Effects of nonlinearity on the estimation of in situ values of acoustic output parameters

Abstract

Water can generate extreme waveform distortion compared to tissue, as indicated by the Gol'dberg number for water, which is 20 times larger than that of tissue at typical diagnostic ultrasound levels. This result was demonstrated by using tofu as a tissue mimicking material. By adjusting transducer voltage drive levels in water to match the peak rarefactional pressures in water to those of waveforms in tofu, a close correspondence was obtained for the peak compressional pressure and time average intensity with depth. A poorer correspondence was found by comparing tofu waveforms with water waveforms that were compensated for broadband attenuation and driven at the same voltage level as tofu. A simplified broadband derating factor, allowing for band-width adjustment, was shown to be more accurate than the standard monochromatic derating. Several new indicators for quantifying the degree of observed nonlinearity are suggested: a field based nonlinearity parameter, a peak pressure ratio p(c)/p(r), and a second harmonic to fundamental frequency spectral ratio. These indicators may have the potential for more consistent characterization of nonlinear relationships among output parameters and drive levels.