Real-time assessment of the parameter of nonlinearity in tissue using "nonlinear shadowing".

A real-time pulse-echo method for assessing tissue nonlinearity is presented. Here, tissue nonlinearity refers to the parameter of nonlinearity, B/A, and not the frequency dependence of the attenuation. Our nonlinearity assessment method is based on two pulse transmissions per beam position. One pulse is transmitted at a high intensity that propagates nonlinearily and, the other, at a low level, that propagates linearly. Echo data are used to estimate a function representing the decay of fundamental frequency amplitude due to nonlinearity along the ultrasound path. This function is then mapped into an image that includes shadows which relate to the nonlinearity of the preceding regions. To delineate a region of interest from the background tissue, a normalization curve is defined and used to compensate for the nonlinearity of the background tissue. Experiments are performed on a tissue-mimicking phantom and in vitro tissue samples including regions of high nonlinearity. Results indicate that the method can effectively identify nonlinear regions and assess the extent of the nonlinearity.