Dependence of inertial cavitation induced by high intensity focused ultrasound on transducer F-number and nonlinear waveform distortion

Pulsed high intensity focused ultrasoundwas shown to enhance chemotherapeutic drug uptake in tumor tissue through inertial cavitation, which is commonly assumed to require peak rarefactionalpressures to exceed a certain threshold. However, recent studies have indicated that inertial cavitationactivity also correlates with the presence of shocks at the focus. The shock front amplitude and corresponding peak negative pressure (p−) in the focal waveform are primarily determined by the transducer F-number: less focused transducers produce shocks at lower p−. Here, the dependence of inertial cavitationactivity on the transducer F-numberwas investigated in agarose gel by monitoring broadband noise emissions with a coaxial passive cavitationdetector (PCD) during pulsed exposures (pulse duration 1 ms, pulse repetition frequency1 Hz) with p− varying within 1–15 MPa. Three 1.5 MHz transducers with the same aperture, but different focal distances ( F-numbers0.77, 1.02, 1.52) were used. PCD signals were proces...