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

In this report, the authors consider the modifications yielded in the Doppler spectrum when acoustic fields of increasing intensities are applied to encapsulated gas bubbles. Their in vitro experimental results show that the spectrum bandwidth 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 generate only positive Doppler frequencies), the Doppler spectrum is enlarged up to the point that negative Doppler shifts also are produced. Possible explanations in terms of either radiation force or streaming are discussed for this asymmetrical bandwidth enlargement.