Analytic and numerical modeling of ultrasonic B-scan and echo decorrelation imaging

Abstract

An numerical model is presented for B-scan images of weakly-scattering,lossy media, based on ultrasound array beam patterns calculated analytically under the Fresnel approximation. Given these beam patterns and a 3D analytic or numerical tissue model, this method yields beamformed A-line signals from which B-scan images are constructed. This approach is further employed to model echo decorrelation imaging, a method for quantitatively mapping transient heat-induced changes in pulse-echo ultrasound images. In echo decorrelation imaging, a normalized decorrelation parameter is computed between A-line signals separated by milliseconds. Maps of this parameter comprise echo decorrelation images, which are potentially useful for monitoring of local tissue coagulation during thermal ablation treatments for cancer therapy. Following previous studies in which scattering cross section has been related to spatial-frequency spectra of tissue sound speed, density, and impedance variations, echo decorrelation is related quantitatively to the local decoherence of these spatial-frequency spectra. Decoherence estimates are validated by simulations employing analytic array beam patterns and random-media models for ablated tissue, and are further applied to quantify tissue structure changes caused by thermal coagulation during in-vitro radiofrequency ablation.