(598g) High-Frequency In Vivo Ultrasound Imaging of Size-Isolated Microbubbles

High-frequency ultrasound imaging using microbubble (MB) contrast agents is becoming increasingly popular in pre-clinical and small animal studies of anatomy, flow and vascular expression of molecular epitopes. Currently, in vivo imaging studies rely on highly polydisperse microbubble suspensions, which may provide a complex and varied acoustic response. In order to study the effect of individual microbubble size populations, microbubbles of 1-2 μm, 4-5 μm, and 6-8 μm diameter were isolated using the technique of differential centrifugation. Size-isolated microbubbles were tested in vivo over a range of concentrations using a Visualsonics Vevo 770 ultrasound imaging system with a 40-MHz probe in fundamental mode. Our results demonstrate that contrast enhancement and circulation persistence are strongly dependent on microbubble size and concentration. Large microbubbles (4-5 and 6-8 μm) strongly enhanced the ultrasound image with positive contrast, while 1-2 μm microbubbles showed little enhancement. For example, the total integrated contrast enhancement, measured by the area under the time-intensity curve (AUC), increased 6.6-fold for 6-8 μm diameter microbubbles at 5 x 10^7 MB/bolus compared to 4-5 μm microbubbles at the same concentration. Interestingly, 1-2 μm diameter microbubbles did not noticeably enhance the signal at any concentration, but rather attenuated the ultrasound signal, indicating that they were weaker scatterers than tissue. The results of this study imply that microbubbles can be tailored for optimal contrast enhancement in fundamental mode imaging.