Applications of Computation in Acoustics: Ultrasound Bioeffects and Underwater Transmission Loss Uncertainty
Patterson, Brandon
2017
Abstract
This dissertation presents work in which computational modeling is used to advance two areas of interest in modern acoustics. First, we develop computational models to study the poorly understood physics underlying Contrast-Enhanced Ultrasound (CEUS) bioeffects and Diagnostic Ultrasound (DUS)-induced lung hemorrhage. A better understanding of these problems is critical to the development of science-based safety guidelines. Because inertial cavitation is thought to cause CEUS bioeffects, we model a spherical bubble in viscoelastic soft tissue, driven by experimentally-measured ultrasound waveforms (1.5 − 7.5 MHz), with known bioeffects thresholds. Bioeffects thresholds were compared with calculated cavitation metrics. Experimentally-determined bioeffects thresholds were found greater than accepted thresholds for inertial cavitation in water, and that the ratio of maximum radius to equilibrium radius correlated strongly with bioeffects thresholds as a function of frequency. Separately, we investigate DUS-induced lung hemorrhage and model an acoustic wave in soft tissue (water) traveling toward an alveolus (air) with a perturbed surface. First, a trapezoidal wave with ultrasonically relevant properties (5.0−12.5 MPa pressures amplitudes, 1.3−6.0 μs wave durations), drives the interface. We showed that acoustic waves may be capable of depositing sufficient baroclinic vorticity to drive significant interface perturbation growth, long after the wave passes. Interface perturbation growth scaled with the circulation density (i.e., the circulation per unit length of the interface) and exhibited power-law behavior at late time. To approximate ultrasound-induced stresses and strains on alveoli, we subjected interfaces of varying initial perturbation amplitude to 1.5 MHz ultrasound pulses with peak amplitudes from 1 − 5 MPa and observed interfacial viscous stress amplitudes up to approximately 60 Pa and interfacial strains up to 38%. While the viscous stresses appeared beneath capillary failure thresholds, the calculated strain was sometimes greater than expected failure thresholds for alveolar epithelium. Since this work considers only a single pulse, and the vorticity driving the deformation is expected to accumulate over multiple pulses, we conclude that vorticity-induced deformation is worthy of further investigation. Finally, we develop area statistics, a computationally efficient method for estimating Probability Density Functions (PDFs) of acoustic Transmission Loss (TL) in uncertain ocean environments. Such PDFs of TL are useful for real-time Naval applications but impractically expensive to obtain via traditional computational methods. This work describes how to approximate the PDF of TL at a point of interest using TL statistics from the field surrounding the point of interest. The idea is that local TL variations due to environmental fluctuations are represented by spatial variations in TL in single baseline TL field calculation. The baseline calculation used the most probable values for uncertain parameters describing the sound speed profile, bathymetry, and geoacoustic bottom-layer properties. Area statistics-generated PDFs were compared with 2000-sample Monte Carlo-generated PDFs via the L1 error, in ten ocean environments with varying properties and uncertainties, for source frequencies of 100, 200, and 300 Hz and depths of 91, 137, 183m. The area statistics-generated PDFs had an L1 error < 0.5 in 91% of the 11,000+ test locations (depths from 20 m - 4.5 km, ranges from <1 km to >70 km). Area statistics PDFs were generated in milliseconds, and required one-millionth the computational effort required for Monte Carlo PDFs and thus suitable for real-time applications.Subjects
acoustics ultrasound transmission loss uncertainty underwater acoustics ultrasound bioeffects computational modeling
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