Nonlinear Mechanisms of Lesion Formation by High Intensity Focused Ultrasound

摘要

Nonlinear mechanisms of lesion formation by high intensity focused ultrasound (HIFU) were investigated experimentally and numerically in a transparent polyacrylamide gel phantom. Numerical predictions were made with a finite-amplitude acoustic propagation model. A 2-MHz transducer of 42-mm diameter and 44.5-mm radius of curvature was operated above the cavitation threshold of the gel phantom at various peak acoustic powers and duty cycles. Acoustic waveforms were recorded in the gel by a fiber optic hydrophone. Bubble activity was detected actively by B-mode diagnostic imaging, passively by a remote focused hydrophone, and optically by CCD and high-speed cameras. Elevated static pressure was applied to suppress bubble activity and increase the boiling temperature, thus isolating the pure effect of acoustic nonlinearity. In overpressure experiment performed at 32 W acoustic power, both cavitation and nonlinear ultrasound propagation accelerated lesion inception and growth, but acoustic nonlinearity, which led to shock formation, played the dominant role. Rapid localized heating, particularly by increased absorption in the shocked wave, led to boiling and then to proximal growth and migration of the lesion even at low overpressure. At 90W acoustic power, boiling was observed and predicted in less than 50 ms.