Effects of Aberration on Super-Resolution Ultrasound Imaging using Microbubbles

Visualizing vasculature beyond the diffraction limit can be achieved using ultrasound super-resolution imaging. Typically, ultrasound scanners model the target medium as homogeneous, assuming a constant speed-of-sound for time-of-flight based calculations. However, variations in ultrasound propagation velocity caused by varying tissue layers affect beamforming operations. This aberration is likely to have considerable effect on super-resolution ultrasound imaging at depth. Here we investigate the effect of aberration on super-resolution ultrasound localization error in soft tissues using simulations. Wave propagation using both linear, B-Mode, and nonlinear, pulse inversion, (PI) imaging acquisition through different tissue media was modelled from a linear array using k-Wave, and the resulting pulse-echo data from a microbubble located at 50 mm depth was beamformed. Media included a control homogeneous propagation medium, and aberrating media typical of liver imaging. Results indicated super-resolution ultrasound localisation errors increased with increasing aberration and were affected by both the imaging method and microbubble sizes. Average localisation errors for PI reached 749 µm axially and 986 µm laterally and 844 µm axially and 734 µm laterally for B-mode. The scale of these errors relative to micro-vascular structures of interest suggests that aberration will have considerable impact on super-resolution ultrasound performance and requires attention to ensure its success.