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.