Journal of Ultrasound in Medicine, Vol 18, Issue 1 81-86, Copyright © 1999 by American Institute of Ultrasound in Medicine
Experimental test of harmonic contribution to the thermal index for soft tissue
I. B. Egerton, G. Vella and S. Barnett
School of Medical Radiation Technology, University of Sydney, Lidcombe, New South Wales, Australia.
Experimental data were gathered for the heating effect of an ultrasound
beam propagating in water and incident on a thermal test object containing
artificial tissue in which a fine-wire thermocouple was embedded close to
the beam entrance. The extent of temperature elevations in the artificial
tissue was limited by using a constant low value for spatial peak temporal
average ultrasound intensity so that the medium did not denature. The
adjusted (for continuous wave conditions) maximum temperatures reached in a
fixed exposure time initially increased linearly with increase in the
spatial peak pulse average ultrasound intensity, but, for the higher values
of this parameter, exceeded that expected from a linear relationship. In
addition to the fundamental frequency, significant higher frequencies were
present in the beam at high intensities, which caused additional
enhancement of the ultrasound heating. Calculations of W(DEG) for
evaluation of the thermal index should include the evaluation of the
heating contributions from all frequencies present in the beam.
