Nonlinear ultrasound propagation in homogeneous and heterogeneous media: Factors affecting the in situ Mechanical Index (MI)
We have previously reported increases in diagnostic shearwave and harmonic SNR using pressures that exceed the US FDA guideline for the Mechanical Index (MI). The MI is calculated using peak rarefaction pressure (PRP) measured in water with linear derating to account for tissue attenuation. However, acoustic propagation through water is a highly nonlinear process that inherently differs in attenuation, nonlinearity, and path heterogeneity from propagation in tissue. We hypothesize that MI overestimates the in situ acoustic output for many clinical scenarios and that PRP can occur in spatially offset locations when imaging through heterogeneous media such as the abdominal wall. Herein we demonstrate how these parameters affect in situ PRP as compared to that predicted by linear derating.
