On the Frequency Dependence of Attenuation in Normal and Fatty Liver

Abstruct-The frequency dependence of the attenuation has been investigated in normal and fatty liver samples in vitro. It was observed that normal liver samples exhibited essentially a linear frequency dependent attenuation, while fatty livers exhibited a power law dependence, where the exponent n was between 1.25-1.4. In order to explain this nonlinear behavior, fatty liver has been modeled as consisting of normal liver tissue which is infiltrated by a large number of tiny fat globules which are 4-10 in size and occasionally coalesce to 100 p, and thus act as Rayleigh scatterers in the diagnostic wavelength range. The addition of these scatterers is consistent with the clinical observation of increased echogenicity in fatty livers. Based on this physical description, the attenuation in fatty liver tissue has been modeled as consisting of a linear absorption term, and a fourthader Rayleigh scattering term. This model predicts that the quantity (n - l) is proportional to the fractional contribution of scattering to the total attenuation. For fatty livers the attenuation contribution due to scatter amounts to 8-13 percent, compared to 21 percent for normal livers.