A Retrospective Look at Retrospective Transmit Beamforming

The advent of software-based ultrasound systems has enabled a new beamforming paradigm known as Retrospective Transmit Beamforming (RTB) -- a major advance in clinical ultrasound imaging. RTB achieves two-way focus everywhere in the image without need for multiple transmit focal zones, yielding triple breakthroughs in image quality, frame rate and ease-of-use. In this review paper, the RTB innovation is traced back to three different analytical approaches: 1) Synthetic focusing; 2) Spatial filtering; and 3) Fast echo acquisition. For each approach, the essential concepts and motivation are described with reference to key publications.