A Double Frequency Doppler Technique for Bubble Size Measurement

The Double Frequency technique of bubble sizina has been shown to be an excelient technique-for the detction ana sizing of microbubbles in fluids. In this technique the bubbles are insonified by two sound fields, a high frequency imaging field and low frequency pumping field. The sum and difference frequency signals generated from the oscillations of the pumped bubbles are used for size measurements. However this technique does lead to some problems from similar signals generated by stationary targets. One way of overcoming this problem is to use the Doppler signals generated by the moving bubbles. In this paper we propose a new technique based on the amplitude measurement of the Doppler spectra generated by the motion of the bubble and the sidebands produced from the Double Frequency technique. Experimental results indicate that size measurements obtained through this method agree with size measuments using Stoke's law. Bubbles are excellent scatters of sound. They can be detected easily by classical ultrasonic imaging systems such as a real time ultrasonic B scan or a Doppler flowmeter ( 1,2,3). Although these techniques have been proposed (for example (3)) to size the bubbles they do not provide accurate size estimates, mainly because there is no way of separating the echoes from bubbles of different size, in other words a group of small bubbles can give rise to the same echo a large bubble. As bubbles exibit a resonant behavior, they have a characteristic frequency inversely proportional to their size. These size dependent oscillations have been already exploited trough different methods including resonant scattering(3), second harmonic generation(4) and double frequency technique(S). Even though the Double Frequency technique where a low frequency pumping field and a high frequency imaging field are used, has better spatial resolution and excellent size descrimination, the presence of stationnary targets may sometimes give rise to signals similar to those of resonating bubbles. Thus any size measurement based on the Double Frequency technique also may lead to erroneous sizes unless steps are made to take these motion dependent changes into account and a suitable sytem designed accordingly. In this paper we propose a new technique based on the amplitude measurement of the Doppler spectra generated by the motion of the bubble and the sidebands produced from the Double Frequency technique. In the first section, we illustrate the principle of the Double Frequency Doppler technique by explaining the formation of the Doppler spectra sidelobes as the bubble goes through its resonanance when excited by a pumping field and an imaging field. The processing of these signals through each stage of the block diagram is discused to explain the extraction of the bubble resonance. Then, with the