Rocking convex array used for 3D synthetic aperture focusing

Volumetric imaging can be performed using 1D arrays in combination with mechanical motion. Outside the elevation focus of the array, the resolution and contrast quickly degrade compared to the azimuth plane, because of the fixed transducer focus. The purpose of this paper is to use synthetic aperture focusing (SAF) for enhancing the elevation focusing for a convex rocking array, to obtain a more isotropic point spread function. This paper presents further development of the SAF method, which can be used with curved array combined with a rocking motion. The method uses a virtual source (VS) for defocused multi-element transmit, and another VS in the elevation focus point. This allows a direct time-of-flight (ToF) to be calculated for a given 3D point. The method is evaluated using simulations from Field II and by measurements using the RASMUS experimental scanner with a 4.5 MHz convex array (GE Kretztechnik, Zipf, Austria). The array has an elevation focus at 60 mm of depth, and the angular rocking velocity is up to 140deg/s. The scan sequence uses an f_prf of 4500 - 7000 Hz allowing up to 15 cm of penetration. The full width at half max (FWHM) and main-lobe to side-lobe ratio (MLSL) is used as quantitative measurements. The elevation FWHM for simulated scatterers placed at depths of 30 to 140 mm of depth were improved by 26.4% on average, and the MLSL ratio was improved by an average of 8.49 dB for the scatterers using 3D SA focusing. The elevation FWHM for a measured wire phantom was improved by 33.8% on average by applying 3D SA focusing. In-Vivo measurements show an improvement in C-scans matching what is found in simulations and wire phantoms. The method has shown the ability to improve the elevation focus and contrast for a convex rocking array. This was shown for simulations and for phantom and In-Vivo measurements using commercially available equipment.