Impact of clutter levels on spatial covariance: Implications for imaging

The Van Cittert-Zernike (VCZ) theorem describes the spatial covariance of an incoherent source, such as the ultra sonic signal that is backscattered by tissue. A simulation tool that describes acoustic propagation, including multiple scattering and nonlinear propagation is used to characterize the effect of tissue structure on the spatial covariance. Simulated spatial covariance curves are compared to in vivo data. It is shown that the non random structure of tissue fundamentally modifies the spatial covariance function. The reverberating echoes generated by the tissue layer add a decorrelated component to the backscattered signal. It is shown that this decorrelation of the short lag spatial covariance (SLSC) can provide a source of contrast to generate ultrasound images at the fundamental and harmonic frequencies. Images of anechoic lesions show that SLSC imaging has better contrast and boundary definition the B-mode images generated from the same echoes.