Ultrasonic imaging with wave mode adaptive weights and global matched coefficient

Abstract

This paper discusses some improvements to ultrasonic synthetic imaging in solids. Specifically, the study proposes new adaptive weights applied to the beamforming array that are based on the physics of the propagating waves, specifically the displacement structure of the propagating longitudinal (L) mode and shear (S) mode that are naturally coexisting in a solid. The wave mode structures can be combined with the wave geometrical spreading to better filter the array and improve its focusing ability compared to static array weights. The paper also proposes compounding, or summing, images obtained from the different wave modes to further improve the array gain without increasing its physical aperture. The wave mode compounding can be performed either incoherently or coherently, in analogy with compounding multiple frequencies or multiple excitations. Furthermore, the introduction of a global matched coefficient computed through the matching of measured and expected times of flight will be presented to show additional improvements in the image reconstruction process. Numerical simulations and experimental testing demonstrate the potential improvements obtainable by the wave structure adaptive weights and the global matched coefficient compared to either static weights in conventional delay-and-sum focusing, or adaptive weights based on geometrical spreading alone in minimum-variance distortionless response focusing.