Properties of ultrasound contrast agents for diagnosis and therapy
Résumé
Ultrasound contrast agents consist of bubbles in the micrometer range encapsulated by nanoshells. These medical microbubbles oscillate linearly while insonified at low acoustic amplitudes, but demonstrate highly nonlinear, even destructive behavior at relatively high acoustic amplitudes. Therefore, medical microbubbles have been investigated for their diagnostic and therapeutic properties. We give an overview of the physical mechanisms associated with this behavior. Microbubbles have been observed to pulsate symmetrically and asymmetrically, translate, coalesce (merge), fragment, crack, and jet (act as a microsyringe) during one single ultrasonic cycle. The temporary formation of a liquid drop inside a microbubble has also been observed. Assuming that fragmentation will occur only if the kinetic energy of a collapsing microbubble exceeds the instantaneous bubble surface energy, we show that the fragmentation threshold is not equal to the threshold at which cavitation occurs. These phenomena have, among others, potential applications in targeting and localized drug delivery.