京都大学 / 新制・課程博士 / 博士(人間健康科学) / 甲第24097号 / 人健博第104号 / 新制||人健||7(附属図書館) / 京都大学大学院医学研究科人間健康科学系専攻 / (主査)教授 黒木 裕士, 教授 青山 朋樹, 教授 松田 秀一 / 学位規則第4条第1項該当 / Doctor of Human Health Sciences / Kyoto University / DFAM

Introduction : L’élastographie shear wave (ESW) est une technique récente d’échographie qui évalue quantitativement la dureté des tissus et permet d’améliorer la caractérisation des lésions mammaires. Comme toute nouvelle technique d’imagerie, l’ESW nécessite une validation préclinique pour définir les conditions d’utilisations et établir les limites des champs d’applications dans lesquelles la technique pourra être considérée comme valide. Matériels et méthodes : Dans une première partie effectuée au laboratoire de recherche en Imagerie nous avons étudié les éléments histologiques sous tendant l’image d’ESW sur un modèle de cancer du sein implanté chez la souris, au cours de sa croissance puis sous traitement. Dans une deuxième partie, nous avons étudié chez des patientes le rôle de la compression manuelle en ESW pour la caractérisation des lésions mammaires. Dans une dernière partie, effectuée en collaboration avec une équipe de l’Institut Langevin Ondes et Images, nous avons étudié la faisabilité d’un nouveau paramètre, le module de cisaillement non linéaire pour l’analyse des lésions mammaires. Résultats : Au laboratoire, nous avons établi des corrélations entre la dureté mesurée en élastographie et les caractéristiques histologiques des tumeurs, y compris sous traitement. Nous avons montré que la fibrose était associée à une dureté élevée et la nécrose à une dureté moindre. Notre étude clinique a montré qu’une compression manuelle minimale était nécessaire pour obtenir de bonnes performances de l’ESW et qu’une pression trop élevée devait être évitée. Enfin nous avons montré la faisabilité en imagerie mammaire d’un nouveau paramètre quantitatif obtenu en élastographie shear wave : le module de cisaillement non linéaire. Conclusion : A partir de travail de thèse, une meilleure compréhension de la part des éléments biologiques et techniques en ESW du sein est possible et des recommandations pour l’utilisation clinique peuvent être formulées. Nos observations cliniques ont entrainé la mise au point d’un nouveau paramètre diagnostique quantitatif : le module de cisaillement non linéaire. / Introduction: Shear Wave Elastography (SWE) is a recent ultrasound technique assessing quantitatively tissue stiffness and improving breast lesions characterization. As every new imaging technique, SWE requires a pre clinical validation in order to define in which conditions it should be used and precise the applications for which the technique is validated. Materials and methods: First, in a research lab we have investigated the pathological features underlying SWE image in a breast cancer model implanted in mice, during tumor growth and under therapy. Secondly, we have studied in patients the role of manual compression in SWE for the characterization of breast lesions. Finally, in collaboration with on team from Institut Langevin Ondes et Images, we have studied the feasibility of a new parameter, the non-linear modulus, for breast lesion assessment. Results: in the research lab, we have shown correlations between stiffness as measured with SWE and pathological features of tumors, even on treatment. We have shown that fibrosis was associated with high stiffness values and necrosis with lowers. Our clinical study, showed that a minimal manual compression was required for optimal performance of SWE and that strong compression should be avoided. Finally, we demonstrated feasibility of a new parameter, derived from SWE, the non-linear modulus. Conclusion: Our work provides a better understanding of biological and technical elements of SWE. On the basis of our results, new recommendations may be made for the use of SWE in clinical practice. From our clinical findings, we developed a new quantitative parameter, which may be useful for the diagnosis of breast lesions, the non-linear modulus.

