Acoustic characterization of high intensity focused ultrasound fields: A combined measurement and modeling approach

TLDR

Acoustic characterization of high‑intensity focused ultrasound (HIFU) fields is essential for accurate prediction of ultrasound‑induced bioeffects and for developing regulatory standards for clinical HIFU devices. The paper proposes a method to determine HIFU field parameters at and around the focus. The method employs nonlinear pressure waveform measurements with a fiber‑optic probe hydrophone in water and a tissue‑mimicking gel phantom for a 2 MHz transducer, and uses the measured low‑amplitude beam plots as inputs to a Khokhlov–Zabolotskaya–Kuznetsov numerical model. Strongly asymmetric waveforms with peak positive pressures up to 80 MPa and peak negative pressures up to 15 MPa were obtained; numerical simulations and measurements agreed well, but at focal intensities above 6000 W cm⁻² steep shocks caused lower measured peak positive pressures due to the hydrophone’s 100 MHz bandwidth, demonstrating that combining measurements and modeling is necessary for accurate HIFU field characterization.

Abstract

Acoustic characterization of high intensity focused ultrasound (HIFU) fields is important both for the accurate prediction of ultrasound induced bioeffects in tissues and for the development of regulatory standards for clinical HIFU devices. In this paper, a method to determine HIFU field parameters at and around the focus is proposed. Nonlinear pressure waveforms were measured and modeled in water and in a tissue-mimicking gel phantom for a 2MHz transducer with an aperture and focal length of 4.4cm. Measurements were performed with a fiber optic probe hydrophone at intensity levels up to 24000W∕cm2. The inputs to a Khokhlov–Zabolotskaya–Kuznetsov-type numerical model were determined based on experimental low amplitude beam plots. Strongly asymmetric waveforms with peak positive pressures up to 80MPa and peak negative pressures up to 15MPa were obtained both numerically and experimentally. Numerical simulations and experimental measurements agreed well; however, when steep shocks were present in the waveform at focal intensity levels higher than 6000W∕cm2, lower values of the peak positive pressure were observed in the measured waveforms. This underrepresentation was attributed mainly to the limited hydrophone bandwidth of 100MHz. It is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields.