Abstract: In the illustrated embodiments, successive overlapping groups of ultrasonic transducer elements are activated to scan along successively offset group symmetry axes. The groups may be selected so that the active symmetry axis is shifted in increments of one-half the element width with a first set of symmetry axes coinciding with a gap between adjacent transducer elements and a second set coinciding with the center of a transducer element. In other embodiments, the successive groups of the first type are sequentially activated in a first cycle and the successive groups of the second type are sequentially activated in a second cycle to produce scanning along interlaced sets of symmetry axes and to produce twice the image display rate.