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Synthetic dynamic transmit focus

Commercial ultrasound scanners employ either a single transmit focus or "multi-zone" focus (in which echo data received from different transmit foci along a given line are detected and then stitched together). However, it is also possible to combine multi-zone echo data prior to detection. An advantage of pre-detection combination is that, by varying the combinatorial weights as a function of range, a continuous (dynamic) transmit focus can be synthesized. Performance can be further enhanced by applying dynamically varying delays prior to combination. The authors present a theoretical basis for this approach. By casting it in terms of the well known sampled data reconstruction process it becomes apparent that the transmit delays required to implement dynamic transmit focus can, within limits, be synthesized on receive using an interpolation process. Formulations for the minimum "sample rate" (focal point spacing) and the dynamic weights and delays are presented. The theoretical predictions are supported by wideband simulations.