The Aloka Patent: US4344327: Electronic Scanning Ultrasound Diagnostic Sysystem. Dec 23, 1980. Yoshihiro Yoshikawa, Takao Katabami, Katsumi Fujinaga, Yoshiaki Kobayashi.

An electronic scanning ultrasonic diagnostic system comprising a probe having ultrasonic beam emitting surface, in which plural ultrasonic wave transducers are convexly arranged at equal distances, emitting desired sector scanning ultrasonic beams, a transmitting and receiving wave control circuit dividing transducers into optional and plural groups and repeatedly performing of transmitting and receiving action with ultrasonic directivities respectively different to each group, and supplying ultrasonic beam scanning signals to an image display section with an assumption that the respective extension lines of each sector scanning ultrasonic beams coincide with the center of the arc shape of the probe.

The picker Patent: US4409982: Ultrasonic Step Scanning ultilizing Curvilinear Transducer Array. Oct 20, 1980. Kiven Plesset, Darvin Adams.

An electronically step scanned real time ultrasonic imaging system and method is disclosed. The system includes a transducer assembly having an array of elements, each with an axis of transmission along which it transmits its main ultrasonic energy when electrically stimulated. The elements are disposed in a curvilinear array, wherein their axes of transmission are approximately coplanar, but divergent in the common plane. This configuration provides a relatively large scanned area without need for either electronic or mechanical sector scanning techniques to steer the incident ultrasonic energy over a large angle.

The Philips Patent: US4281550: Curved array of Sequenced Ultrasound Transducers. Dec 17, 1979. Kenneth Erikson.

An array of ultrasound transducers for pulsed sector-scan operation includes a plurality of transducer elements disposed on an arc of a circle and oriented to emit and receive ultrasound radiation in the direction of the center of the arc. A group of adjacent transducers within the array is active for each ultrasound pulse. The position of the group in the array is incrementally shifted along the arc, one transducer at a time, to effect scanning. The inherent focussing effect of a curved group of transducers is compensated with time delays or a negative lens to provide a parallel, sector-scanned radiation beam.

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