Analysis of Reverberations in Medical Ultrasound

Models and methods for investigation of reverberations in nonlinear imaging techniques are presented in this thesis. Four independent papers provide insight on nonlinear propagation effects, a theoretical description of reverberations and the effects of reverberations on conventional and second-harmonic ultrasound imaging. Two papers, Paper A and C, describe methods used to investigate nonlinear distortion and reverberations, whereas Paper B and D concentrate on acoustic phenomena and performance of conventional and second-harmonic imaging.Paper A provides a comparison of Field II, the Texas code and Abersim; three freely available simulation tools. If analytic solutions exist, these are used as gold standards for the comparison, and when they do not; high resolution Field II or Texas code simulations are defined to be the gold standard. The comparison suggests that Abersim performs equivalent or better than the two other methods in solving diffraction, attenuation and nonlinear distortion.In Paper B, the effects of transmit beamforming and safety regulations on second-harmonic generation at two different frequencies are investigated. The safety regulations are imposed through a limitation of the maximum mechanical index of the transmit beam. Abersim is used as the simulation tool. The results suggest that the two frequencies perform equivalently when the transmit beamforming is equal in terms of wavelengths and the medium has a linear-in-frequency attenuation. Nonlinear frequency dependent attenuation and heterogeneous effects are suggested to be the main cause of the reduced improvements of second-harmonic imaging at higher frequencies.Paper C presents a time-domain Spectral Element Method for nonlinear propagation in a finite spatial domain. The method is shown to perform well when compared with analytic plane wave solutions and in a two-dimensional comparison with Abersim. The Spectral Element Method is suggested to be accurate for heterogeneous media, but this is not investigated or verified.The last paper concentrate on reverberations. A mathematical description of reverberations is presented along with a classification system. The main results state that reverberations always act in reciprocal pairs, and that secondharmonic suppression of reverberations is a combined effect of transmit beam intensity and a reverberation weight filter presented in the paper.The thesis provides insight on the description of reverberations and how they can be investigated. The influence of reverberations on ultrasound imaging is suggested to be more severe in applications where the object of interest is fully submerged in heterogeneous tissue. Deeper understanding of ultrasound acoustics may lead to new nonlinear imaging techniques where the noise contribution can be separated from the first-order echo. In turn, this might provide better gray scale images and ultrasound diagnoses.