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This thesis addresses the underlying physics associated with nonlinear ultrasonic field propagation, measurements of the nonlinear properties of materials, and mechanisms contributing to the observed systematic variation of backscattered ultrasonic energy from the heart over the heart cycle. Studies were performed to address the reliability of the methods of measurement used for the quantitative characterization of nonlinear phenomena and to explore the utility of these methods. This thesis examines the measurement of nonlinear acoustic properties of materials using several methods, including the transmission of ultrasound through the material, as well as the backscattered signal from a region of interest within the material. A method of transmitting ultrasound into the diffractive far field with a negligible amount of distortion is described, along with the consequences of working with different frequencies (and subsequently different diffractive regimes). This thesis also describes studies designed to assess the nature of backscattered ultrasound from the heart obtained by using harmonic imaging, which utilizes nonlinear phenomena to improve the overall quality of clinical ultrasonic images. Several investigators have previously reported a systematic cyclic variation in the backscattered ultrasonic signal from the tissue of the heart. However, a discrepancy in the reported magnitude of this variation seems to be present in the literature. This discrepancy is examined in the context of the multiple methods used to characterize the variation. Furthermore, the characteristics of this systematic variation of backscatter are compared with the dynamics of the left ventricle described using a damped harmonic oscillator model as an approach for identifying the underlying causes of the observed variation.

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9781243775382

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