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

The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental $\left(1.1\mathrm{MHz}\right)$ or third harmonic $\left(3.3\mathrm{MHz}\right)$ frequency were measured by a fiber optic probe hydrophone (FOPH). At $1.1\mathrm{MHz}$ when the electric power applied to the transducer was increased from $1.6\text{to}125\mathrm{W}$ , the peak positive∕negative pressures at the focus were measured to be $p^{+}=1.7–23.3\mathrm{MPa}$ and $p^{-}=-1.2–-10.0\mathrm{MPa}$ . The corresponding spatial-peak pulse-average $\left(I_{\mathrm{SPPA}}\right)$ and spatial-average pulse-average $\left(I_{\mathrm{SAPA}}\right)$ intensities were $I_{\mathrm{SPPA}}=77–6000\mathrm{W}∕{\mathrm{cm}}^2$ and $I_{\mathrm{SAPA}}=35–4365\mathrm{W}∕{\mathrm{cm}}^2$ . Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced $-6\mathrm{dB}$ beam size $(\mathrm{L}\times \mathrm{W})$ of the compressional wave $(11.5\times 1.8–8.8\times 1.04\mathrm{mm})$ but an increased beam size of the rarefactional wave $(12.5\times 1.6–13.2\times 2.0\mathrm{mm})$ . Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at $3.3\mathrm{MHz}$ produced higher peak pressures $(p^{+}=3.0–35.1\mathrm{MPa}$ and $p^{-}=-2.5–-13.8\mathrm{MPa})$ with smaller beam size $(0.5\times 4\mathrm{mm})$ . Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.