Universal REFoCUS Beamforming With Spatial Weighting

REFoCUS (Retrospective Encoding For Conventional Ultrasound Sequences) offers great flexibility by enabling synthetic aperture beamforming from conventional ultrasound sequences. This flexibility is beneficial for many aspects in medical ultrasound beamforming, including e.g. combination of different transmit waves, distributed sound speed estimation and common-midpoint gathers. REFoCUS beamforming also has image quality comparable to state-of-art methods such as Retrospective Transmit Beamforming (RTB). However, the previously published implementations of REFoCUS do not address clutter from sidelobes and grating lobes present in the data before the recovery. This reduces image quality due to potentially strong sidelobes and grating lobes, particularly when using REFoCUS in combination with micro-beamforming and matrix array probes. Recordings from micro-beamforming probes may thus not be compliant with the existing REFoCUS methods. We propose to solve the sidelobes and grating lobe issues by introducing a reformulation of REFoCUS that performs multistatic data recovery and beamforming in the time domain, allowing spatial weighting to remove clutter and noise. Spatial weighting is based on common beamforming principles and incorporates element directivity, dynamic F-number, beam geometry weighting, and grating lobe suppression. We also discuss how aperture sampling affects beamforming with REFoCUS. Spatially Weighted REFoCUS (SWR) and critical sampling of the transmit aperture show suppression of receive grating lobes in an in vivo setting with two different micro-beamforming matrix-array probes, leading to an increase in gCNR contrast from 0.44 to 0.96 in a fetal image and from 0.39 to 0.89 in a cardiac image.