong relationships between indentation and SWE stiffness outcomes, regardless of testing conditions. It is recommended to match loading parameters to in vivo conditions. Small sample geometries and low indentation rates appeared to optimize agreement between indentation and SWE outcomes. The fourth and final study of this dissertation examined the in vivo reliability of uterine SWE. Institutional research ethics board approval was obtained (20170872-01H, H-08-18-790) and data were acquired from 32 participants with no active uterine disorders. SWE reliability was good to excellent, except in the anterior myometrium, where poor reliability was attributed to difficulty standardizing SWE sites between days. Phase in the menstrual cycle did not impact SWE outcomes. Site depth exhibited significant correlations with stiffness for multiple cases and heterogeneity in uterine stiffness was observed across sites. Anecdotal observations of confounding factors require further investigation. The findings of this thesis suggest that quasi-static flat-tip indentation testing is a suitable benchmark standard to validate SWE under a variety of parameters. However, endoprobe SWE of the uterus does not appear to be valid at target depths of 3cm and beyond. Even at depths less than 3cm, linear or curvilinear probe SWE outperformed endoprobe SWE. Results suggest that further development of endoprobe SWE for uterine applications is required prior to clinical investigation of gynecological SWE for diagnostic and treatment planning purposes.

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.

This thesis contributes to recent interest within medical imaging regarding the development and clinical application of magnetic resonance elastography (MRE) to the human brain. MRE is a non-invasive phase-contrast MRI technique for measurement of brain mechanical properties in vivo, shown to reflect the composition and organisation of the complex tissue microstructure. MRE is a promising imaging biomarker for the early characterisation of neurodegeneration due to its exquisite sensitivity to variation among healthy and pathological tissue. Neurodegenerative diseases are debilitating conditions of the human nervous system for which there is currently no cure. Novel biomarkers are required to improve early detection, differential diagnosis and monitoring of disease progression, and could also ultimately improve our understanding of the pathophysiological mechanisms underlying degenerative processes. This thesis begins with a theoretical background of brain MRE and a description of the experimental considerations. A systematic review of the literature is then performed to summarise brain MRE quantitative measurements in healthy participants and to determine the success of MRE to characterise neurological disorders. This review further identified the most promising acquisition and analysis methods within the field. As such, subsequent visits to three brain MRE research centres, within the USA and Germany, enabled the acquisition of exemplar phantom and brain data to assist in discussions to refine an experimental protocol for installation at the Edinburgh Imaging Facility, QMRI (EIF-QMRI). Through collaborations with world-leading brain MRE centres, two high-resolution - yet fundamentally different - MRE pipelines were installed at the EIF-QMRI. Several optimisations were implemented to improve MRE image quality, while the clinical utility of MRE was enhanced by the novel development of a Graphical User Interface (GUI) for the optimised and automatic MRE-toanatomical coregistration and generation of MRE derived output measures. The first experimental study was performed in 6 young and 6 older healthy adults to compare the results from the two MRE pipelines to investigate test-retest agreement of the whole brain and a brain structure of interest: the hippocampal formation. The MRE protocol shown to possess superior reproducibility was subsequently applied in a second experimental study of 12 young and 12 older cognitively healthy adults. Results include finding that the MRE imaging procedure is very well tolerated across the recruited population. Novel findings include significantly softer brains in older adults both across the global cerebrum and in the majority of subcortical grey matter structures including the pallidum, putamen, caudate, and thalamus. Changes in tissue stiffness likely reflect an alteration to the strength in the composition of the tissue network. All MRE effects persist after correcting for brain structure volume suggesting changes in volume alone were not reflective of the detected MRE age differences. Interestingly, no age-related differences to tissue stiffness were found for the amygdala or hippocampus. As for brain viscosity, no group differences were detected for either the brain globally or subcortical structures, suggesting a preservation of the organisation of the tissue network in older age. The third experiment performed in this thesis finds a direct structure-function relationship in older adults between hippocampal viscosity and episodic memory as measured with verbal-paired recall. The source of this association was located to the left hippocampus, thus complementing previous literature suggesting unilateral hippocampal specialisation. Additionally, a more significant relationship was found between left hippocampal viscosity and memory after a new procedure was developed to remove voxels containing cerebrospinal fluid from the MRE analysis. Collectively, these results support the transition of brain MRE into a clinically useful neuroimaging modality that could, in particular, be used in the early characterisation of memory specific disorders such as amnestic Mild Cognitive Impairment and Alzheimer's disease.

