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Nonlinear propagation effects on broadband attenuation measurements

It has been anticipated that the frequency dependency of the attenuation coefficient represents a useful parameter for tissue characterization. Two well established frequency methods to measure this frequency dependency are the so called "log spectral difference" and the "central frequency shift" methods which are both techniques using broadband pulses. It is known that large amplitudes of the emitted wave can bias the estimated frequency dependency of the attenuation coefficient due to nonlinear propagation of finite amplitude waves. The authors investigated the influence of the central frequency and bandwidth of the emitted pulse on this phenomenon and tried to apply these results to in vivo attenuation measurements of liver. Firstly, tissue mimicking phantoms were used in a transmission substitution experiment. The frequency dependency of their attenuation coefficient was defined at different energy levels of the emitted pulse (ranging from 50-400 kPa), changing center frequency and bandwidth. Secondly, a commercial echocardiographic scanner was used-to image the liver of healthy volunteers at different acoustic power outputs. Radiofrequency data was acquired and the frequency dependency of the attenuation coefficient was estimated. The in vitro and in vivo experiments showed that nonlinear propagation effects can bias the estimate of the attenuation coefficient. In substitution measurements (as in the authors' in vitro experiments), the extracted frequency dependency of the attenuation coefficient decreased with increasing amplitude of the emitted pulse, while, as expected, the opposite occurred in reflection measurements (as in the authors' in vivo experiments).