Experiments on the heating and mechanical effects of high intensity focused ultrasound

Abstract

Free field experimental measurements of the temperature rise of water and tissue deformation in the focal region of a 2 MHz high intensity focused ultrasound (HIFU) transducer were performed. Millisecond pulsed HIFU was operated at acoustic focal intensities in the range of 1,200 W-cm2 and up to 18,500 W-cm2, resulting in nonlinear wave propagation and shock wave formation. Pulsed, planar laser-induced fluorescence (LIF) was used as a fast-rise-time, non-intrusive, temperature measurement method of the water present in the focal region. The laser light sheet was oriented transverse to the acoustic axis. Cross-sectional, instantaneous temperature field measurements within the HIFU focal volume revealed that the water temperature increased steadily with increasing the HIFU input power. We measured heating rates of 4000-7000 °C -sec within the first millisecond of the HIFU burst. Increasing the length of the burst initially resulted in an increase in the water temperature within the HIFU focal spot (up to 2.5 ms), after which it steadied or slightly dropped. Acoustic streaming was measured and shown to be consistent with the reduction in heating with increased burst length due to the convective cooling effect. Furthermore, the deformation and mechanical effects on tissue due to pulse HIFU is studied. Deformation field were measured using Laser Digital Image Correlation technique. Compressive deformation mechanisms in the micron scale were observed at the focal point.