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The utilization of the bubble pressure dependent harmonic resonance frequency for enhanced heating during high intensity focused ultrasound treatments

The use of bubbles in High Intensity Focused Ultrasound (HIFU) enhances tissue heating; however, localization of the treatment is difficult due to harmful effects at non-focal regions. In this study the bubble dynamics were studied through the utilization of resonance curves and bifurcation diagrams of the normalized bubble oscillation amplitude (NBOA) (time varying radius/initial radius) by solving the Hoff model. This method allowed us to study the bubble dynamics in wide ranges of control parameters. Results indicate that, by sonicating the bubbles with their pressure dependent harmonic resonance frequency (e.g. 3.1 MHz for a 5 micron bubble with shell shear modulus of 50 MPa and 100nm thickness), the (NBOA) is negligible below a threshold pressure (PT) (∼1.5 MPa). For pressures above the PT, the NBOA increases and remains constant for a large pressure range concomitant with a significant increase in higher harmonic emissions. Because of the steep spatial pressure gradient of the HIFU transducer, the n...