Tissue-induced ultrasonic wavefront distortion

A quantitative description of wavefront distortion due to tissue inhomogeneities is desirable in order to assist in the development of aberration correction procedures for medical ultrasound imaging. A through-transmission in vitro measurement procedure was used to estimate the ultrasonic wavefront distortions of five different tissue types: bovine liver, porcine liver, bovine skin and bovine muscle for two different orientations (sound direction parallel and perpendicular to fiber orientation). Water and Dow Corning 710 were used for distortionless references. Known thickness tissue samples were vacuum packed in a thin plastic bag at the time of acquisition and were measured in a 20/spl deg/C degassed water tank. Each sample was positioned perpendicular to the beam axis, immediately adjacent to a PVDF membrane hydrophone which was positioned in the 3 MHz circular transducer's focal region to receive the ultrasonic signals that propagated through the tissue sample. For each tissue sample, a total of 625 individual waveforms (25/spl times/25 rectangular grid in 1 mm steps) were collected. Three classes of calculations were performed to evaluate tissue inhomogeneities: correlation coefficient between two waveforms, arrival time shift between two waveforms, and the insertion loss for each waveform relative to water. Each was evaluated as a function of two waveform spatial location configurations. One was typical of phased array imaging (linear distance along each data row) and the other was based in eight waveform separation distances through the sample (1 to 5.66 mm). Results show that waveform distortion increased as a function of spatial location separation for the tissue samples and was significantly greater for the tissue samples compared to the reference materials.