Synthetic transmit focusing for ultrasonic imaging

Medical ultrasound imaging systems use large transmit apertures for improved azimuthal resolution. Uniform imaging characteristics are achieved by employing multiple transmit focal points. The separation between the focal points has to be small enough so that the beam degradation at the transition point is acceptable. The method presented here improves the beam characteristics away from the transmit foci, in particular at the transition points. The proposed method synthesizes transmit foci in between two real focal points. A synthesized signal is formed as the weighted sum of the echoes resulting from the transmissions to the real focal points. Since the first focal point is closer to the imaging aperture than the target point it creates a parabolic phase error. The other focal point is located beyond the target and creates a phase profile of opposite curvature. By properly combining the echo signals the opposing phase curvatures partially cancel each other. The reduced phase errors in the combined signal improve the shape of the synthesized beam. The weighting factor for the signal combination depends on the location of the target point with respect to the real focal points. Analytically, the weights are determined with a least squares method that minimizes the combined phase errors created by the pair of transmit focal points