Les caillots sanguins sont responsables de nombreuses pathologies, telles que les thromboses veineuses profondes (TVP), les embolies pulmonaires (EP) ou les accidents vasculaires cérébraux (AVC). La thromboembolie veineuse, regroupant les thromboses veineuses profondes et les embolies pulmonaires, ainsi que les accidents vasculaires cérébraux sont parmi les principales causes de décès. La formation d’un caillot sanguin survient suite à une lésion de la paroi endothéliale, d’un changement hémodynamique de l’écoulement sanguin ou de conditions d’hypercoagulation du sang. Lors de situations pathologiques, le caillot ne se dissout pas naturellement et des anticoagulants sont prescrits. Leur but est d’empêcher l’évolution du caillot et de prévenir des risques de récidives. Dans le cas d’un accident vasculaire cérébral ou de certaines embolies pulmonaires, de l'activateur tissulaire du plasminogène recombinant (rt-PA) peut être prescrit pour dissoudre directement le caillot. Ces traitements comportent des risques d’hémorragie interne, et une incertitude persiste encore sur leur durée d’administration. La caractérisation des caillots sanguins vise à mieux comprendre leur développement pour pouvoir adapter les traitements et potentiellement à confirmer leur effet. Un moyen de caractériser les tissus est d’utiliser des méthodes ultrasonores. L’élastographie, une de ces méthodes, permet de calculer la rigidité d’un tissu, caractérisé par son module d’Young. On appelle élastographie dynamique les méthodes d’élastographie basées sur la propagation d’ondes de cisaillement dans le tissu pour calculer son module d’Young (relié à la vitesse de l’onde). Plusieurs études ont montré que le module d’Young de caillots sanguins augmentait avec le temps et que des caillots plus vieux et plus denses étaient plus résistants à la dissolution. La dispersion de l’onde de cisaillement reliée à la contribution visqueuse du tissu est un paramètre nouvellement disponible sur certains échographes cliniques qui est peu étudié dans le cadre des caillots sanguins. L’atténuation de l’onde de cisaillement est également un paramètre acoustique, non explorée dans le cas des caillots sanguins. L’utilisation de l’atténuation de l’onde de cisaillement a montré des résultats encourageant en simulations et sur fantômes. Cependant, avec les méthodes actuelles, le calcul de l’atténuation d’onde de cisaillement suppose un milieu homogène ne favorisant pas la présence de résonances acoustiques, ce qui n’est pas le cas pour des caillots cylindriques dans des vaisseaux sanguins. L’enveloppe ultrasonore peut également être étudiée de manière statistique et les paramètres issus de ces statistiques reflètent la microstructure des tissus imagés. L’objectif général de cette thèse est de caractériser les caillots sanguins pendant leur évolution à l’aide des différents paramètres découlant des ultrasons. Cette caractérisation vise à permettre aux soignants d’adapter l’administration du traitement. Un autre objectif consiste à évaluer l’effet du traitement sur les différents paramètres ultrasonores afin d’attester son efficacité. Pour réaliser ces objectifs l’étude se sépare en trois parties. La première partie est une étude in vivo en partenariat avec le centre hospitalier universitaire de Grenoble qui vise à étudier le module d’Young et les paramètres d’ultrasons statistiques au cours du temps chez des patients présentant des thromboses veineuses profondes. Les patients étaient sous anticoagulants ; le module d’Young n’a pas montré de changement significatif entre les mesures aux jours 0, 7 et 30. Les paramètres statistiques (étudiés suivant la distribution statistique homodyne K) ont montré une augmentation de l’intensité des rétrodiffuseurs (probablement liée à une augmentation du nombre de brins de fibrine et/ou du nombre de globules rouges) et une désorganisation des rétrodiffuseurs. Une des conclusions est que les paramètres statistiques pourraient permettre d’améliorer la caractérisation de caillots sanguins même quand le module d’Young ne varie pas significativement. La deuxième partie se concentre sur l’analyse du phénomène de résonance acoustique présent lors du passage d’ondes de cisaillement dans le caillot. Le but était de trouver une région d’intérêt où le calcul de l’atténuation de l’onde de cisaillement présente le moins de variation associée à ce phénomène. Les paramètres liés à la résonance, le module d’Young, la dispersion et l’atténuation de