Improved framerate with synthetic transmit aperture imaging using prefocused subapertures

A method using subapertures that are prefocused near the middle of the region of interest Is proposed as a substitute for composite focusing commonly used in abdominal ultrasound imaging. In that method M transmit zones are stitched together in order to achieve dose to dynamic transmit focusing. The depth of focus or each zone is proportional to the inverse square of the aperture D, and the focused region is approximately M times that. In the proposed method, the aperture is divided into non-overlapping subapertures of size D/N. Each subaperture transmits a pre-focused beam with dynamic focusing on the full aperture on reception. Data from the subapertures is combined using synthetic aperture processing. The depth of focus for each subaperture is N*N times that of the full aperture. Thus the equivalent of composite focusing with M=N*N zones is achieved with N subapertures. Thus a framerate increase of N is achieved. In addition to the increase in framerate the focusing is of more uniform quality since it degrades gradually away from the pre-focus, and does not have any zone patterns. RF-data from point targets and tissue-equivalent phantoms has been processed and compared with composite zone focusing and the focusing has been verified to follow the predicted pattern. Examples of point responses and images are given.