Introduction: The Elastography Software Library (ESL) was developed to achieve clinically feasible, super-resolution (SR) Magnetic Resonance Elastography (MRE). ESL was created by accomplishing four objectives: 1. perform a critical analysis of MRE inversion, using discrete-time Fourier transform (DTFT) methods, to enable selection of the wave inversion approach most suitable to high- and SR MRE (Chapter 2) 2. develop a new method for real-time 4D phase unwrapping, to enable large acquisitions to be processed in clinical work ow (Chapter 3) 3. develop a new inversion pipeline that recovers fine features in elastograms (Chapter 4) 4. extend this pipeline with a novel interpolation technique to achieve super-resolution (Chapter 5) The results of these experiments were combined to make the ESL. Over the course of the work, two objectives also resulted in software applications in their own right (PhaseTools for phase unwrapping, and Elastography Software Pipeline (ESP) for fine feature elasticity map recovery). Methods: Critical Analysis: Two-filter cascades were designed to model the signal processing pipelines found in the present MRE literature. These models were subjected to DTFT-based analysis to determine the relative advantage of various mathematical approaches to the MRE inverse problem. Phase Unwrapping: A test data set was developed to measure algorithm performance in 4D on data sets with varying levels of wrap, gradient and noise. The algorithms that performed most accurately and efficiently on test data were then applied to in vivo brain, liver, and muscle data, of both moderate and severe wrap, and inspected for wrap failure. Fine Feature Recovery: A new MRE image processing pipeline was developed that incorporates wavelet-domain denoising, image-driven noise estimation, and feature detection. ESP was first validated using simulated data, including viscoelastic Finite Element Method (FEM) simulations, at multiple noise levels. ESP images were then compared with Multifrequency Dual Elasto-Visco Inversion (MDEV) pipeline images in three ten-subject cohorts of brain, thigh, and liver acquisitions. Finally the proportion of spectral energy at fine frequencies was quantified using the Reduced Energy Ratio (RER) for both ESP and MDEV. Super-Resolution: An extension of the ESP pipeline was developed that incorporated a new image fusion technique to combine non-redundant information. The algorithm was validated on an analytic simulation program developed for the study. An in vivo cross-validation was performed between 1X, 2X and 4X magnification levels measuring both spectral gains and shear modulus values. Results: Critical Analysis: The more complex, heterogeneous FEM models were found to only outperform Algebraic Helmholtz Inversion (AHI) in very low noise, with Gaussian smoothing of σ > 0:8px or Butterworth low-pass cutoffs of < 0:8π negating any advantages from assumption of local heterogeneity. Phase Unwrapping: Three algorithms were determined to perform with sufficient robustness in real-time on 4D data sets with challenging phase wrap. These algorithms were then applied to in vivo brain, skeletal muscle, liver and phantom data and shown to successfully resolve heavy phase wrap within a \real-time" criterion of under 3 minutes. Fine Feature Recovery: For FEM inversions, mean values of background and soft target simulated results remained within 8% of prescribed up to σ = 10% for both jG*j and ϕ, though inspection of the ϕ image showed scatter- and boundary-related artefacts around the soft target. Hard target results showed jG*j means within 7% of prescribed up to σ = 5% but unreliable ϕ means, and inspection showed showed scatter- and boundary-related artefacts. For the in vivo cohorts, ESP results showed mean correlation of R = 0:83 with MDEV and liver stiffness estimates within 7% of Local Frequency Estimation (LFE) results. Finally, ESP showed statistically significant increase in fine feature spectral energy as measured with RER for both jG*j (p < 1X10-9) and ϕ (p < 1X10-3). Super-Resolution: At 4X SR, both brain and liver cohorts showed a highly significant (p ≤ 10-6) increase in both number of recovered frequencies and normalised spectral energy in those recovered frequencies. Both the 2X and 4X SR techniques showed a decrease in stiffness estimate from the original resolution (mean decrease of 11.6% and 14.0%) respectively; however cohort correlations between SR and original values were upwards of R = 0:988. Discussion: Established as a technique highly sensitive to important tissue changes, MR Elastography is now also a finely-featured super resolution technique in two parameters, enabling new clinical and research applications. Future work includes statistical mapping of both localised and diffuse soft tissue changes, rapid computation on heterogeneous processing architectures, and two-parameter super-resolution MRI-based lesion mapping.

