Nonlinearity Parameter Estimation Method in Pulse-Echo Using a Reference Phantom

The nonlinearity parameter (B/A) of a fluid-like media could be useful to assess structural changes, e.g., for tissue identification. To estimate the B/A, the 2nd harmonic generated signal recorded using a dual transducer could be used. However, in many instances, only the fundamental band signal acquired, for example with a clinical linear array transducer, might be available. In our approach the 2nd harmonic is related to the depletion in the fundamental band signal when comparing echo signal envelopes collected using two different excitation pressures: low and high. Specifically, we use the energy conservation principle by relating the 2nd harmonic to the loss of power at the fundamental band as it propagates in lossy media. Our method assumes that for propagation in a lossy medium, energy from the fundamental band is transferred mainly to the 2nd harmonic, and that propagation at low power settings are quasi-linear. Data were taken from numerical phantoms by simulating a typical linear array and 3D random density media excited with a broadband 5-MHz Gaussian pulse at two excitation peak pressures, i.e., low and high of 100 kPa and 1 MPa, respectively. A well-characterized reference phantom with similar speed of sound and attenuation coefficient as the sample and known B/A was used for calibration of the unknown scattering properties of the assessed sample. An expression for estimation of the B/A of a nonuniform medium is presented. The interrogated media for testing was simulated as a nonuniform phantom with a background B/A=6 and an 18-mm diameter circular inclusion (B/A=9). The result obtained was a parametric image of the B/A versus depth that captures nonlinear changes at the location of the inclusion and below it. The results reflect the cumulative nature of the nonlinearity parameter due to the shadowing effect observed below the location of the inclusion. In summary, the method is able to derive a quantitative B/A map using a reference phantom, with potential to characterize nonlinear media with a simple pulse-echo acquisition setup.