The Linear and Nonlinear Ultrasound Field of Convex Arrays Operating in a Diverging Wave Mode

Abstract

High frame rate (ultrafast) ultrasound imaging has received a lot of attention recently. Ultrafast imaging facilitated innovations such as shear wave elastography, super-resolution imaging, and ultrafast contrast-enhanced Doppler imaging. It requires the use of large apertures to send plane or diverging waves to insonify the whole imaged area in one transmit event. Combining diverging wave imaging (DWI) with tissue harmonic imaging (THI) may offer improvements in image quality comparable to those obtained in conventional focused imaging. No studies have reported simulations of the nonlinear acoustic fields produced by diagnostic arrays in diverging wave mode. The aim of this study is to model convex arrays operating in diverging wave mode with the KZK equation. We considered 2 specific problems: (a) increased bubble destruction during ultrafast contrast imaging due to the nature of the diverging field

and (b) whether the second harmonic generated in DWI is adequate for THI. In linear simulations, we showed that using convex arrays for ultrafast contrast imaging produced pressures that are greater in the nearfield and lower in the farfield than those of focused beams, and may induce more nearfield bubble destruction. In nonlinear simulations, the second harmonic produced by ultrafast THI was found to be 2-13 dB lower than that of focused beams when operated at the same maximum MI. This moderate difference of the second harmonic between DWI and focused ultrasound suggests it may be feasible to combine DWI and THI.