Angular Spectrum Decomposition Analysis of Second Harmonic Ultrasound Propagation and its Relation to Tissue Harmonic Imaging

An angular spectrum decomposition method (ASM) for calculating second-harmonic propagation in fluid-like media is discussed. This method is based on a perturbation solution of Westervelt’s equation, which can be augmented to take into account inhomogeneity of the medium. ASM can be applied to the analysis of tissue harmonic imaging (THI), a diagnostic method that utilizes second harmonics for imaging. One of the advantages of THI is the second harmonic’s ability to improve image quality by reducing aberration caused by inhomogeneity in the body wall. The body wall aberration can be modeled as a localized phase screen in the wave propagation path. The results show that the interactions of angular spectrum components of the fundamental field generate a second-harmonic field that is relatively immune to the aberration. An experiment was performed to demonstrate this phenomenon by using a phase screen created from low density Polyethylene material. The measurements were in close agreement with calculations using ASM.