P1B-7 Experimental and Theoretical Study of Strongly Focused High Intensity Ultrasound

The model developed by the authors for strongly focused HIFU [2] was verified experimentally. The verification was performed for 1.03 MHz focusing transducer loaded by the water. The transducer comprised spherical piezo-element immersed in the mineral oil and had aperture diameter 84 mm. and focal radius 54 mm. At the first step, acoustic field distribution in a plane, which was close and parallel to the focal plane, was measured at 10 W of input electric power. Using this data, the normal velocity distribution over the plane which is tangent to the centre point of the spherical radiator was reconstructed. This distribution was further scaled and served as boundary conditions for calculation of high intensity field distribution using approach described in [2]. At the second step the model predictions were compared with the data extracted from the acoustical pressure waveforms measured for different values of the output acoustic power. In addition to usually extracted pressure harmonic content, the spatial distributions of harmonics of on-axis projection of particle velocity have been obtained from pressure harmonic distributions with the angle spectrum expansion, providing connection between pressure and particle velocity harmonics. The predictions of the pressure positive and negative peaks, harmonic content and dependence of the harmonic effective propagation angle on the harmonic number fitted closely the corresponding experimental results. The proposed approach allows accurate prediction of strongly focused HIFU fields based on the measurements of low-intensity field distributions.