Optical observations of microbubble oscillation in small tubes

In diagnostic medicine, microbubbles are used as contrast agents to image blood flow. However, the effect of microvessels surrounding the oscillating microbubbles (MBs) is under investigated. Here, oscillations of Definity (Lantheus Imaging) microbubbles in tubes with inner diameter of 25 ¿m and 200 ¿m are compared in response to low acoustic driving pressure, using the ultrahigh speed Brandaris 128 camera, at frame rates of ~12 million frames per second. A significant reduction in oscillation was observed for MBs in the smaller 25 ¿m tube, when compared to those in a 200 ¿m tube. In the 25 ¿m tube, at 50 kPa an increased incidence of MBs which did not oscillate above the noise level of the system was observed (56% of MBs compared to 8% in the 200 ¿m tube), indicating an increased oscillation damping. Unlike theoretical predictions no significant difference was observed between the resonance frequency curves calculated for MBs in 25 ¿m and 200 ¿m tubes. While previous investigators have shown the effect of microvessels on MB oscillation at high ultrasound pressures, the present study provides the first optical images of low amplitude microbubble oscillations in small tubes.