l’onde de cisaillement ont pu être reportés dans des fantômes de différents diamètres ainsi que dans des caillots in vitro pendant leur coagulation. Une région d’intérêt minimisant la résonance lors du calcul de l’atténuation de l’onde de cisaillement a pu être trouvée. La troisième étude porte sur l’effet de traitement avec du rt-PA sur le module d’Young, la dispersion de l’onde de cisaillement, l’atténuation de l’onde de cisaillement et les paramètres d’ultrasons statistiques. Du sang de porc a été collecté et 3 caillots par échantillon ont été préparés, chacun subissant une condition différente de traitement (pas de traitement, traitement à 20 minutes après le début de la coagulation et traitement à 60 minutes après cette phase initiale). Les résultats montrent que le module d’Young diminue significativement avec l’ajout du traitement, la dispersion de l’onde de cisaillement augmente pour les caillots traités à 60 minutes comparés à ceux non traités, l’atténuation de l’onde de cisaillement augmente pour les caillots traités à 20 minutes comparés à ceux non traités et l’intensité des rétrodiffuseurs diminue significativement dans les deux conditions de traitement. Ces paramètres pourraient aider à confirmer l’effet du traitement et déceler si des patients y sont résistants ou non. Cette thèse a permis une meilleure compréhension des propriétés mécaniques et acoustiques des caillots sanguins au cours de la coagulation et en fonction de traitements. Tout ceci constitue autant de propriétés qui pourraient être utilisées afin d’adapter la dose et la durée d’un traitement afin de réduire les effets secondaires lors de la prise en charge des patients. / Blood clots are the cause of numerous pathologies, including deep vein thrombosis (DVT), pulmonary embolism (PE) and stroke. Venous thromboembolism, which includes deep vein thrombosis and pulmonary embolism, and stroke are among the leading causes of death. Blood clots form as a result of damage to the endothelial lining, hemodynamic changes in blood flow or hypercoagulable blood conditions. In pathological situations, the clot does not dissolve naturally, and anticoagulants are prescribed. Their purpose is to prevent the clot from progressing and to reduce the risk of recurrence. In the case of stroke or certain pulmonary embolisms, recombinant tissue plasminogen activator (rt-PA) may be prescribed to dissolve the clot. While these treatments can be effective, they do pose a risk of causing internal bleeding, and there is currently uncertainty regarding the optimal duration for administering them. Blood clot characterization aims to better understand how blood clots develop, so as to be able to adapt treatments and potentially confirm their effect. One way of characterizing tissues is to use ultrasound methods. Elastography, one such method, calculates the stiffness of a tissue characterized by its Young's modulus. Elastography methods based on shear wave propagation in a tissue to calculate the Young's modulus (related to the wave velocity) are known as dynamic elastography approaches. Several studies have shown that the Young's modulus of blood clots increases with time, and that older and denser clots are more resistant to dissolution. Shear wave dispersion related to the viscous contribution of a tissue is a newly available parameter on some clinical ultrasound scanners. However, little is known in the context of blood clots. Shear wave attenuation is also an acoustic parameter related to tissue viscosity and it is unexplored in the case of blood clots. The use of shear wave attenuation has shown encouraging results in simulations and on phantoms. However, with current methods, the calculation of the shear wave attenuation assumes a resonance-free homogeneous medium, which is not the case for cylindrical clots in blood vessels. The ultrasound envelope can also be studied statistically, and the parameters derived from these statistics reflect the microstructure of the tissues imaged. The general aim of this thesis is to characterize blood clots as they evolve, using various parameters derived from ultrasound. This characterization is intended to enable caregivers to adapt treatment administration. Another objective is to evaluate the effect of treatment on the various ultrasound parameters, in order to attest its effectiveness. To achieve these objectives, the study is divided into three parts. The first part is an in vivo study in partnership with Grenoble