Phase Coherence Imaging: Principles, applications and current developments

Abstract

Phase Coherence Imaging was proposed in 2009 as a new adaptive beamforming method, based on measuring the instantaneous phase dispersion of the aperture data. Low phase dispersion is indicative of a focused reflector, while large phase dispersion is produced by out of focus reflectors, reverberations and the grained structure of some materials. Based on a phase dispersion metric, a coherence factor can be defined and used to weight the beamformer output, in such a way that focused reflectors are maintained (low phase dispersion), while side-lobes, grating-lobes, reverberations and grain noise are reduced (large phase dispersion). Besides increasing the image dynamic range by reducing the sidelobes and background noise, lateral resolution is simultaneously improved by reducing the main-lobe width. Several phase coherence factors have been defined, based on different dispersion metrics with different computational complexity and noise reduction levels. In particular, a realization based on the sign of the received signals (Sign Coherence Factor) allows implementing the method in real-time with very low hardware resources requirements. The coherence factor can be used to weight the beamformer output or as an image itself, with the property of representing reflectors with amplitudes independent of their reflectivity.