" ..... Albert Macovski patented a method (US3918024: Ultrasonic array for reflection imaging, June 24, 1974) of controlling a circular, or linear array of transducer elements to generate a dynamically focused beam that could also be swept through space simply by adjusting the delays applied to the array. This eliminated the need for mechanical scanning of the transducer to form a B-scan, and was one of the first ultrasound phased arrays. The linear array included per-channel fixed steering delay lines that could be changed from firing to firing to sweep a sector, as well as a quadratically tapped delay line for focusing. This array configuration, with long slender array elements, has become the standard in commercial phased array scanners due to its flexibility, good acoustic sensitivity, and simple construction. Most transducers include a fixed focus lens in the out-of-plane (y) direction to reduce slice thickness and improve sensitivity.
Macovski also proposed a combined circular and linear array configuration. The circular array was used during transmit to sweep the beam in the x direction and yet provide focusing in the y direction. The linear array at the center received signals along a dynamically focused axial beam but relied on the sinc beam profile or a fixed lens in both the x and y directions. The product of the transmit and receive beams is the total pulse-echo beam profile. The difficulty with this configuration was the low acoustic sensitivity resulting from the small element size. There were various implementations of fixed and dynamic delay lines for these annular and linear phased arrays. Macovski in his 1975 patent proposed using Charge Coupled Devices (CCDs) to move "packets" of charge along a delay line at clock rates proportional to the depth (R). Melen, at Stanford, built such a CCD-based beamformer and later that same year published a paper about it. This technique suffered from several technical issues including low charge transfer efficiency (CTE) of CCD devices available at the time and artifacts resulting from the different clocks (one for each channel) used to drive the delay lines. ..... "
Excerpted from a brief history of Ultrasound Beamforming by Steve Freeman from the Biomedical Ultrasonics Laboratory at the University of MIchigan
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