A robust, reliable method to non-invasively measure in-vivo mechanical properties of large tissue areas was unavailable until the advent of a new Magnetic Resonance Imaging (MRI) technique known as Magnetic Resonance Elastography (MRE). MRE is a phase-contrast imaging technique that enables quantification of tissue mechanical properties by capturing the motion of induced shear waves via a synchronised Motion Encoding Gradient (MEG). The complex shear modulus is determined via mathematical inversion and reported as the magnitude of the complex shear modulus, |G*|, and phase angle, φ. The work reported in this thesis focuses on the development of MRE data acquisition and analysis protocols optimised to study thigh muscle mechanical properties. The protocols are subsequently applied in healthy volunteers to study natural phenomena such as contraction and ageing and interventions such as an experimental protocol to produce Exercise Induced Muscle Damage (EIMD). Methodological advances made throughout this work include moving from 2D to 3D MRE data acquisition protocols and the application of advanced inversion software to extract muscle viscoelastic properties from the acquired MRE data. Results obtained include observation of reduced muscle stiffness in 6 elderly subjects (>80 years old) compared to 4 young subjects in the Vastus Lateralis (32%), quadriceps muscle group (22%) and entire thigh cross-section (19%), higher resting stiffness of agonist quadriceps compared to antagonist hamstrings (18%) and an increase in quadriceps stiffness (40%) during a leg raise task in 11 healthy subjects. Variability in muscle group recruitment patterns during the contraction were also observed, with the phase angle of the Vastus Intermedius (VI) increasing significantly during contraction. The final experiment involved the recruitment of 20 healthy male subjects to perform an eccentric exercise protocol designed to induce EIMD. Subjects who displayed a Maximum Voluntary Contraction (MVC) force deficit of >10% were considered to have experienced EIMD. A further severe EIMD group were defined based on the presence of hyper-intense signal on T2 weighted imaging following the protocol. The T2 hyper-intensity was found to occur exclusively in the Rectus Femoris (RF) and VI muscle groups. Increased muscle stiffness was observed in the RF muscle in subjects who experienced moderate EIMD (6%). A greater increase in RF stiffness (48%) was observed in the severe EIMD group. The severe EIMD group also displayed significantly increased VI stiffness (14%). The experiments carried out provide several novel findings which can support the development of beneficial strategies to promote both healthy ageing and rehabilitation in athletes, and potentially contribute to improving muscle performance evaluation tests which will expand the opportunities for clinical applications of muscle MRE.

