Measurements of Ultrasonic Differential and Total Scattering Cross Sections for Tissue Characterization

Measurements of power scattered from several nontissue random media and calf liver have been made as a function of both frequency and scattering angle v. The average differential scattering cross section per unit volume (sigma)(,sd)(v) was obtained by normalizing the measured scattered power to remove effects introduced by the measurement system. The normalization procedure utilized in this study was derived, from first principles, for the narrowband, quadrature detecting system employed in the experiments. The derivation provided a statement of conditions required for proper measurement of (sigma)(,sd). The scattering properties of three groups of nontissue media were studied. First, a referenced medium consisting of glass spheres in agar was studied to verify the accuracy and precision of the data obtained with our equipment and processing procedures. Then, studies were conducted on two media composed of graphite particles in gelatin, one with twice as many particles as the other. Finally, studies were carried out on a set of media composed of Sephadex spheres in water. Three samples were employed, each with a different range size of Sephadex spheres. These last two groups were used to simulate some of the changes in tissue morphology that may occur with disease. The results obtained from these groups suggest that information about scatterer structure (e.g. number density and size) can be obtained reliably from analysis of the absolute magnitude, angular dependence, and frequency dependence of (sigma)(,sd)(v). Finally, measurements were made of power scattered from normal calf liver. The results indicated that calf liver is a weak scatterer with the majority of power scattered in the forward direction ((VBAR)v(VBAR) < 90(DEGREES)). It was also discovered that the angular dependence of (sigma)(,sd) is nearly constant over 3 to 7 MHz, and the frequency dependence of (sigma)(,sd) is approximately f('1.3), regardless of scattering angle. By integrating (sigma)(,sd)(v) over all the measured scattering angles, it was determined that scattering loss contributed approximately 2% to the total attenuation of ultrasound in calf liver over 3 to 7 MHz. Further analysis of (sigma)(,sd)(v) suggested that the correlation length of the tissue was on the order of 1 mm.