Synthetic Aperture High Quality B-mode Imaging with a Row-Column Array Compared to Linear Array Imaging

Row-column (RC) matrix probes can yield 3-D volumetric imaging using a number of receiving elements similar to traditional linear arrays for commercial scanners. Some doubts have, however, been raised on the B-mode image quality of RC probes. It is hypothesized that synthetic aperture (SA) RC imaging can yield a better volumetric resolution than commercial spatially translated linear arrays, and at the same time attain volume rates similar to frame rates for single slice linear array scanning. A Vermon 6 MHz $128\times 128$ elements RC array with $\lambda$ pitch was used on a Verasonics Vantage 256 scanner. A SA sequence with $2\times 96$ emissions on the rows and columns and reception on all 128 orthogonal elements were employed giving a 62.5 Hz volume rate for a pulse repetition frequency of 12 kHz. The resolution and contrast were compared to an optimized SA sequence for a 256-elements GE L3-12D 6 MHz linear array probe, where the volume was acquired using a linear probe translation in the elevation direction. Imaging was conducted on a 3-D printed point spread function (PSF) phantom with scattering voids in a $6\times 4\times 4$ grid with a 2.05 mm spacing in all three directions. The exposed kidney of a Sprague-Dawley rat was also scanned in vivo with the RC probe. B-mode images of the 3-D printed PSF phantom were shown with a 40 dB dynamic range along with the linear array scans. An isotropic resolution of $(1.05\lambda, 1.10\lambda,0.62\lambda)=(x, y, z)$ was obtained for the row-column probe. The linear array probe had an elevation resolution determined by the geometric elevation focus of the array, and therefore the four rows of point scatterers could not be differentiated in the elevation direction due to the fixed elevation focus. The data rates were identical for the two arrays, but the RC array yielded an isotropic PSF with an improved contrast, and a 62.5 Hz volume rate comparable to normal linear array imaging. In vivo kidney images for the three orthogonal planes were shown with a 60 dB dynamic range demonstrating the isotropic speckle pattern in all three directions for all depths. The SA imaging RC sequence thus yielded a PSF independent of orientation and depth. Any slice plane in the volume therefore had a uniform speckle pattern, contrast, and resolution, demonstrating that RC arrays can yield higher quality B-mode images than linear arrays. The penetration depth of the probe and sequence was also measured to be $550\lambda$ corresponding to 141 mm.