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 (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 positive/negative 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 (I-SPPA) and spatial-average pulse-average (I-SAPA) intensities were I-SPPA=77-6000 W/cm(2) and ISAPA =35-4365 W/cm(2). Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced -6 dB beam size (L X W) of the compressional wave (11.5 x 1.8-8.8 X 1.04 min) but an increased beam size of the rarefactional wave (12.5 X 1.6-13.2 x 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 X 4 mm). Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement. (c) 2006 Acoustical Society of America.