On the interaction between ultrasound and contrast agents during Doppler investigations.

Knowledge of interaction mechanisms between ultrasound (US) and contrast agents (CA) suspended in blood is important for a correct interpretation of clinical investigation results. Experiments performed in different laboratories have shown that, as a consequence of primary radiation force, CA tend to move away from the US transducer. Accordingly, Doppler spectra produced by particles suspended in moving water turn out to be significantly altered from what is theoretically expected. The purpose of this paper is twofold. First, an original model describing the bubble dynamics as the outcome of the balance between US radiation force and fluid drag force is validated for the case in which bubbles are suspended in blood. The high fluid viscosity is shown to prevent significant bubble deviations from the unperturbed fluid streamlines so that, in large vessels, a residual spectral distortion may exist only at the highest intensity levels permitted by current regulations. Finally, the relative importance and differences between the effect of primary radiation force and streaming mechanisms that, in principle, could lead to similar effects, are discussed.