The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental (1.1 MHz) or third harmonic (3.3 MHz) frequency were measured by a fiber optic probe hydrophone (FOPH). At 1.1 MHz when the electric power applied to the transducer was increased from 1.6 to 125 W, the peak positiveegative pressures at the focus were measured to be p+=1.7–23.3 MPa and p−=−1.2–−10.0 MPa. The corresponding spatial-peak pulse-average (ISPPA) and spatial-average pulse-average (ISAPA) intensities were ISPPA=77–6000 W/cm2 and ISAPA=35–4365 W/cm2. Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced −6 dB beam size (L×W) of the compressional wave (11.5×1.8–8.8×1.04 mm) but an increased beam size of the rarefactional wave (12.5×1.6–13.2×2.0 mm). Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at 3.3 MHz produced higher peak pressures (p+=3.0–35.1 MPa and p−=−2.5–−13.8 MPa) with smaller beam size (0.5×4 mm). Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.
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