Investigation of ultrasound plane-wave imaging for optimal synthetic focusing

Plane-wave imaging for ultrafast ultrasound, revolutionized by substantially enhancing frame rates, has paved the way for numerous applications in medical ultrasound. To improve spatial resolution and contrast, plane-wave compounding is typically utilized. Plane-wave compounding, referred to as plane-wave synthetic focusing (PW-SF) in this study, coherently sums the echoes from plane waves steered at multiple angles. To further improve the image quality and reduce the computational complexity for PW-SF, this study investigates the impact of plane-wave angle distributions and beamforming grids. Plane-wave angles with evenly selected in the radian space and in the sine of radian space, referred to as θ-even and α-even angles, respectively, are analytically derived. For the beamforming grids, we apply a plane-wave pixel-based synthetic focusing (PW-PBSF) approach, directly focusing echoes on display pixels, to achieve the optimal image quality in terms of resolution and computational complexity. From the simulation and phantom experiment, the largest improvement in lateral resolution was observed with PW-PBSF [0.16 mm], which produced 0.29 mm compared with 0.35 mm for PW-SF-256, corresponding to approximately a 17% enhancement. The α-even angle discretization approach also provided higher contrast, contrast-to-noise ratio (CNR), and generalized contrast-to-noise ratio (gCNR) values, measured as 5.72 dB, 1.54, and 0.841 dB, respectively whereas θ-even yielded 5.58 dB, 1.51, and 0.814. Consistent with the phantom findings, In vivo imaging of carotid artery also demonstrated that the PW-PBSF can provide improved resolution of plane-wave images without incurring redundant computational costs.