Journal of Ultrasound in Medicine, Vol 18, Issue 1 33-41, Copyright © 1999 by American Institute of Ultrasound in Medicine
Effects on nonlinearity on the estimation of in situ values of acoustic output parameters
T. L. Szabo, F. Clougherty and C. Grossman
Imaging Systems Division, Hewlett Packard, Andover, Massachusetts 01810, USA.
Water can generate extreme waveform distortion compared to tissue, as
indicated by the Goldberg 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 pc/pr, 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.
