Measurement and modeling of nonlinear waveforms in high-intensity focused ultrasound fields

Abstract

Direct measurement of HIFU fields in situ is important for the accurate prediction of thermal and mechanical bioeffects, as well as for the development of standards for medical systems. An experimentally validated numerical model can be an effective tool in both laboratory and clinical settings when direct measurements are not possible. Calculations with a KZK-type model and measurements with a fiberoptic probe hydrophone were employed together to characterize HIFU fields in water and in a tissue-mimicking gel. To determine the boundary conditions for simulations, the normal velocity distribution on the transducer surface was reconstructed using acoustic holography and combined with acoustic power measurements. At the focus, highly nonlinear waveforms (+700 and −150 bars peak pressures) were obtained both experimentally and numerically, which differed significantly from waveforms linearly extrapolated from low-amplitude results. Strongly distorted shock waveforms were localized in an axial region much smaller than the half-maximum beamwidth of the transducer excited at low level. At the highest excitation levels, the simulations predicted frequency content higher than was measurable in our configuration. Simulations also show that if these frequencies are not included, predicted heating rates are significantly lower. [Work is supported by NIH DK43881, NSBRI SMS00402, and RFBR.]