University Hospital to investigate the Young's modulus and statistical ultrasound parameters over time in patients with deep vein thrombosis. The patients were on anticoagulants, and the Young's modulus showed no significant changes between measurements on days 0, 7 and 30. Statistical parameters (studied according to the homodyne K statistical distribution) showed an increase in the intensity of scatterers (probably linked to an increase in the number of fibrin strands and/or number of red blood cells) and a disorganization of scatterers. One conclusion is that statistical parameters might be used to improve the characterization of blood clots even when the Young's modulus does not vary significantly. The second part focuses on the analysis of the resonance phenomenon present when shear waves pass through the clot. The aim was to find a region of interest where the calculation of the shear wave attenuation shows the least possible variation due to this behavior. Parameters related to resonance, the Young's modulus, shear wave dispersion and shear wave attenuation were plotted in phantoms of different diameters and in in vitro blood clots during coagulation. A region of interest was found, which minimized resonances when calculating shear wave attenuation. The third study investigates the effect of in vitro treatments with rt-PA on the Young's modulus, shear wave dispersion, shear wave attenuation and statistical ultrasound parameters. Porcine blood was collected and 3 clots per sample were prepared, each undergoing a different treatment condition (no treatment, treatment at 20 minutes after the beginning of coagulation and treatment at 60 minutes after the onset of coagulation). The Young's modulus decreased significantly with treatment, the shear wave dispersion increased for clots treated at 60 minutes compared to those untreated, the shear wave attenuation increased for clots treated at 20 minutes compared to those untreated, and the backscatter intensity decreased significantly in both treatment conditions. These parameters might help confirm the effect of a treatment and detect whether or not patients are resistant to it. This thesis has led to a better understanding of the mechanical and acoustic properties of blood clots during coagulation and as a function of a treatment. These properties can be used to adapt the dose and duration of treatment to reduce side effects in the management of patients.

Subsurface volume loss takes place in many geotechnical situations, and it is inherently accompanied by complex stress and displacement fields that may influence the performance of engineered geosystems. This research is a deformation-centered analysis, it depends on soil compressibility and it is implemented using finite elements. Soil stiffness plays a central role in predicting ground deformation. First, an enhanced Terzaghi’s soil compressibility model is proposed to satisfy asymptotic conditions at low and high stress levels with a small number of physically meaningful parameters. Then, the difference between small and large strain stiffness is explored using published small and large-strain stress-strain data. Typically, emphasis is placed on the laboratory-measured stiffness or compressibility; however, there are pronounced differences between laboratory measurements and field values, in part due to seating effects that prevail in small-thickness oedometer specimens. Many geosystems are subjected to repetitive loads; volumetric strains induced by drained repetitive ko-loads are experimentally investigated to identify shakedown and associated terminal density. The finite element numerical simulation environment is used to explore the effect of localized subsurface mass loss on free-surface deformation and shallow foundations settlement and bearing capacity. A stress relaxation module is developed to reproduce the change in stress associated to dissolution features and soft zone formation. The comprehensive parametric study is summarized in terms of dimensionless ratios that can be readily used for engineering applications. Field settlement data gathered at the Savannah River Site SRS are back-analyzed to compare measured values with predictions based on in situ shear wave velocity and strain-dependent stiffness reduction. The calibrated model is used to estimate additional settlements due to the pre-existing cavities, new cavities, and potential seismic events during the design life of the facility.

