Frequency dependence of backscatter coefficient versus scatterer volume fraction

Various groups are using the frequency dependence of backscattering to characterize tissue. In most cases, sparse scatterer concentrations are assumed in relating scattering parameters to tissue properties. This study addresses the relationship between backscatter frequency dependence and scatterer volume fraction. Backscatter coefficients (BSC) in the 2.5 to 9.0 MHz frequency range were measured for agar-gel phantoms containing Sephadex scatterers (mean diameter 42 /spl mu/m) at volume fractions ranging from 5 to 50%. The BSC increased with scatterer volume fraction at low scatterer concentrations; at higher concentrations, the BSC reached a maximum with concentration and then decreased with yet increasing scatterer concentrations, as has been noted by previous authors. In addition, the volume fraction for the maximum backscatter coefficient varied with frequency, generally being greater at higher frequencies than lower frequencies, and the frequency dependence of the backscatter coefficient was greater for the higher scatterer concentrations than for the lower concentrations. These results are predicted by continuum models, where the spatial autocorrelation function depends on scatterer volume fraction.