Production of harmonics in vitro by high-intensity focused ultrasound.

Experiments were performed to investigate the production of harmonics by high-intensity focused ultrasound (HIFU) produced by a spherical bowl; spherical radius 15 cm, frequency 1.7 MHz, as a function of beam power in excised bovine liver. The intensity of the nth harmonic, in both water and the tissue sample, varied approximately as the nth power of the incident intensity up to the point at which irreversible changes were produced in the sample. The greatest observed axial power absorption enhancement factor was approximately 6.3, and the greatest observed total absorbed power enhancement was approximately 2.3. These enhancements may have some effect on the onset of lesioning, but not much effect on its subsequent development. In water, at an intensity of about 120 W/cm2 at the focus, the -3-dB beam diameters of harmonic components were observed to vary approximately as the inverse square root of the harmonic number.