Unexpected Doppler effects from microbubbles moving through an ultrasound beam

Encapsulated microbubbles crossing an ultrasound field are known to be sensitive to radiation force, i.e. they tend to move along the direction of the incident beam. In this paper, a simple simulation model is introduced and shown capable of predicting the displacements imposed to microbubbles when they simultaneously experience the ultrasound radiation force and the drag force of the fluid where they are suspended. Based on the total particle displacement estimated between consecutive transmitted pulses, Doppler spectra corresponding to different intensity levels were computed. They were then compared to the experimental spectra obtained in vitro by using Levovist ((R)Schering, Berlin, Germany) particles suspended in distilled water flowing at a steady rate. The excellent agreement between experimental and simulation results confirm the validity of the proposed model