Effects of abdominal wall morphology on ultrasonic pulse distortion

Wavefront propagation through the abdominal wall was investigated by measurements employing full-thickness specimens and their individual fat and muscle layers. These measurements confirm that both fat and muscle produce significant wavefront distortion and focus degradation. However, the spatial characteristics of the distortion produced by fat and muscle differ, and the total distortion produced by the abdominal wall is not the arithmetic sum of that produced by the individual layers. The interface between the muscle and subcutaneous fat layers, which consists of a thin layer of connective tissue, was found to be smooth or slightly curved. Distortion produced by different tissues was further characterized by simulations using a finite-difference time-domain implementation of the linearized wave propagation equations for a lossless inhomogeneous fluid. Scanned images of six human abdominal wall cross sections provided the data for the propagation media in the simulation. The images were mapped into regions of fat, muscle, and connective tissue, each of which was assigned a uniform sound speed and density obtained from literature values. The computed wavefronts contained arrival time, energy level, and wave shape distortion similar to that in measurements. Visualization of wave propagation within the tissue cross sections suggests that most arrival time distortion is produced by acoustic path length differences while amplitude and waveform variations are the result of scattering from inhomogeneities. Many arrival time fluctuation and energy level fluctuation features could be traced to specific anatomical structures. These results indicate that wavefront distortion, apart from bulk attenuation effects, is primarily caused by tissue structures and inhomogeneities rather than refraction at layer interfaces.