Ultrasound imaging guidance and evaluation in ultrasound therapy.

High intensity ultrasound can create hyperechoic regions in an ultrasound image due to local generation of bubbles. We explore the link between the temporal extent of these hyperechoic regions and tissue damage caused by ultrasound therapy. The decay rate of increased echogenicity from the focal zone in insonated live exteriorized canine kidney was quantified and correlated to the spatial extent of tissue damage. The decay half-time, t_half, defined as the time for echogenicity enhancement to decay by a factor of 2, was observed in all cases to be greater than 41 seconds in spatial zones where extensive histological damage was observed. In cases where the measured t_half was less than 11 seconds, the damage was limited to minor hemorrhage, or was not detected. These t_half discrimination boundaries of 41 and 11 seconds were not statistically different for cases where contrast agent was used to enhance therapeutic efficiency. 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 damping model. As a result, the overall damping term for dual frequency operation 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 size range to generate a 1 MHz frequency from 4 MHz and 5 MHz sound sources is 2.6--3.0 mum. Furthermore, this range constitutes the primary distribution of a specific ultrasound contrast agent: Optison. When a chamber of 0.1% diluted Option was sonified by 4 and 5 MHz sound beams with 80° incident angle between them, an approximately 100 Pa, 1 MHz frequency signal can be measured approximately 10 cm away. Furthermore, the received 1 MHz difference frequency signal shows omni-directional 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.