Abstract
We constructed an experimental setup of focused ultrasound Field for water with uniformly distributed lipid microbubbles, and measured nonlinear propagation properties caused by oscillations of microbubbles. An ultrasound transducer with a concave PZT ceramic element with a focal length of 40 mm was fixed in a water tank. The therapeutic sensitizer Sonazoid with a diameter of 2 to 3 μm was used. A 30 mm thick polyacrylamide gel containing microbubbles was fixed to include the focus to suppress bubble mobility during the measurement. Elements with four center frequencies l, 2.23, 3.35, and 4.32 MHz, and a moderately weak ultrasound intensity (focal pressure 0.15 MPa) that suppresses the destruction of microbubbles, were used. Then, we conducted one-dimensional measurement and the results showed that the waveform attenuated with the increasing incident frequency and void fraction, which indicates the nonlinearity of the oscillations of microbubbles. Higher harmonics were generated at low frequencies. While focal-pressure increased with the applied voltage to the transducer at high frequencies (near resonance), no increase in focal-pressure was observed in the case of low frequencies.
