Errors in ultrasonic scatterer size estimates due to mixed scatterer populations

Ultrasonic scatterer size estimation provides an accurate measure of actual scatterer size when those sizes are narrowly distributed about a single, mean value. Although often the case, there are instances in tissue where two or more scatterer types with significantly different sizes are believed to contribute to the same signal. The purpose of this work is to characterize the errors in the size estimates obtained for one scatterer type when contaminating scatterers of a second type are present. Theoretical results for the error are compared with simulation and experimental results for uniform phantoms containing a binary mixture of scatterers. These results indicate that errors can be significant for the frequency bands typically used in size estimation, especially when contaminant scatterers are larger than the scatterers of interest. Results also indicate, however, that these errors can be reduced by shifting the frequency band used to estimate size. A technique for correcting the errors is also described and applied to the phantom data. Although effective, the method requires prior knowledge of the backscatter coefficient of the contaminant scatterers, and the variability of the corrected values can limit its utility as contaminant scattering strength increases.