Research On Acoustic Wave Regulation And Photoacoustic Imaging In Random Scattering Medium

Photoacoustic imaging as an emerging biomedicine diagnostic technique that has been developed quickly in the past decade, it bridges traditional optical method and acoustic method. It utilizes the optical absorption coefficient as the majoy imaging patameter, as a result, it inherits the high sensitivity of optical imaging in evaluating tissue chemical and physiological information; at the same time, the information of tissue is carried by ultrasonic wave, therefore, it also inherits the high spatial resolution of ultrasonography in imaging deep tissue; at last, as a non-ionizing method, it can guarantee the safty to tissue. For these virtues, photoacoustic imaging received widely and increasing attentions in various fields, such as optics, acoustic, and biomedical engineering. One of the majoy challenge of this technology is:the application of photoacoustic imaging is usually limited to the medium with relatively homogeneous acoustical properties, but the real tissue is imhomogeneous. The propagation of acoustic wave in imhomogeneous medium, especially in the medium with random scattering is a fascinating issue to acoustic researchers, it is of great importance in acoustic imaging and detecting. To this end, this thesis focuses on the fundamental of photoacoustic tomography and the wave’s propagation. By means of numerical simulation and phantom experiment, the improvement of photoacoustic tomography in random scattering medium and dynamic focusing of acoustic wave utilizing a randomly scattering lens and single transducer are achieved. This thesis consists of seven chapters:In chapter one, we give an introduction for the verious biomedicine imaging techniques, and the comparison with photoacoustic imaging. After that, we introduce the regulation and focusing of both optical and acoustic wave in random scattering medium. At last, a brief outline of this thesis is also given.In chapter two, we give an discussion for the theoretical background of photoacoustic imaging, including the generation of photoacoustic wave and the reconstruction of the optical absorption coefficient’s distribution in the tissue. After that, we introduce the majoy system and a discussion of their features is also given.In chapter three, based on the previous research, a reconstruction method for Green’s function in random scattering medium is proposed. There are two limitations of the previous work:first, there is no discussion for the situation that the two points are lacated at the opposite sides of the scattering lay; second, real transducers are needed at the two points. This thesis propose a "virtual source" method to retrieve the Green’s function, and break the limitations mentioned above.In chapter four, we combine time-reversal with Green’s function’s reconstruction, and improve the photoacoustic tomography in random scattering mediun significantly. Many other methods have been developed to break the limitation set by medium’s inhomogeneity, but they need prior properties of tissue, and are not suitable for random scattering. The method proposed in this chapter reconstruct Green’s function of the medium with the method introduced in the last chapter, the the initial acoustic pressure is obtained by time-reversal. Numerical simulation and phantom experiment show that, this method effectively decreases the false contrast, noise, and position deviation of images induced by the multiple scattering.In chapter five, we demonstrate with a random scattering lens, one fixed ultrasound transducer can dynamically focus the acoustic wave. This acoustic lens which composed of randomly-distributed scatterers ulitize the mutli-pathes effect of the scattering. The key issues of this method is to determine the Green’s function of the lens with the method proposed in chapter three, then, dynamically focusing to an arbitrary point by the lens and a fixed transducer can be achieved by time-reversal. In comparison with other acoustic lenses, this lens neither needs special structure nor a real acoustic source at the focusing point. Moreover, it can dynamically focus the acoustic wave at any desired point without resetting the parameter or structure of the lens. Numerical experiments are carried out to validate the proposed method.In chapter six, we apply the random scattering lens to photoacoustic imaging, and photoacoustic imaging is achieve by single fixed transducer. Theoretical analysis indicates that, if we can determine the Green’s function of the scattering lens, it can bring super-resolution effect in acoustic imaging. In this situation, the effective aperture is determined by physical size of the transducer array and the parameter of the lens. Theoretically, photoacoustic imaging can be achieved by single fixed transducer. Based on this theory, we realize single fixed transduer photoacoustic imaging in both numerical and phantom experiments.In chapter seven, a summary of this thesis and some outlook for future investigation are presented.In summary, centered around the fundamental of photoacoustic tomography and the wave’s propagation in random scattering medium, this thesis proposes a Green’s function’s retrieving method, combine this method and time reversal, we improve the photoacoustic imaging in random scattering medium; and a random scattering lens is also introduced, with which we achieve dynamic focusing of the acoustic wave and photoacoustic super-resolution imaging. Researches given in this thesis are meaningful in perspective of both theoretical exploration and practical application.