Measurement of high intensity focused ultrasound fields by a fiber optic probe hydrophone

TLDR

The acoustic fields of a high‑intensity focused ultrasound (HIFU) transducer operating at 1.1 MHz or its third harmonic (3.3 MHz) were measured using a fiber‑optic probe hydrophone (FOPH). Measured peak pressures ranged from 1.7–23.3 MPa (1.1 MHz) to 3.0–35.1 MPa (3.3 MHz), with spatial‑peak pulse‑average intensities up to 6000 W cm⁻², while nonlinear propagation reduced the compressional beam size and increased the rarefactional beam size, and absorbed power density rose from 1.0 to 3.0, demonstrating that FOPH is a convenient and reliable tool for HIFU exposimetry.

Abstract

The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental (1.1MHz) or third harmonic (3.3MHz) frequency were measured by a fiber optic probe hydrophone (FOPH). At 1.1MHz when the electric power applied to the transducer was increased from 1.6to125W, the peak positive∕negative pressures at the focus were measured to be p+=1.7–23.3MPa and p−=−1.2–−10.0MPa. The corresponding spatial-peak pulse-average (ISPPA) and spatial-average pulse-average (ISAPA) intensities were ISPPA=77–6000W∕cm2 and ISAPA=35–4365W∕cm2. Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced −6dB beam size (L×W) of the compressional wave (11.5×1.8–8.8×1.04mm) but an increased beam size of the rarefactional wave (12.5×1.6–13.2×2.0mm). Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at 3.3MHz produced higher peak pressures (p+=3.0–35.1MPa and p−=−2.5–−13.8MPa) with smaller beam size (0.5×4mm). Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.