In this study, ambient noise wavefield was used for the first time to image spatial and temporal upper crustal seismic structures in NW-Bohemia/Vogtland region. The data come from 111 stations and were collected from continuous recordings of the permanent station networks of Germany and Czech Academy of Sciences as well as temporary stations of the BOHEMA and PASSEQ experiments. Rayleigh and Love waves travelling between each station-pair are extracted by cross-correlating long time series of ambient noise data recorded at the stations. Group velocity dispersion curves are obtained by time-frequency analysis of cross-correlation functions between 0.1 and 1 Hz, and are tomographically inverted to provide 2-D group velocity maps. At shorter periods Rayleigh wave group velocity maps are in good agreement with surface geology where low velocity anomalies appear along Mariánské Lázně Fault and Eger rift. A low velocity zone is observed at the northern edge of Mariánské Lázně Fault which shifts slightly to the south with increasing period and correlates well with the main focal zone of the earthquake swarms at 5 s period. We invert the 2-D group velocity maps into a 3-D shear wave velocity model. In this step Love waves were excluded from further analysis because of their high level of misfit to modelled dispersion curves. Horizontal and vertical sections through the model reveal a clear low velocity zone above the Nový Kostel seismic focal zone which narrows towards the top of the seismic activity and ends above the shallowest hypocenters at 7 km depth. We investigate temporal variation of seismic velocity within and around the Nový Kostel associated with 2008 and 2011 earthquake swarms by employing Passive Image Interferometry method using 7 continuous seismograms recorded by the WEBNET network. The results reveals stable seismic velocities without a clear post seismic velocity change during earthquake swarms in the Nový Kostel area.

A method for numerical simulation of train induced track vibration and wave propagation in subgrade has been proposed. The method uses a mass to simulate the bogie of a train and considers the effect of rail roughness. For this method, rail roughness is considered as a randomly generated signal and a filter is used to block the undesired components. The method predicts the particle velocity around the track and can be applied to many kinds of railroad trackbeds including traditional ballast trackbed and modern Hot mix asphalt (HMA) trackbed. Results from ballast and HMA trackbeds are compared and effects of HMA layer on damping track vibration and dissipating wave propagation are presented. To verify the credibility of the method, in-track measurements were also conducted. Site measurements included performing geophysical tests such as spectral analysis of surface wave test and seismic refraction test to determine the subsurface conditions at the test site. Ballast and HMA samples were tested in the laboratory by resonant column test to obtain the material properties. Particle velocities were measured and analyzed in the frequency domain. Results from in-track tests confirm the applicability of the numerical method. The findings and conclusions are summarized and future research topics are suggested.

Bulk rock strength is greatly dependent on fracture density, so that reductions in rock strength associated with faulting and fracturing should be reflected by reduced shear coupling and hence S-wave velocity. This study is carried out along the Canterbury rangefront and in Otago. Both lie within the broader plate boundary deformation zone in the South Island of New Zealand. Therefore built structures are often, , located in areas where there are undetected or poorly defined faults with associated rock strength reduction. Where structures are sited near to, or across, such faults or fault-zones, they may sustain both shaking and ground deformation damage during an earthquake. Within this zone, management of seismic hazards needs to be based on accurate identification of the potential fault damage zone including the likely width of off-plane deformation. Lateral S-wave velocity variability provides one method of imaging and locating damage zones and off-plane deformation. This research demonstrates the utility of Multi-Channel Analysis of Surface Waves (MASW) to aid land-use planning in such fault-prone settings. Fundamentally, MASW uses surface wave dispersive characteristics to model a near surface profile of S-wave velocity variability as a proxy for bulk rock strength. The technique can aid fault-zone planning not only by locating and defining the extent of fault-zones, but also by defining within-zone variability that is readily correlated with measurable rock properties applicable to both foundation design and the distribution of surface deformation. The calibration