Nonlinear bulk elasticity imaging using dual frequency ultrasound

The nonlinear acoustic bulk properties of tissue, e.g., the coefficient of nonlinearity, βn, or the nonlinear bulk elasticity, β p = β n κ 0, have been shown to be promising parameters for tissue characterization due to their sensitivity to tissue structure. Previously developed methods for imaging these parameters using single frequency ultrasound have shown success in a laboratory setting using the transmission mode. In the pulse-echo mode, however, unknown absorption, diffraction, and speckle produce unreliable estimates and instability, causing these methods to have achieved no clinical relevance. In this paper, a pulse-echo method for measurement of the nonlinear bulk elasticity is presented using a dual frequency approach. The method is less sensitive to diffraction and absorption due to a separate low frequency manipulation wave. The technique is tested in both simulations and in vitro in a heterogeneous phantom with two regions of different nonlinear properties. Both in simulations and in vitro, a spatial βp map is produced where the two regions are clearly distinguished. In addition, the quantitative estimates of βp obtained are close to the expected values, making the method a promising first step toward in vivo imaging of nonlinear bulk properties.