Le but de cette étude était d'évaluer la faisabilité et la reproductilité de l'élastographie ShearWave par voie transabdominale d’organes fœtaux et du placenta chez les babouines gestantes.Matériels et méthodes: Les échographies fœtales de tous les babouines gestantes ont été réalisées prospectivement pendant 9 mois dans un seul centre de recherche sur les primates. La visualisation des organes foetaux cibles (foie, poumons proximal et distal, substance blanche du cerveau et matière grise périventriculaire) et du placenta a été initialement effectuée en utilisant l'échographie 2D pour le repérage des organes, puis le mode élastographique a été activé. Pour chaque organe, trois mesures ont été effectuées par deux opérateurs. On a calculé les coefficients de corrélation intra-classe (CCI) intra-observateur et inter-observateur.Résultats: Au cours de la période d'étude (03/2013 - 12/2013), 21 babouines gestantes ont été inclus correspondant à 21 foetus. Une à trois échographies ont été réalisées pour chaque fœtus. Les mesures ont été réalisables par les deux opérateurs dans tous les cas. Le CCI intra-observateur et inter-observateur était de 0,654 - IC 95% (0,606 à 0,699) et 0,645 - IC 95% (0,553 à 0,721), respectivement.Conclusion: L’élastographie ShearWave par voie transabdominale des organes fœtaux et du placenta peut être réalisée chez les babouines gestantes. La reproductibilité intra-observateur et inter-observateur est correcte mais varie selon les organes visés. / The purpose of this study was to assess the feasibility and reproducibility of transabdominal ShearWaveTM elastography of fetal organs and placenta in pregnant baboons.Materials and methods: Fetal ultrasounds of all pregnant baboons in a single primate research center were performed prospectively during 9 months. The visualization of fetal targeted organs (liver, proximal and distal lungs, brain white substance and periventricular gray substance) was initially performed using 2D ultrasound, and then elastography mode was activated. For each organ, three measurements were carried out by two operators. Intra-observer and inter-observer intra-class correlation coefficients (ICC) were calculated.Results: During the study period (03/2013–12/2013), 21 pregnant baboons (21 fetuses) were included. One to three ultrasound scans were performed for each fetus. The measurements were feasible by the two operators in all cases. The intra-observer and inter-observer ICC were 0.654, 95% CI (0.606 to 0.699) and 0.645, 95% CI (0.553 to 0.721) respectively.Conclusion: Transabdominal ShearWaveTM Elastography of fetal organs can be achieved in pregnant baboons. The intra-observer and inter-observer reproducibility is correct but vary according to the targeted organs.ves: The purpose of this study was to assess the feasibility and reproducibility of transabdominal ShearWaveTM elastography of fetal organs and placenta in pregnant baboons.

Introduction: Implicit within spinal manipulative therapy is the assumption that treatment loads are effectively transcribed to actuate consistent mechanisms for expected clinical results. There is conflicting evidence between the mechanistic understandings and the physiologic responses from experimental evidence. Greater clarity on how loads are transferred through tissues to the target sites would be useful in enhancing utilization and efficacy of spinal manipulative procedures. Purpose: Directly monitor displacement of tissue in strata at sequential depths between the load application site and target articulation in the thoracic spine. Tissue displacement served as a surrogate for evidence of load transmission. Methods: Ultrasound elastography techniques monitored displacement in sequential strata while electromyographic signals, force, kinematic motions were monitored synchronously. Volunteers were placed prone on a treatment table, while a typical spinal manipulative pre-load maneuver was applied in the thoracic spine. Results: When applying a therapeutic load to the skin the results demonstrate with increasing depth of tissue there is a sequentially decreasing rank order in the mean cumulative displacement with each layer being significantly greater than the deeper adjacent layer. Superficial loose connective tissue layer (0.34 mm ± 0.15) vs. intermediate muscle layer (0.28 mm ± 0.11), p=0.004. Intermediate muscle layer (0.28 mm ± 0.11) vs. deep muscle layer (0.16 mm ± 0.6), p<0.0001. Filtered myoelectric signals were linearly correlated with tissue strata cumulative displacements, but the relationship was not strong (-0.23 < r < 0.46). Conversely, Pearson correlation analysis revealed strong and relatively stable correlations (0.74 < r < 0.90) for the association between displacement at the load application site and tissue layers. Conclusion: The sequential tissue motion demonstrates that some degree of load transfer through layers occurs. Both direct and indirect stimulation of tissues across both depth and breadth is feasible, to an extent consistent with the stimulation of mechanoreceptors. / Thesis / Master of Science (MSc)

It is well known in medicine that changes in tissue elasticity may be related to pathological phenomena such as cancer and other disease. Physicians routinely use palpation as means of inspecting the thyroid, prostate, and breast, where a palpably hard mass can often indicate the presence of a malignant lesion. Magnetic Resonance Elastography (MRE) has emerged as a relatively new elasticity imaging technique which can be used to spatially map and measure displacement patterns resulting from harmonic shear-wave propagation in soft tissue. Displacement fields are then used in reconstructing the tissue’s elastic property distributions. The feasibility of using MRE as a noninvasive means of characterizing the mechanical properties of silicone phantom mimicking human breast, was investigated though experiments involving MRE acquisitions of four phantoms. To achieve sufficient excitation of the phantom tissue, an acoustic actuator was developed. The results of these studies have shown the MRE acquisition to be successful in capturing sufficient data for elastic parameter reconstruction. Another different type of actuator has been developed and tested in the laboratory. The results show the potential for future use of this actuator in MRE experiments.

