Directional Coherence Factor for Volumetric Ultrasound Imaging With Matrix Arrays

Matrix arrays with small apertures limit spatial and contrast resolutions of volumetric ultrasound imaging. Coherence-based beamformers are prevalent for sidelobe suppression and resolution improvement. While the spatial coherence of a matrix array is fundamentally a 2-D function, conventional coherence factor (CF) methods neglect the directional variation of an ${M} \times {N}$ matrix array when calculating volumetric coherence. We hereby propose a projection-based directional CF (DCF) to exploit the 2-D nature of volumetric coherence function. Instead of computing the coherent and incoherent summations across the entire 2-D aperture, DCF projects aperture data onto azimuthal, elevational, diagonal, and anti-diagonal directions and subsequently calculates the CFs for each direction separately. The orthogonal coherence pairs, i.e., azimuth and elevation, and diagonal and anti-diagonal, are multiplied to obtain DCFRC and DCFDiag, respectively. The Jaccard similarity of DCFRC and DCFDiag is used to derive the final DCF to weigh the reconstructed images. We evaluated the performance of DCF beamforming in point-target simulations, multipurpose phantom experiments, and in vivo muscle imaging and compared it to delay-and-sum (DAS) and CF beamformers. Our DCF achieved sidelobe reduction throughout the entire volume compared to conventional CF. Moreover, diagonal weighting significantly improved, on average, the azimuthal resolution by 41.3% versus DAS and 7.35% versus CF as well as the elevational resolution by 40.4% versus DAS and 38.7% versus CF. Our proposed DCF offers a practical solution for resolution and contrast enhancement of volumetric imaging in 2-D matrix array configurations.