Abstract : Several methods have been proposed to estimate the viscoelastic properties of soft biological tissues using forced low frequency vibrations (10-500Hz). Those methods are based on the measurement of phase velocity of the shear waves using ultrasound. This thesis shows that the measurements of velocity as well as attenuation are subjected to bias. Those bias are related to reflected waves created on boundaries, to the non negligible size of the piston source which causes diffraction effects and to the influence of a low frequency longitudinal wave. Indeed, a theoretical analysis of the field radiated by a point source explains how mechanic vibrations of a piston generates a transverse wave with a longitudinal component and how this component can interfere with a low frequency longitudinal wave. However, by using a low frequency transient excitation, those bias can be avoided. This method, the « Transient Elastography », is applied to an agar-gelatin phantom, to a muscle and to a dairy produce; the shear wave velocities are found to be 2.7, 5.3 and 0.5 m.s-1. Then the precise numerical values of elasticity and viscosity can be deduced. However, generation of shear waves by acoustic vibrators is a relatively complex problem and the directivity pattern of shear waves produced by usual vibrators are more complicated than those obtained for longitudinal ultrasonic transducers. In order to extract shear modulus parameters from the shear wave propagation in soft tissues, it is important to understand and to optimize the directivity pattern of shear wave vibrators. This thesis is devoted in the second part to a careful studying of the theoretical and the experimental directivity pattern produced by a point source in soft tissues. Both theoretical and experimental measurements show that the directivity pattern of a point source vibrator presents two very strong lobes for angle around 35°. This second part also point out the impact of the near field in the problem of shear wave generation. The whole results show that transient elastography may be very useful in rheology and in medecine.
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Stefan Catheline. INTERFEROMETRIE-SPECKLE ULTRASONORE : APPLICATION A LA MESURE D'ELASTICITE. Physique [physics]. Université Paris-Diderot - Paris VII, 1998. Français. ⟨tel-00378129⟩



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