A Review of Tissue Characterization from Ultrasonic Scattering

Ultrasonic scattering is related to tissue architecture by a model which expresses the scattered wave pressure as the product of a frequency-dependent factor and the three-dimensional Fourier transform of a spatially windowed scattering function. The scattering function is the sum of the variations in compressibility and the variations in density, the latter being weighted by the cosine of the scattering angle. An analogous Fourier transform relation describes the average scattered intensity in terms of the correlation of the scattering function between points in a scattering volume. These Fourier transforms permit different scattering measurements to be related via trajectories in wavespace. Backscatter measurements made on blood, eye, liver, spleen, brain, and heart show the feasibility of differentiating tissues and determining spacing. Ultrasonic measurements of random medium model properties have demonstrated that average differential scattering cross sections per unit volume can be found accurately and precisely, and that scattering cross sections per unit volume can be used to make quantitative statements about changes in scattering properties. Measurements of calf liver ultrasonic differential and total scattering cross sections show that calf liver is a weak scatterer with nearly one-half of the total scattered power occurring between scattering angles of 25-45°. Measurements of scattering by fixed pig and human liver show qualitative agreement with predictions of scattering from architecture observed optically. Results presently accumulated indicate the promise of scattering measurements for tissue characterization, but extensive additional in vitro and in vivo development is required before their clinical utility for diagnosis is known.