Sidelobe reduction for limited diffraction pulse-echo systems

Conventional focused transducers have a sharp focal spot with low sidelobes, but they also have a short depth of field. Commercial medical scanners obtain increased depth of field by combining several images in a montage, each obtained at a different focal depth. Therefore, to get low sidelobes over a large depth of field several transmits must be used, which decrease the frame rate. Limited diffraction beams such as Bessel beams and X waves obtain good resolution over very large depth of field, but they have high sidelobes. A summation-subtraction method developed for decreasing the sidelobes is presented. The method requires three transmits, decreasing the frame rate to one third. The authors apply this technique to limited diffraction beams and obtain an analytic description.