sites presented here have well defined field relationships and known fault-zone exposure close to potential MASW survey sites. They were selected to represent a range of progressively softer lithologies from intact and fractured Torlesse Group basement hard rock (Dalethorpe) through softer Tertiary cover sediments (Boby’s Creek) and Quaternary gravels. This facilitated initial calibration of fracture intensity at a high-velocity-contrast site followed by exploration of the limits of shear zone resolution at lower velocity contrasts. Site models were constructed in AutoCAD in order to demonstrate spatial correlations between S-wave velocity and fault zone features. Site geology was incorporated in the models, along with geomorphology, river profiles, scanline locations and crosshole velocity measurement locations. Spatial data were recorded using a total-station survey. The interpreted MASW survey results are presented as two dimensional snapshot cross-sections of the three dimensional calibration-site models. These show strong correlations between MASW survey velocities and site geology, geomorphology, fluvial profiles and geotechnical parameters and observations. Correlations are particularly pronounced where high velocity contrasts exist, whilst weaker correlations are demonstrated in softer lithologies. Geomorphic correlations suggest that off-plane deformation can be imaged and interpreted in the presence of suitable topographic survey data. A promising new approach to in situ and laboratory soft-rock material and mass characterisation is also presented using a Ramset nail gun. Geotechnical investigations typically involve outcrop and laboratory scale determination of rock mass and material properties such as fracture density and unconfined compressive strength (UCS). This multi-scale approach is espoused by this study, with geotechnical and S-wave velocity data presented at multiple scales, from survey scale sonic velocity measurements, through outcrop scale scanline and crosshole sonic velocity measurements to laboratory scale property determination and sonic velocity measurements. S-wave velocities invariably increased with decreasing scale. These scaling relationships and strategies for dealing with them are investigated and presented. Finally, the MASW technique is applied to a concealed fault on the Taieri Ridge in Macraes Flat, Central Otago. Here, high velocity Otago Schist is faulted against low velocity sheared Tertiary and Quaternary sediments. This site highlights the structural sensitivity of the technique by apparently constraining the location of the principal fault, which had been ambiguous after standard processing of the seismic reflection data. Processing of the Taieri Ridge dataset has further led to the proposal of a novel surface wave imaging technique termed Swept Frequency Imaging (SFI). This inchoate technique apparently images the detailed structure of the fault-zone, and is in agreement with the conventionally-determined fault location and an existing partial trench. Overall, the results are promising and are expected to be supported by further trenching in the near future.

Seismic anisotropy at continental regions, mainly at stable areas, gives important information about past and present tectonic events, and helps us in understanding patterns of upper mantle flow in a way not achieved by other methods. The measurement of shear wave splitting (SWS), at individual stations, from core refracted phases (such as SKS phases), indicates the amount and orientation of the seismic anisotropy in the upper mantle. Previous studies of SWS in South America concentrated mainly along the Andes and in southeast Brazil. Now we add extra measurements extending to all Brazilian territory, especially in the Pantanal and Paraná-Chaco basins, as part of the FAPESP 3-Basins Thematic Project. The results from both temporary deployments and from the Brazilian permanent network provide a more complete and robust anisotropy map of the South America stable platform. In general the fast polarization orientations have an average E-W orientation. Significant deviations to ESE-WNW or ENE-WSW are observed in many regions. We compare our results with different anisotropy proxies: absolute plate motion given by the hotspot reference frame HS3-NUVEL-1A, a recent model of time dependent upper mantle flow induced by the Nazca plate subduction, global anisotropy from surface wave tomography, and geologic trends. We observe a poor correlation of the anisotropy directions with geological trends, with the exception of a few stations in northern Brazil and a better correlation with the mantle flow model. Therefore, our observed anisotropy is mainly due to upper-mantle flow, with little contribution from frozen lithospheric anisotropy. Also, deviations from the mantle flow model, which includes a thicker