Compound Lens Design and Beamforming for Wide-Field Volumetric Imaging Using a Row-Column Array

Row-column addressed (RCA) ultrasound arrays enable volumetric imaging with a significantly reduced channel count compared to fully addressed 2-D arrays; however, their field of view (FOV) is typically limited to the transducer footprint. Diverging acoustic lenses can extend the FOV, but accurate beamforming remains challenging when the lens's finite thickness and multilayer structure are not properly modeled. This work presents a compound acoustic lens design that simultaneously achieves a large FOV, reduced edge-wave artifacts, and acceptable sensitivity loss, together with a beamforming approach that accurately accounts for wave propagation through multilayer lens structures. The proposed approach combines ray tracing with a Normal Path (RT-NP) approximation and determines the optimal propagation path using a two-dimensional Newton-Raphson optimization, enabling accurate time-of-flight (TOF) estimation while significantly reducing computational complexity compared to conventional two-interface ray-tracing methods. The method is validated experimentally using a 128+128 Vermon RCA transducer operating at 6 MHz and equipped with a BiQuadratic compound lens composed of TPX and RTV-PTFE materials. Pressure-field measurements validate the predicted wavefront expansion, while simulations indicate an FOV expansion to approximately 54.5°, enabling imaging well beyond the transducer aperture while preserving high spatial resolution and image quality. Hydrophone and RF-data measurements demonstrate sub-wavelength TOF accuracy of the proposed beamformer. The proposed system achieves a penetration depth of 75 mm compared with 90 mm without the lens, with only minor degradation in spatial resolution. These results demonstrate that compound-lens RCA systems, combined with accurate and efficient beamforming, enable wide-field volumetric imaging of organ-scale structures while preserving the practical advantages of RCA arrays, with a theoretical volume rate of approximately 104 Hz, indicating the potential for real-time 3-D visualization.