Contactless Remote Induction of Shear Waves in Soft Tissues Using a Transcranial Magnetic Stimulation Device

This study presents the first observation of shear wave induced remotely within soft tissues. It was performed through the combination of a transcranial magnetic stimulation device and a permanent magnet. A physical model based on Maxwell and Navier equations was developed. Experiments were performed on a cryogel phantom and a chicken breast sample. Using an ultrafast ultrasound scanner, shear waves of respective amplitude of 5 and 0.5 micrometers were observed. Experimental and numerical results were in good agreement. This study constitutes the framework of an alternative shear wave elastography method

Imaging of Shear Waves Induced by Lorentz Force in Soft Tissues

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this 5 study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 um. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model 10 using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample

Super-resolution in near-field acoustic time reversal using reverberated elastic waves in skull-shaped antenna

International audienceWe investigate the potential of using elastic waves for near-field acoustic time reversal, and in doing so evaluate the possibility of reconstructing sound source positions at below-wavelength distances from a skull-shaped acoustic antenna. Our work is based on a conceptual processing model that translates elastic waves conducted and reverberated in an elastic object into source position, through a time reversal analysis. Signals are recorded by passive sensors glued on a replica of a human skull, measuring solely its mechanical vibrations, and not sensitive to airborne sound. The sound source is placed along the azimuthal and sagittal planes for distances to the skull between 5 and 100 cm. We reconstruct the source position for signals with frequencies in the physiological hearing range with a resolution indirectly proportional to the distance between source and skull across all measurements in the far-field. Measurements in the near-field show –3 dB widths smaller than half a wavelength (super-resolution) with highest resolutions of down to λ/15 measured in front of the orbital cavities. We infer that these anatomical details give rise to complex features of the skull's Green's function, that in turn enhance resolution in a direction-dependent manner

Acousto-electrical speckle pattern in Lorentz force electrical impedance tomography

Ultrasound speckle is a granular texture pattern appearing in ultrasound imaging. It can be used to distinguish tissues and identify pathologies. Lorentz force electrical impedance tomography is an ultrasound-based medical imaging technique of the tissue electrical conductivity. It is based on the application of an ultrasound wave in a medium placed in a magnetic field and on the measurement of the induced electric current due to Lorentz force. Similarly to ultrasound imaging, we hypothesized that a speckle could be observed with Lorentz force electrical impedance tomography imaging. In this study, we first assessed the theoretical similarity between the measured signals in Lorentz force electrical impedance tomography and in ultrasound imaging modalities. We then compared experimentally the signal measured in both methods using an acoustic and electrical impedance interface. Finally, a bovine muscle sample was imaged using the two methods. Similar speckle patterns were observed. This indicates the existence of an "acousto-electrical speckle" in the Lorentz force electrical impedance tomography with spatial characteristics driven by the acoustic parameters but due to electrical impedance inhomogeneities instead of acoustic ones as is the case of ultrasound imaging

Observation of natural flexural pulse waves in retinal and carotid arteries for wall elasticity estimation

The risk of cardiovascular events is linked to arterial elasticity that can be estimated from the pulse wave velocity. This symmetric wave velocity is related to the wall elasticity through the Moens-Korteweg equation. However, ultrasound imaging techniques need improved accuracy, and optical measurements on retinal arteries produce inconsistent results. Here, we report the first observation of an antisymmetric pulse wave: the flexural pulse wave. An optical system performs in vivo wave velocity measurements on retinal arteries and veins. Velocity estimation ranges between 1 and 10 millimeter per second. The theory of guided waves confirms the existence of this wave mode and its low velocity. Natural flexural waves can also be detected at the bigger scale of a carotid artery using ultrafast ultrasound imaging. This second natural pulse wave has great potential of becoming a biomarker of blood vessel aging

Elastographie et retournement temporel des ondes de cisaillement (application à l'imagerie des solides mous)

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.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

One channel spatio-temporal inversion of acoustic wave in reverberant cavities

International audienceIt has been recently shown that it was possible to optimally recover the Green functions from a complex wave field despite of a non-isotropic distribution of noise sources. The method used is based on a particular use of the inverse filter (IF) formalism which is called the passive IF. Based on this formalism, we have investigated the possibility to control the spatio-temporal degrees of freedom in a reverberant cavity for the focusing of waves (active processes). The understanding of this phenomenon can be very useful in a lot of different applications like in acoustical imaging, seismology, or telecommunications. In the present work, the spatio-temporal focalization of ultrasounds in reverberant cavities is studied using medical arrays and water tanks. Through experiments, a complete spatio-temporal inversion is realized to synthesize optimized emitting signals. The result generalizes the focalization control over a spatial vector and during an arbitrary time window

Friction experiments with elastography: the slow slip and the super-shear regimes

International audienceTo get an insight into the processes underlying dynamic friction that plays an important role in seismic sources for example, we developed a sliding dynamic experiment coupled to elastography imaging. This experimental setup permits to observe simultaneously the frictional interface and the waves emitted in the bulk during slipping. We use soft solids made of hydro-organic gel of PVA, in contact with either glass or sandpaper. The huge interest of such soft solids is that ultrasonic waves allows to observe in real time the rupture nucleation and propagation, as well as shear waves themselves inside the medium. We investigate the friction in two different cases. In the case of friction on sand paper, links are formed between the gel and the sand paper by local pinning. The breaking of these links emits a characteristic wave pattern, and their occurrence is related to the local sliding velocity. In a very different way, when the gel slide on a glass surface, with an interlayer of sand grains, the slip occurs as successive rupture events, with a rupture front crossing the whole surface. We can study then the rupture velocity, and in the cases of ruptures faster than the shear wave velocity, we observe a Mach cone of shear waves