Experimental test of harmonic contribution to the thermal index for soft tissue

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.