Spatio-Angular Coherence Factor MVDR for Plane-Wave Ultrasonic Phased Array Total Focusing Method Imaging

Abstract

Abstract. Total focusing method (TFM) imaging is widely adopted in nondestructive evaluation (NDE) using ultrasonic phased arrays (UPA) due to its high imaging resolution in detecting and identifying subsurface defects. Plane-wave TFM (PWI-TFM) combines plane-wave transmission with full-aperture reception, which can enhance acoustic energy in the region of interest and improve the signal-to-noise ratio (SNR) of defect echoes. However, the use of a finite steering range and discrete angular steps in PWI transmission often leads to nonuniform acoustic energy coverage, which results in spatially varying gains and elevated sidelobes. To address these limitations, this article proposes a spatio-angular coherence factor minimum variance distortionless response (SACF-MVDR) adaptive weighting method for PWI-TFM imaging. The proposed method combines coherence information from the receive-aperture and steering-angle domains with diagonally loaded MVDR beamforming to improve robustness against noise and clutter. Simulations for near-surface defect imaging in railway wheels indicate that the proposed method provides better background suppression and defect separation than conventional TFM, Hanning-windowed TFM, phase coherence factor (PCF), sign coherence factor (SCF), and MVDR. Among the compared methods, SACF-MVDR achieves the lowest image entropy while maintaining high lateral resolution.