The Relationship of Cavitation to the Negative Acoustic Pressure Amplitude in Ultrasonic Therapy

The relationship between the cavitation and acoustic peak negative pressure in the high-intensity focused ultrasound (HIFU) field is analyzed in water and tissue phantom. The peak negative pressure at the focus is determined by a hybrid approach combining the measurement with the simulation. The spheroidal beam equation is utilized to describe the nonlinear acoustic propagation. The waveform at the focus is measured by a fiber optic probe hydrophone in water. The relationship between the source pressure amplitude and the excitation voltage is determined by fitting the measured ratio of the second harmonic to the fundamental component at the focus, based on the model simulation. Then the focal negative pressure is calculated for arbitrary voltage excitation in water and tissue phantom. A portable B-mode ultrasound scanner is applied to monitor HIFU-induced cavitation in real time, and a passive cavitation detection (PCD) system is used to acquire the bubble scattering signals in the HIFU focal volume for the cavitation quantification. The results show that: (1) unstable cavitation starts to appear in degassed water when the peak negative pressure of HIFU signals reaches 13.5 MPa; and (2) the cavitation activity can be detected in tissue phantom by B-mode images and in the PCD system with HIFU peak negative pressures of 9.0 MPa and 7.8 MPa, respectively, which suggests that real-time B-mode images could be used to monitor the cavitation activity in two dimensions, while PCD systems are more sensitive to detect scattering and emission signals from cavitation bubbles.