Spatial distributions of acoustic parameters in high-intensity focused ultrasound fields

Different peak and average acoustic parameters determine the efficiency of different physical mechanisms of high-intensity focused ultrasound (HIFU) interaction with biological tissue. Spatial distributions of these parameters are therefore important for transducer calibration and extrapolation of measurements in water to application in tissue. In the case of linear focusing, all parameters of the acoustic field can be obtained from the spatial distribution of the wave amplitude. However, in nonlinear focused beams, each parameter has its own characteristic spatial structure, which changes with the increase of the HIFU power level. This work compares the focal size and location calculated for the peak positive and peak negative pressure, mean intensity, and effective acoustic energy absorption in water and in tissue. Numerical solutions, obtained with the KZK-type model, are analyzed for various regimes of linear, quasilinear, and strongly nonlinear propagation which includes formation of shocks. The results of simulations are validated by comparison with measurements performed with a fiber-optic probe hydrophone in water and in a tissue-mimicking phantom. The peak positive pressure and effective absorption are finely focused, whereas the negative pressure, responsible for cavitation, is broad and displaced towards the transducer. [Work supported by NSBRI SMS00402, RFBR, and NIH DK43881.]