Frequency dependence of cavitation activity in a rotating tube exposure system compared to the mechanical index

Cavitation was produced by focused and unfocused ultrasonic beams in a 60-rpm rotating-tube exposure system. Hemolysis, free radical generation, and hydrogen peroxide were assayed after exposure to determine the dependence of cavitation activity on pressure-amplitude p and frequency f. For unfocused exposure, hemolysis rose rapidly from an apparent threshold pressure amplitude, to a maximum value and then declined gradually at higher amplitudes. Cavitation activity decreased with increasing ultrasonic frequency for a fixed pressure amplitude. Apparent thresholds for unfocused exposure were similar for hemolysis after 15 s and H2O2 production after 15 min. These thresholds increased roughly in proportion to increasing ultrasonic frequency. Cavitation activity was lower and thresholds were higher for focused exposures than for unfocused exposures. The approximate pf-1 = 0.20 MPa/MHz dependence of the experimental threshold of H2O2 production from 0.745 to 2.95 MHz differs from the pf-1/2 = 0.24 MPa/MHz1/2 dependence of the theoretical "mechanical index" threshold for inertial cavitation.