Minimum Variance beamforming for closely spaced microbubbles

The Minimum Variance beamformer (MVB) is known to outperform the conventional Delay-and-Sum (DAS) beamformer in terms of lateral resolution. Super-resolution ultrasound imaging (SRI) relies on localising several microbubbles in each image, and thus the objective of this work is to assess how the distance between two scatterers can affect their apparent size or shape on the image by using the MVB. The MVB method was employed on simulated point scatter data with a 8 MHz (λ=192 μm) 128-element linear array probe. Two scatterers were placed in variable positions in three dimensions and closely spaced to each other. The lateral Full-Width-Half-Maximum (FWHM) was used for performance evaluation. It was found that the FWHM of each scatterer was affected by the presence of another, and decreased as the distance between two scatterers increased. Relative positioning in axial (0.1mm apart) or azimuthal (1 mm apart) positions provided a tilted scatterer appearance. As the MVB differentiates scatterers at closer distances compared to the DAS beamformer, it is concluded that a larger number of microbubbles can be counted using the former.