BUBBLE DETECTION AND SIZING WITH A DOUBLE FREQUENCY DOPPLER SYSTEM

The detection and sizing of microbubbles have applications in many fields. In this paper it is shown that resonant moving bubbles introduce Doppler shifts on the sidebands generated at the sum and difference frequency of a pumping frequency field which is swept through the resonance frequency of the bubble, together with a higher frequency imaging field. These so-called double frequency Doppler shifts are equal to the conventional Doppler shifts induced on the imaging frequency, with an amplitude which is maximum when the pumping frequency is equal to the resonance frequency of the bubble. A new device was constructed to detect and size resonant bubbles passing through the ultrasonic beam in a flowing liquid by monitoring these double frequency Doppler shifts. It is shown experimentally that moving bubbles can be accurately sized even when they are surrounded by stationary targets, and that the variation of the displacement velocity does not affect the accuracy of the size estimation.