Acoustical investigation of freely moving single microbubbles

A new approach to characterize full populations of contrast agents suspended in a water tank is presented. In particular, the movements of single bubbles due to ultrasound (US) radiation force are observed. The backscattered signal and bubble displacement measured in various experimental conditions are compared to predictions of a non-linear model describing both radial and translational bubble motion. Two different types of experiments have been performed to estimate viscoelastic properties and radial dimensions of thermoplastic shelled microballoons and BR14. First, maximum bubble velocities experienced by a full population insonified at frequencies ranging from 2 to 8 MHz have been measured. Such velocities have then been compared to those predicted by the model for resonant bubbles. The modeled shell properties have finally been updated to obtain a good fit over all frequencies. The diameters have been estimated by exploiting the non-linear behavior of single bubbles. In this case, the bubbles have been insonified by a 20 cycle 2 MHz burst with 490 kPa rarefactional pressure. The received relative second harmonic signal level and the bubble velocity are compared to values predicted by simulation for incrementing bubble radii, until a unique radius can be read.

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