The measurement of backscatter from individual contrast agent microbubbles

In vitro experimentation still has some way to go to achieve a thorough understanding of the dynamic properties of microbubbles. This paper describes an experiment to detect RF backscatter from individual microbubbles using a hydrodynamically focused flow. A fine capillary was placed at the centre of the focused flow and single microbubbles were released along a precise vertical path coincident with the axis of the ultrasound beam. Six period pulses were used in combination with two different pulse amplitudes (550 kPa and 980 kPa p.p.). The RF signal intensities of individual scattering events for the contrast agent Definity were calculated and normalised using calibration data previously obtained from solid copper particles. The usable receive frequency range of the system was 1.0 - 4.5 MHz. At 550 kPa, a dominant resonance was observed at a transmit frequency of 1.5 MHz for both the fundamental and second harmonic backscatter. At 980 kPa the fundamental backscatter increased monotonicaly with transmit frequency, whilst the second harmonic backscatter was almost independent of transmit frequency. The experimental system demonstrates considerable flexibility of control and the ability to produce a large output of backscatter data from individual microbubbles.