A theoretical investigation of the effect of nonlinear propagation on measurements of mechanical index.

Safety parameters for diagnostic ultrasound scanners are calculated from measurements made in water, which are derated to account for the attenuation of tissues. Sound is attenuated less by water than by tissue, and so the effects of nonlinear propagation are greater in water. This study compares mechanical index (MI) and derated intensity with the analogous quantities in idealised soft tissue, for simplified models of scanners with source amplitudes up to 2.5 MPa. As expected, MI is much smaller than implied by linear extrapolation from low-amplitude measurements but, in a system with moderate gain, the reduction in tissue is commensurate with that in water, MI and derated intensity underestimating the values in tissue by at most 20%. Determining MI at the location of peak negative pressure halves the error. In high gain systems, however, MI can be less than 60% of the value in tissue.