P3G-11 Generation and Aberration of Second-Harmonic Ultrasound Beams in Heterogeneous Tissue

A simulation study with different f-numbers shows results concerning generation and aberration of three-dimensional second-harmonic beams using a constant aperture. The second-harmonic is compared to the transmit first-harmonic and a same frequency fundamental. Heterogeneity was implemented as a 20 mm body wall model using a series of delay screens designed to match human abdominal body wall characteristics. Transmit pressures for the simulations are estimated using a constant MI of 1.1. The amount of second-harmonic generated increases for high f-numbers improving the signal-to-noise-ratio of the second-harmonic. A substantial part of the aberrated second-harmonic total energy at the focal point is generated beyond the body wall. Aberration reduces the amount of second-harmonic energy generated by 30-50% compared to the homogeneous case. The second-harmonic transmit beam is nonetheless aberrated. Comparison of aberrated transmit beam profiles shows that the side-lobe level of the second-harmonic is lower than for the fundamental beam, and about the same as the first-harmonic. The shape of the second-harmonic beam profiles resembles the first-harmonic more and more for an increasing f-number. Thus, second-harmonic imaging improves image quality for deeper organs due to the improved signal-to-noise-ratio and lower side-lobe level for both a homogeneous and a heterogeneous case