Doppler spectra from contrast agents crossing an ultrasound field

When contrast agents are injected in a fluid, it is implicitly assumed that they move at the same velocity as the fluid itself. However, a series of i n vitro tests performed by using air-filled microbubbles suspended in disti lled water, have shown that the Doppler spectrum generated in this case may be notably different from that obtained from non-resonating scatterers. In this paper, we show, through a simple simulation model, that the actual mo vement of microbubbles may be predicted as the result of the complex balanc e between two forces: the ultrasound radiation force, which tends to move t he particles along the sound beam direction, and the fluid drag force, whic h tends to move the particles along the fluid stream. The contrast agents t urn out to be displaced only during the passage of the ultrasound burst; du ring the remaining time, they are maintained at the fluid velocity by the d rag force. Based on the total particle displacement estimated between conse cutive pulses, a series of Doppler spectra corresponding to different inten sity levels was computed. This series was shown to be in excellent agreemen t with the experimental spectra obtained in vitro using Levovist(R) (Scheri ng AG, Berlin, Germany) particles suspended in distilled water Rowing at a steady rate.