A comparative evaluation of three hydrophones and a numerical model in high intensity focused ultrasound fields

The pressure fields of two different high intensity focused ultrasound (HIFU) transducers operated in burst mode were measured at acoustical power levels of 25 and 50 W (continuous wave equiva- lent) with three different hydrophones: A fiber-optic displacement sensor, a commercial HIFU nee- dle hydrophone, and a prototype of a membrane hydrophone with a protective coating against cavitation effects. Additionally, the fields were modeled using a freely available simulations soft- ware package. The measured waveforms, the peak pressure profiles, as well as the spatial-peak tem- poral-average intensities from the different devices and from the modeling are compared and possible reasons for differences are discussed. The results clearly show that reliable pressure meas- urements in HIFU fields remain a difficult task concerning both the reliability of the measured val- ues and the robustness of the sensors used: Only the fiber-optic hydrophone survived all four exposure regimes and the measured spatial-peak temporal-average intensities varied by a factor of up to 1.5 between the measurements and the modeling and between the measurements among them- selves.