There has been great interest in recent times in the use of elastography for the characterization of human tissue. Digital Image Elasto-Tomography is a novel breast cancer pre-screening technique under development at the University of Canterbury, which aims to identify and locate stiff areas within the breast that require further investigation using images of the surface motion alone. A calibrated array of five digital cameras is used to capture surface motion of the breast under harmonic actuation. The forward problem, that is the resulting motion for a given mechanical property distribution, is calculated using the Finite Element Method. The inverse problem is to find the mechanical properties which reproduce the measured surface motion through numerical simulation. A reconstruction algorithm is developed using a shape based description to reduce the number of parameters in the inverse problem. A parallel Genetic Algorithm is developed for parameter optimization. A geometric method termed Fitness Function Analysis is shown to improve the inclusion location optimization problem. The ensemble of solutions generated using the Genetic Algorithm is used to produce an optimal and a credible region for inclusion location. Successful single frequency phantom reconstructions are presented. An effective way of combining information from multi-frequency phantom data by examining the characteristics of the measured surface motion using data quality metrics is developed and used to produce improved reconstructions. Results from numerical simulation datasets and a two inclusion phantom used to test the optimization of multiple and ellipsoidal inclusions indicate that although two inclusions can be successfully reconstructed, the single inclusions assumption may suffice even in irregular, heterogeneous cases. This assumption was used to successfully locate the stiffest inclusion in a phantom containing multiple inclusions of differing stiffness based on three multi-frequency datasets. The methods developed in phantoms are applied to three in vivo cases for both single and multi-frequency data with limited success. This thesis builds on previous work undertaken at the University of Canterbury. The original contributions in this work are as follows. A new reconstruction algorithm combining a genetic algorithm with fitness function analysis is developed. The most realistic tissue mimicking phantoms to date are used. An ellipsoidal shape-based description is presented, and applied to the first multi-inclusion reconstructions in DIET. This work presents the first reconstruction using meshes created directly from data using a meshing algorithm developed by Jonas Biehler. A multi-frequency cost function is developed to produce the first multi-frequency and in vivo reconstructions using DIET data.

Mudanças nas características mecânicas de tecidos biológicos geralmente estão relacionadas com algum tipo de patologia. Técnicas de imagens elastográficas são métodos quantitativos de se estimar as propriedades mecânicas de tecidos. Em geral, o objetivo destas técnicas de imagem é medir o movimento do tecido provocado por uma força interna ou externa. Por meio desse movimento, parâmetros viscoelásticos do meio em análise são reconstruídos. A força de excitação pode ser tanto quasi-estática, como dinâmica. O trabalho apresentado nesta tese aborda as técnicas de elastografia dinâmica e quasi-estática. Na abordagem quasi-estática, a elasticidade não-linear é estudada através de phantoms com características que simulam as do tecido humano. Na abordagem dinâmica, o movimento dinâmico promovido por força de radicação acústica é avaliado através de técnicas ultrassônicas e magnéticas. O desenvolvimento de materiais para serem usados como phantoms para elastografia por ultrassom é descrito. O comportamento elástico não-linear desses materiais foi analisado através de resultados de ensaios mecânicos. Esses materiais foram desenvolvidos para apresentarem uma relação tensão/deformação que não dependesse do módulo de cisalhamento para pequenas deformações, e foram projetados para serem usados em phantoms em que configurações heterogêneas são empregadas (por exemplo, phantoms com inclusões esféricas). O efeito da não-linearidade elástica dos materiais sobre o contraste, a relação sinal ruído e a relação contraste ruído de imagens elastográficas de