Aberration correction by nonlinear beam-mixing: Generation of a pseudo point sound source

Nonlinear beam mixing with microbubbles was explored to create a pseudo point source for aberration correction of therapeutic ultrasound. A damping coefficient for a bubble driven by a dual frequency sound field was derived by revisiting Prosperetti's linearized damping model. As a result, the overall damping term for dual frequency was obtained by linear summation of two damping terms for each frequency. The numerical simulation based on the bubble model suggests that the most efficient bubble radius to generate a 1 MHz frequency from 4 MHz and 5 MHz sound sources is 2.6 - 3.0 μm. Furthermore, this size range constitutes the primary distribution of a specific ultrasound contrast agent: Optison (Amersham Health, Princeton, NJ). When a chamber of 0.1% diluted Optison was insonified by 4 MHz and 5 MHz sound beams with 80° incident angle between them, an approximately 100 Pa, 1 MHz difference frequency signal was measured approximately 10 cm away. In addition, the received 1 MHz difference frequency signal shows omnidirectional characteristics, even though the overlap zone of the two sound beams is on the order of the difference frequency wavelength. Therefore, the induced sound source can be considered as a pseudo point source and is expected to be useful for aberration correction for therapeutic ultrasound.