Browsing by Subject "Acoustic radiation force"
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Item Acoustic radiation force and torque on nonspherical objects(2022-04-15) Jerome, Thomas Samuel; Hamilton, Mark F.; Haberman, Michael R; Porter, Tyrone M; Wilson, Preston SThis dissertation presents two theories for acoustic radiation force and torque on nonspherical objects. Developed first is a semi-analytical theory for acoustic radiation force and torque on compressible spheroids. The analytical framework generalizes a previous theory for radiation force on a sphere, and expresses the radiation force and torque on objects of arbitrary shape and composition in terms of products of coefficients in spherical wave expansions of the incident and scattered fields. The theory applies to homogeneous compressible spheroids of arbitrary aspect ratio, size, and specific acoustic impedance. Boundary conditions are satisfied using spheroidal coordinates and spheroidal wave functions to obtain a system of equations for the coefficients in the expansions of the scattered and transmitted fields. The spheroidal wave expansion coefficients are related analytically to the required spherical wave expansion coefficients. Results are presented for radiation force and torque exerted on compressible and rigid prolate spheroids by progressive and standing plane wave incidence. Developed second is the Born approximation of acoustic radiation force and torque, which applies to soft objects of arbitrary shape and allows for inhomogeneous material properties. The Born approximation expresses the radiation force and torque as integrals over the volume of the object. The approximation is shown to be accurate for objects with sizes up to the order of a wavelength subject to two restrictions. First, the compressibility and density of the object must be similar to the corresponding properties of the surrounding fluid, such that scattering from the object is weak, and second, the incident field must be sufficiently distinct from a progressive plane wave that the dominant contribution to acoustic radiation force is provided by gradients of the time-averaged potential and kinetic energy densities. Simplified analytical solutions and numerical results are obtained for objects with a variety of shapes and material property distributions, including homogeneous and inhomogeneous spheres, cylinders, prolate spheroids, and a canonical representation of a red blood cell. Results reveal effects of the geometry, orientation, and material inhomogeneity of an object on the acoustic radiation force and torque. The theory is validated for spheres and prolate spheroids in a standing plane wave, and for spheres in standing and progressive spherical wave fields.Item Acoustic radiation force due to sound beams incident on spherical scatterers in soft tissue-like media(2019-05) Treweek, Benjamin Charles; Hamilton, Mark F.; Haberman, Michael R; Ilinskii, Yurii A; Kallivokas, Loukas F; Wilson, Preston SThis dissertation presents a theory for acoustic radiation force on a spherical scatterer embedded in a soft elastic medium that supports the propagation of shear waves. Existing theories for acoustic radiation force on a sphere are restricted to a fluid surrounding the sphere. Potential applications reside in biology and medicine. For example, the mechanical properties of soft tissue, in particular its shear stiffness, are a useful proxy for tissue health and can be used for non-invasive tissue characterization. The present work investigates the effect that shear elasticity in the surrounding medium has on the radiation force on an embedded spherical scatterer. The theory is developed in Lagrangian coordinates, instead of Eulerian coordinates that are traditionally used for a fluid surrounding the sphere. It is assumed that a compressional wave is incident on the sphere. Coefficients in the spherical harmonic expansions describing the incident compressional wave field, and the scattered compressional and shear wave fields, are examined in detail. The radiation force is separated into two contributions, one that corresponds to the scattered compressional waves and the other to the scattered shear waves, both of which displace the sphere from its initial position. The compressional wave contribution can be determined analytically, and a variety of material properties for the sphere and incident beam patterns are examined for this contribution. The shear wave contribution has yet to be determined analytically, and is investigated numerically. A third contribution, corresponding to the static deformation of the surrounding medium due to the scattered shear wave, is also investigated numerically. A finite element method is used to determine the full effect of the scattered shear wave on the sphere displacement, and it is found that effects due to the scattered shear wave may be of the same order as those due to the scattered compressional wave.Item Estimation of the mechanical properties of soft tissues using a laser-induced microbubble interrogated by acoustic radiation force(2012-05) Yoon, Sangpil; Emelianov, Stanislav Y.; Hamilton, Mark F.; Aglyamov, Salavat R.; Wilson, Preston S.; Karpiouk, Andrei B.; Larin, KirillThis dissertation introduces a new approach to measure the mechanical properties of soft tissues. A laser-induced microbubble, created by focusing a single nanosecond laser pulse with a custom-made objective lens, was created at desired locations inside a tissue sample. An acoustic radiation force was generated by a low frequency transducer to displace the microbubble. A custom-built high pulse repetition frequency (PRF) ultrasound system, consisting of two 25 MHz single element transducers, was used to track the dynamics of the microbubble. Reconstruction of the mechanical properties at the specific location in a tissue sample was performed using a theoretical model, which calculated the dynamics of a microbubble under an externally applied force in a viscoelastic medium. The theoretical model and the high PRF ultrasound system were successfully validated in both gelatin phantoms and ex vivo bovine crystalline lenses. Age-related sclerosis of the crystalline lenses from bovine was clearly detected, which might be linked to changes in the crystalline. Location-dependent variation explained that the outer cortex and the inner nucleus had different mechanical properties. In the old and young porcine vitreous humors, age-related changes were not found. However, local variations of the mechanical properties were discovered, which may coincide with the different distributions of the molecular compositions. The laser-induced microbubble approach shows potential for future research into the origin of physiological phenomena and the development of inherent disorders in the eye. I hope that further studies – in the development of a more suitable theoretical model for the microbubble dynamics, in extension to in vivo applications, and in defining the relationship of the mechanical properties to molecular components in the eye – may provide a plan for the therapeutic treatment of eye-related diseases.