Quantitative relationship between tissue composition and scattering of ultrasound

This study establishes quantitative laws by which the magnitude of ultrasonic scatter is related to the gross chemical composition of tissues, viz., water, fat, and collagen. Tissues are modeled as immiscible mixtures, the constituents of which are uniformly dispersed over the volume. Each component of the mixture causes the ultrasonic velocity of the medium to fluctuate about the mean value. Such localized variations in the refractive index of a medium cause propagating sound waves to scatter. It is proposed that the contribution of each component to the total fluctuation is in proportion to its volume fraction. The analysis shows that the scatter of ultrasound exhibits a complex nonlinear dependence on the concentrations of the individual components. From the knowledge of sound speed of individual components, total velocity fluctuations are determined for binary and three-component models of the tissue. Comparison of these values, with those estimated from scatter measurements, shows a close correlation between the two sets. Collagen, although present in relatively small amounts in soft tissues, contributes a significant portion of the total scatter. However, both fat and collagen fractions must be taken into consideration to account for the major fraction of the observed scatter. In conclusion, there is a direct relationship between tissue composition and scatter from tissues. Such dependence must also be taken into consideration, in addition to the correlation function and correlation lengths to explain backscatter or loss in a transmitted signal due to scatter from tissues.