Toward the optimization of a diverging-wave acquisition sequence for pulse-echo ultrasound*

Conventional ultrasound imaging utilizes a sequence of focused transmit (FT) beams swept across a region of interest (ROI). Plane-wave transmit (PWT) imaging uses unfocused, planar wavefronts at multiple angles, allowing for rapid compounding and retrospective focusing; however, PWT imaging is restricted by its limited region of overlapping insonification. Diverging-wave transmit (DWT) schemes have also been shown to increase frame rates by compounding beams with broad curvature using virtual sources. To compare the imaging capability among FT, PWT, and DWT schemes, we performed simulations in Field II. Data was simulated using a Verasonics P4-2v cardiac ultrasound probe geometry with N = 64 elements, width w = 0.25 mm, height h = 14 mm, and kerf = 0.05 mm. A center frequency of 2.75 MHz with 74% fractional bandwidth at -6 dB were used for all cases. An ROI was populated with 35 point scatterers at x(0,20) mm by z(5,35) mm with 5-mm spacing in x and z. The full synthetic aperture dataset was calculated for the P4-2v probe and the defined medium. Each imaging case was studied by applying the delay profiles for a fixed focus or virtual source regime. Results showed an increase in lateral resolution of point-spread functions (PSFs) at 15-mm depth for single PWT and DWT beams compared to FT, as expected. Analysis of the PSF contours for all 35 point scatters were used to determine the optimal parameters required to image within a given ROI for each case investigated.