um phantom contendo inclusões esféricas, sofrendo altas deformações (até 20%) foi investigada. Foi demonstrada a viabilidade de se medir movimentos vibratórios induzidos por feixes acústicos confocais através de um ultrassom Doppler que utiliza ondas contínuas. A interferência de feixes de ultrassom com pequena diferença de frequências provoca o aparecimento de uma força dinâmica no alvo. Foi demonstrada a formação de imagens de uma esfera rígida imersa em um phantom viscoelástico, através da varredura de ambos os transdutores (confocal e Doppler) pelo plano focal do transdutor confocal. O comportamento dinâmico de uma esfera magnetizada induzido por força de radicação acústica foi investigado. A esfera foi suspensa em água em configuração de pêndulo. Forcas estática de longa (poucos segundos) e curta (poucos milisegundos) duração foram utilizadas. O movimento da esfera foi medido através de um sensor magnetoresistivo. A partir da nova posição de equilíbrio em resposta à força de radicação de longa duração, a amplitude dessa força foi estimada. Para se estimar a viscosidade da água, o movimento de relaxação da esfera após a força ter sido desligada foi ajustado a um modelo de movimento-harmônico amortecido. O movimento de uma esfera rígida imersa em um phantom feito de gelatina, deslocada por força de radicação acústica, foi avaliado por meio de ecos ultrassônicas obtidos com um sistema pulso/eco. A teoria utilizada para se estimar os parâmetros viscoelásticos do phantom, usando o movimento induzido na esfera, é uma extensão da teoria usada para se estimar a viscosidade da água. / Changes in the mechanical properties of soft tissues may be related with pathological disorders. Elasticity imaging is a quantitative method of estimating the mechanical properties of the tissue. In general, the aim of this technique is to measure tissue motion caused by external or internal forces and use it to reconstruct the viscoelastic parameters of the medium. The excitation stress used can be (quasi-) static or dynamic. Both elastographic approaches are explored in this thesis work. In the quasi-static approach, the nonlinear elasticity is studied through tissue-mimicking phantom experiments. In the dynamic approach, the dynamic motion provided through acoustic radiation force is evaluated using ultrasonic and magnetic techniques. The development of phantom materials for elasticity imaging is reported. These materials were specifically designed to provide nonlinear stress/strain relationship that can be controlled independently of the small strain shear modulus of the material, and were designed for use in phantoms where heterogeneous configurations (e.g, spherical targets in a uniform background) are employed. The effects of phantom materials nonlinearity over the strain contrast, signal-to-noise ratio and contrast-to-noise ratio of a phantom containing spherical inclusions undergoing large deformations (up to 20%) were investigated.The feasibility of measuring vibration movement, through a mono-channel continuous wave Doppler system, induced by focused confocal beams, is demonstrated. The interference of two ultrasonic beams promotes a dynamic force to the target. The ability to form images of a rigid spherical inhomogeneity embedded in viscoelastic phantom by scanning both ultrasonic transducers (confocal and Doppler) across the confocal transducer focal plane is presented. The dynamic behavior of a rigid magnetic sphere induced by an acoustic radiation force was investigated. The sphere was suspended in water in a simple pendulum configuration. Steady forces of long (few seconds) and short (few milliseconds) durations were used. The movement of the magnetic sphere was tracked using a magnetoresistive sensor. From the new equilibrium position of the sphere in response to the long-duration static radiation force, the amplitude of this force was estimated. To access the water viscosity, the relaxation movement after the acoustic force had stopped was fitted to a harmonic-motion model. The motion of a rigid sphere embedded in gelatin phantom, displaced by acoustic radiation force, was evaluated using the ultrasonic echoes from a pulse-echo system. The theory used to estimate the viscoelastic parameters of the phantom, from the oscillation of the rigid sphere is an extension of the relation used to estimate the water viscosity.