Journal of Ultrasound in Medicine, Vol 18, Issue 1 69-80, Copyright © 1999 by American Institute of Ultrasound in Medicine
Nonlinear propagation and the output indices
E. L. Carstensen, D. Dalecki, S. M. Gracewski and T. Christopher
Department of Electrical, Engineering, The Rochester Center for Biomedical Ultrasound, The University of Rochester, New York 14627, USA.
By ignoring the effects of nonlinear propagation, current exposimetry
protocols 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 values to obtain estimates of fields in situ. The mechanical index
was designed to provide an indication of temporal peak acoustic fields for
use in prediction of nonthermal biological effects in tissues. At low
outputs, the mechanical index, together with the frequency, gives the peak
negative pressure near the focus of the field. As currently formulated,
however, the pressure used in the mechanical index may be far from the
focus at high output levels. Recommendations of the World Federation of
Ultrasound in Medicine and Biology avoid the underestimate associated with
nonlinear propagation as well as other problems with the mechanical index
and may be preferable in dealing with non-thermal bioeffects. The thermal
indices that are implemented currently in the Output Display Standard
(American Institute of Ultrasound in Medicine/National Electrical
Manufacturers' Association) are affected less seriously by nonlinear
propagation.