lithosphere at the Amazon craton, are mainly due to flow surrounding cratonic nuclei not used in the model: the keel of the São Francisco craton and a possible cratonic nucleus beneath the northern part of the Paraná Basin (called Paranapanema block). Large delay times at the Pantanal Basin may indicate a stronger asthenospheric channel, a more coherent flow, or a thicker asthenosphere. Small delays beneath the northern Paraná Basin and central Amazon craton may indicate thinner anisotropic asthenosphere. / Anisotropia sísmica em regiões continentais, principalmente em áreas estáveis, nos dá informações importantes sobre eventos tectônicos do passado e do presente, e nos ajuda a entender padrões de fluxo do manto superior de forma não alcançada por outros métodos geofísicos. A medida de separação de ondas cisalhantes (SWS), em estações individuais, de fases refratadas no núcleo (fases SKS, por exemplo), indica a intensidade e orientação da anisotropia sísmica no manto superior. Estudos prévios de SWS na América do Sul se concentraram principalmente ao longo dos Andes e no sudeste do Brasil. Agora adicionamos medidas extras que se extendem por todo território Brasileiro e alguns países vizinhos, especialmente nas bacias do Pantanal e do Chaco-Paraná, como parte do \"Projeto Temático 3-Bacias\" da FAPESP. Os resultados tanto das estações temporárias quanto da rede permanente Brasileira mostram um mapa de anisotropia mais robusto e completo da plataforma estável da América do Sul. Em geral, as direções de polarização rápida tem em média direção L-O. Desvios significantes nas direções LSL-ONO ou LNL-OSO são observadas em muitas regiões. Comparamos nossos resultados com diferentes representantes da anisotropia: movimento absoluto de placa dado pelo sistema de referência de hotspot HS3-NUVEL-1A, um modelo recente dependente do tempo de fluxo do manto superior induzido pela subducção da placa de Nazca, anisotropia global de tomografia de ondas de superfície, e tendências geológicas. Observamos pouca correlação das direções de anisotropia com tendências geológicas, com exceção de algumas estações no norte do Brasil e uma melhor correlação com o modelo de fluxo do manto. Portanto, nossa anisotropia observada é devida principalmente a fluxo do manto superior, com pouca contribuição de anisotropia \"congelada\" litosférica. Também, desvios do modelo de fluxo do manto, o qual inclui uma litosfera mais espessa no cráton da Amazônia, são devido ao fluxo ao redor de núcleos cratônicos não usados no modelo: a quilha do cráton do São Francisco e um possível núcleo cratônico abaixo da região norte da bacia do Paraná (chamado bloco do Paranapanema). Atrasos de tempo grandes na bacia do Pantanal podem indicar um canal astenosférico mais forte, um fluxo mais coerente ou uma astenosfera mais espessa. Pequenos atrasos abaixo da parte norte da bacia do Paraná e no centro do cráton da Amazônia podem indicar uma astenosfera mais fina.

Le travail mené dans cette thèse s’inscrit dans la continuité des recherches sur l’élastographie par onde de cisaillement. Après un rappel bibliographique, la présentation d’une étude expérimentale montre que la résolution en élastographie par ultrasons est du même ordre de grandeur que la résolution en échographie et que sa limite dépasse la longueur d’onde de cisaillement. L’originalité de ce travail repose sur l’utilisation d’un réseau de sources mécaniques pour la génération et le contrôle des ondes de cisaillement. Un miroir à retournement temporel de six vibreurs est d’abord mis en place. Ce dispositif montre un contrôle spatio-temporel du champ élastique dans un gel gélatine-graphite. Comparé à l’utilisation d’un seul vibreur, le réseau de sources, proposé dans ce travail, améliore de 10dB le rapport signal sur bruit. L’application de cette méthode sur un modèle du crâne humain montre la possibilité de délivrer et contrôler les ondes de cisaillement dans le cerveau par conduction osseuse. Enfin, l’application de cette méthode à l’élastographie des couches abdominales est présentée par une étude sur un modèle synthétique et in vivo sur un volontaire sain / This thesis is in line with shear-wave elastography research. After, a bibliographic review, an experimental study shows that the resolution on ultrasound elastography is of the same order of magnitude of the echography resolution and its limit exceeds the shear-wavelength. The original aspect of this work is the use of a phased array of mechanical sources to generate and control shear waves. A time reversal mirror of six shakers is set up. This device shows a space-time control of shear-wave field in gelatin-graphite phantom. Compared to the use of a single source, this phased array of shear-wave improves by 10dB the signal to noise ratio. Using this method on human skull model shows the possibility to deliver and control shear waves in brain using bone conduction. Finally, the application of this method on shear-wave elastography of abdominal layers is shown by a study on synthetic model and in vivo on a healthy volunteer

