Nonlinear propagation and the output indices

By ignoring the effects of nonlinear propagation, current exposimetry proto cols may yield significant underestimates of the acoustic pressure in situ. This problem can be avoided simply by (1) extrapolating pressures linearly from low amplitude measurements in water and (2) linearly derating these v alues to obtain estimates of fields insitu. The mechanical index was design ed to pros ide an indication of temporal peak acoustic fields for use in pr ediction of nonthermal biological effects in tissues. At low outputs, the m echanical index, together with the frequency, gives the peak negative press ure near the focus of the field. As currently formulated, however, the pres sure used in the mechanical index may be far from the focus at high output levels. Recommendations of the World Federation of Ultrasound in Medicine a nd Biology avoid the underestimate associated with nonlinear propagation as well as other problems with the mechanical index and may be preferable in dealing with nonthermal bioeffects. The thermal indices that are implemente d currently in the Output Display Standard (American Institute of Ultrasoun d in Medicine/National Electrical Manufacturers' Association) are affected less seriously by nonlinear propagation.