Dependence of inertial cavitation induced by high intensity focused ultrasound on transducer <i>F</i>-number and nonlinear waveform distortion.
Ter Haar, G
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Pulsed high intensity focused ultrasound was shown to enhance chemotherapeutic drug uptake in tumor tissue through inertial cavitation, which is commonly assumed to require peak rarefactional pressures to exceed a certain threshold. However, recent studies have indicated that inertial cavitation activity also correlates with the presence of shocks at the focus. The shock front amplitude and corresponding peak negative pressure (<i>p</i> <sup>-</sup>) in the focal waveform are primarily determined by the transducer <i>F</i>-number: less focused transducers produce shocks at lower <i>p</i> <sup>-</sup>. Here, the dependence of inertial cavitation activity on the transducer <i>F</i>-number was investigated in agarose gel by monitoring broadband noise emissions with a coaxial passive cavitation detector (PCD) during pulsed exposures (pulse duration 1 ms, pulse repetition frequency 1 Hz) with <i>p<sup>-</sup></i> varying within 1-15 MPa. Three 1.5 MHz transducers with the same aperture, but different focal distances (<i>F</i>-numbers 0.77, 1.02, 1.52) were used. PCD signals were processed to extract cavitation probability, persistence, and mean noise level. At the same <i>p</i> <sup>-</sup>, all metrics indicated enhanced cavitation activity at higher <i>F</i>-numbers; specifically, cavitation probability reached 100% when shocks formed at the focus. These results provide further evidence supporting the excitation of inertial cavitation at reduced <i>p</i> <sup>-</sup> by waveforms with nonlinear distortion and shocks.
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The Journal of the Acoustical Society of America, 2018, 144 (3), pp. 1160 - ?