Seismic anisotropy at continental regions, mainly at stable areas, gives important information about past and present tectonic events, and helps us in understanding patterns of upper mantle flow in a way not achieved by other methods. The measurement of shear wave splitting (SWS), at individual stations, from core refracted phases (such as SKS phases), indicates the amount and orientation of the seismic anisotropy in the upper mantle. Previous studies of SWS in South America concentrated mainly along the Andes and in southeast Brazil. Now we add extra measurements extending to all Brazilian territory, especially in the Pantanal and Paraná-Chaco basins, as part of the FAPESP 3-Basins Thematic Project. The results from both temporary deployments and from the Brazilian permanent network provide a more complete and robust anisotropy map of the South America stable platform. In general the fast polarization orientations have an average E-W orientation. Significant deviations to ESE-WNW or ENE-WSW are observed in many regions. We compare our results with different anisotropy proxies: absolute plate motion given by the hotspot reference frame HS3-NUVEL-1A, a recent model of time dependent upper mantle flow induced by the Nazca plate subduction, global anisotropy from surface wave tomography, and geologic trends. We observe a poor correlation of the anisotropy directions with geological trends, with the exception of a few stations in northern Brazil and a better correlation with the mantle flow model. Therefore, our observed anisotropy is mainly due to upper-mantle flow, with little contribution from frozen lithospheric anisotropy. Also, deviations from the mantle flow model, which includes a thicker lithosphere at the Amazon craton, are mainly due to flow surrounding cratonic nuclei not used in the model: the keel of the São Francisco craton and a possible cratonic nucleus beneath the northern part of the Paraná Basin (called Paranapanema block). Large delay times at the Pantanal Basin may indicate a stronger asthenospheric channel, a more coherent flow, or a thicker asthenosphere. Small delays beneath the northern Paraná Basin and central Amazon craton may indicate thinner anisotropic asthenosphere. / Anisotropia sísmica em regiões continentais, principalmente em áreas estáveis, nos dá informações importantes sobre eventos tectônicos do passado e do presente, e nos ajuda a entender padrões de fluxo do manto superior de forma não alcançada por outros métodos geofísicos. A medida de separação de ondas cisalhantes (SWS), em estações individuais, de fases refratadas no núcleo (fases SKS, por exemplo), indica a intensidade e orientação da anisotropia sísmica no manto superior. Estudos prévios de SWS na América do Sul se concentraram principalmente ao longo dos Andes e no sudeste do Brasil. Agora adicionamos medidas extras que se extendem por todo território Brasileiro e alguns países vizinhos, especialmente nas bacias do Pantanal e do Chaco-Paraná, como parte do \"Projeto Temático 3-Bacias\" da FAPESP. Os resultados tanto das estações temporárias quanto da rede permanente Brasileira mostram um mapa de anisotropia mais robusto e completo da plataforma estável da América do Sul. Em geral, as direções de polarização rápida tem em média direção L-O. Desvios significantes nas direções LSL-ONO ou LNL-OSO são observadas em muitas regiões. Comparamos nossos resultados com diferentes representantes da anisotropia: movimento absoluto de placa dado pelo sistema de referência de hotspot HS3-NUVEL-1A, um modelo recente dependente do tempo de fluxo do manto superior induzido pela subducção da placa de Nazca, anisotropia global de tomografia de ondas de superfície, e tendências geológicas. Observamos pouca correlação das direções de anisotropia com tendências geológicas, com exceção de algumas estações no norte do Brasil e uma melhor correlação com o modelo de fluxo do manto. Portanto, nossa anisotropia observada é devida principalmente a fluxo do manto superior, com pouca contribuição de anisotropia \"congelada\" litosférica. Também, desvios do modelo de fluxo do manto, o qual inclui uma litosfera mais espessa no cráton da Amazônia, são devido ao fluxo ao redor de núcleos cratônicos não usados no modelo: a quilha do cráton do São Francisco e um possível núcleo cratônico abaixo da região norte da bacia do Paraná (chamado bloco do Paranapanema). Atrasos de tempo grandes na bacia do Pantanal podem indicar um canal astenosférico mais forte, um fluxo mais coerente ou uma astenosfera mais espessa. Pequenos atrasos abaixo da parte norte da bacia do Paraná e no centro do cráton da Amazônia podem indicar uma astenosfera mais fina.