Short-lag spatial coherence of backscattered echoes: imaging characteristics

Conventional ultrasound images are formed by delay-and-sum beamforming of the backscattered echoes received by individual elements of the transducer aperture. Although the delay-and-sum beamformer is well suited for ultrasound image formation, it is corrupted by speckle noise and challenged by acoustic clutter and phase aberration. We propose an alternative method of imaging utilizing the short-lag spatial coherence (SLSC) of the backscattered echoes. Compared with matched B-mode images, SLSC images demonstrate superior SNR and contrast-to-noise ratio in simulated and experimental speckle-generating phantom targets, but are shown to be challenged by limited point target conspicuity. Matched B-mode and SLSC images of a human thyroid are presented. The challenges and opportunities of real-time implementation of SLSC imaging are discussed.

I. Introduction

In ultrasound, spatial coherence is a measure of the similarity of backscattered echoes received by individual transducer elements at a given time, as a function of element separation. The spatial coherence of backscattered ultrasound waves is described by the van Cittert-Zernike (VCZ) theorem, a fundamental tenet of modern optics. The VCZ theorem predicts the mutual intensity (also termed spatial covariance or mutual coherence evaluated at zero delay) of a wave field produced by an incoherent source [1]. According to this theorem, the spatial covariance in an observation region is the scaled Fourier transform of the intensity distribution of an incoherent source.