Applications of the Double Frequency Technique in Bubble Sizing and Pressure Measurements in Fluids

Abstract

The detection and sizing of microbubbles in fluids has potential applications in industry and medicine. Non-invasive techniques using ultrasound appear attractive compared to optical techniques. Different methods employing ultrasound have been shown to be capable of detecting bubbles in fluids and of estimating their sizes. These include those using the Doppler effect (Nishi, 1972)¹ , resonant scattering (Fairbanks and Scully, 1977)² and second harmonic generation (Miller, 1981)³. There are some major drawbacks in these techniques. In the Doppler technique there is no way of distinguishing echoes from a cluster of small bubbles from those of a large bubble occupying the same volume. The Doppler technique is also incapable of detecting stationary bubbles, such as those trapped in tissues. The resonant scattering and the second harmonic generation techniques work on the basis of the fact that bubbles have a characteristic resonant frequency dependent on their sizes. In resonant scattering, however, it is found that larger bubbles give rise to a stronger echo than bubbles at resonance, giving rise to ambiguities which would make it difficult to measure the size distribution of bubbles covering a range of sizes. Both the resonant scattering and the second harmonic generation methods suffer from poor spatial resolution.