Beamformer for a Lensed Row–Column Array in 3-D Ultrasound Imaging

Abstract

Row-column (RC) arrays typically suffer from a limited field of view (FOV), with the imaging area confined to a rectangular region equal to the footprint of the probe. This limitation can be solved by using a diverging lens in front of the probe. Previous studies have introduced a thin lens model for beamforming lensed RC arrays, but this model inaccurately assumes the lens to be infinitely thin, leading to degraded resolution and contrast due to errors in the time of flight (TOF) calculations. This article presents a beamformer based on ray tracing for accurate TOF calculation. A Verasonics Vantage 256 scanner was equipped with a Vermon RC probe with ${128}+{128}$ elements, $\lambda $ pitch, and a ${6}~\textit {MHz}$ center frequency. A synthetic aperture ultrasound sequence with 96 virtual sources and 32 active elements for each emission with row elements was employed, and all column elements were used for acquiring data. This method was tested with a polystyrene (PS) lens with a spherical shape and polymethyl methacrylate (PMMA) in a bicylindrical shape. Based on pressure field measurements, these two lenses provide a 20° and 33° FOV, respectively. The thin lens model had a lateral resolution of around $17.4\lambda $ for the bicylindrical lens, whereas the new method achieves a resolution of around $3.8\lambda $ , representing a 4.6-fold improvement. The contrast is enhanced from 23.1 to 29.8 dB for the bicylindrical lens while preserving the FOV.