Improved plane-wave high frame rate imaging using retrospective transmit focusing and filter-derived coherence-index weighting

Recently, a wide spectrum of new applications in biomedical ultrasound such as real time 3D ultrasound and transient elastography has triggered the need of high frame-rate imaging. One of the methods to achieve high frame-rate imaging is to use plane wave excitation (PWE) with which a single insonification is sufficient to form an image. However, due to the lack of transmit focusing, the signal-to-noise-ratio (SNR), contrast, and spatial resolution of the resultant images are lower compared with a conventional focused transmission. To solve this problem, we propose a filter-based retrospective focusing technique combined with filter-derived coherence-index weighting (FRF+FCI weighting) for high frame-rate imaging with PWE. A 2-D filter is designed and applied to beamformed baseband data to retrieve transmit focusing. Then a filter-derived coherence index (FCI) at each imaging point is used as a weighting factor to further improve the filter-retrieved focusing quality. Here FCI is a coherence measure of the filtered signal samples in the sliding filter kernel, Due to the sidelobe-suppression nature of the designed 2-D filter, FCIs are low at sidelobes; thus FCI weighting can further suppress the sidelobes in the filtered signal samples. Simulation results demonstrated that our proposed FRF+FCI weighting technique can enhance the image quality of high frame-rate imaging with single PWE — reducing the sidelobes, and improving the SNR and spatial resolution while being capable of retaining the high frame rate. Since it is applied over high frame-rate imaging with single PWE, FRF+FCI weighting performed after beamforming is inherently insusceptible to motion artifacts. In addition, coded excitation techniques can be integrated into our technique to further improve SNR.