Quantitative analysis of Doppler spectrum modifications yielded by contrast agents insonified at high pressure

In this report, we consider the modifications yielded in the Doppler spectr um when acoustic fields of increasing intensities are applied to encapsulat ed gas bubbles. Our in vitro experimental results show that the spectrum ba ndwidth is nearly proportional to the incident acoustic pressure, when its amplitude is maintained below about 200 kPa. At higher pressure levels, it even may happen that, in a steady, unidirectional flow (which should genera te only positive Doppler frequencies), the Doppler spectrum is enlarged up to the point that negative Doppler shifts also are produced. Possible expla nations in terms of either radiation force or streaming are discussed for t his asymmetrical bandwidth enlargement.