Experimental imaging of the acoustic nonlinearity parameter B/A for biological tissues via a parametric array.

The acoustic nonlinearity parameter B/A is an important parameter in nonlinear acoustics and has been suggested to be a novel parameter for ultrasound tissue characterization. In this paper, the principle and experimental results for B/A imaging by using a parametric array are presented. As two primary waves with different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the two primary waves. The axial sound pressure amplitude curve for the generated difference frequency wave in the near field is measured and agrees well with the theoretical calculation. Unlike the second harmonic wave, the difference frequency component of the parametric array grows almost linearly with distance from the piston source and goes through fewer oscillations in the near field. It therefore provides a better source for acoustic nonlinearity parameter tomography. On the basis of the finite amplitude insert substitution method and the conventional CT technology, an experimental imaging system for acoustic nonlinearity parameter B/A via a parametric array is set up. B/A images for several biological specimens, including normal and pathological tissues, are obtained.