Improved Plane Wave Imaging with Retrospective Transmit Focusing and Weighting Based Post Filtering

The new application of ultrasonic imaging to transient elastography has triggered the emergence of high frame-rate imaging. Using plane wave excitation (PWE), a single insonification is sufficient to form an image; thus PWE is one of the methods for achieving high frame-rate imaging. 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 those using conventional focused transmission. To solve this problem, we propose a filter-based retrospective transmit focusing technique combined with weighting based post filtering - filter-derived coherence-index weighting (FRF+FCI weighting) for plane wave imaging. 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 filter-coefficient-weighted baseband data 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 sidelobe contribution in the filtered baseband data. It is demonstrated that our proposed FRF+FCI weighting technique can improve plane wave imaging quality - reducing the sidelobes, and improving the SNR and spatial resolution while being capable of retaining the high frame rate. In addition, the proposed method is inherently less susceptible to motion artifacts (because being applied over ultrafast plane wave imaging) and phase aberration (because the overall wavefront of the transmit plane wave mitigates the phase aberration on transmit).