An annular array ultrasonic transducer is provided which comprises a plurality of concentric rings of piezoelectric material having widths chosen for operation at different depths of focus. The total aperture of the transducer is divided into a plurality of contiguous focal depths. In the near field...http://www.google.fr/patents/US4537074?utm_source=gb-gplus-shareBrevet US4537074 - Annular array ultrasonic transducers
