Production of harmonics in vitro by high-intensity focused ultrasound

Experiments were performed to investigate the production of harmonics by hi gh-intensity focused ultrasound (HIFU) produced by a spherical bowl; spheri cal 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 tis sue 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 approx imately 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/cm(2) at the focus, the -3-dB beam diameters o f harmonic components were observed to vary approximately as the inverse sq uare root of the harmonic number. (C) 1999 World Federation for Ultrasound in Medicine & Biology.