SIMULATION OF ULTRASONIC PULSE-PROPAGATION THROUGH THE ABDOMINAL-WALL

Ultrasonic pulse propagation through the human abdominal wall has been simulated using a model for two-dimensional propagation through anato mically realistic tissue cross sections, The time-domain equations for wave propagation in a medium of variable sound speed and density were discretized to obtain a set of coupled finite-difference equations, T hese difference equations were solved numerically using a two-step Mac Cormack scheme that is fourth-order accurate in space and second-order accurate in time. The inhomogeneous tissue of the abdominal wall was represented by two-dimensional matrices of sound speed and density val ues. These values were determined by processing scanned images of abdo minal wall cross sections stained to identify connective tissue, muscl e, and fat, each of which was assumed to have a constant sound speed a nd density, The computational configuration was chosen to simulate tha t of wavefront distortion measurements performed on the same specimens . Qualitative agreement was found between those measurements and the r esults of the present computations, indicating that the computational model correctly depicts the salient characteristics of ultrasonic wave front distortion in vivo. However, quantitative agreement was limited by the two-dimensionality of the computation and the absence of detail ed tissue microstructure, Calculations performed using an asymptotic s traight-ray approximation showed good agreement with time-shift aberra tions predicted by the full-wave method, but did not explain the ampli tude fluctuations and waveform distortion found in the experiments and the full-wave calculations. Visualization of computed wave propagatio n within tissue cross sections suggests that amplitude fluctuations an d waveform distortion observed in ultrasonic propagation through the a bdominal wall are associated with scattering from internal inhomogenei ties such as septa within the subcutaneous fat. These observations, as well as statistical analysis of computed and observed amplitude fluct uations, suggest that weak fluctuation models do not fully describe ul trasonic wavefront distortion caused by the abdominal wall. (C) 1997 A coustical Society of America.