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

Experimental data were gathered for the heating effect of an ultrasound bea m propagating in water and incident on a thermal test object containing art ificial tissue in which a fine-wire thermocouple was embedded close to the beam entrance. extent of temperature elevations in the artificial tissue wa s limited by using a constant low value for spatial peak temporal average u ltrasound intensity so that the medium did not denature. The adjusted (for continuous wave conditions) maximum temperatures reached in a fixed exposur e 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 fun damental 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 pr esent in the beam.