1 Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada.
2 Physical Sciences Department, Sunnybrook Research Institute, Toronto, Ontario, Canada.
3 Department of Physics, Concordia University, Montreal, Quebec, Canada.
4 Department of Biology, Concordia University, Montreal, Quebec, Canada.

Objectives: The development of low boiling point liquid droplets as phase-change contrast agents allows for the local creation of microbubbles at a point of interest in vivo. Although there are many possible applications, few investigations have used selectively created microbubble boluses to measure volumetric flowrate. In this study, the flow ratio between two vessels is calculated by vaporizing droplets in each vessel individually.

Methods: Proof of principle is demonstrated in vitro by an imaging sequence that vaporizes droplets using a high mechanical index pulse, then images the transit of the resulting microbubbles at a high frame rate using low mechanical index plane waves.

Results: It is shown that a linear relationship exists between the concentration of droplets and enhancement of the resulting microbubble bolus. In vitro flow is measured with a mean error of 8% in a 0.66 cm diameter vessel and with a mean error of 33% in a 0.49 cm diameter vessel. The relative volumetric flow between two adjacent vessels is calculated with a mean percentage error of 25% when imaging the region of droplet vaporization for flow ratios between 0.25 and 4.

Conclusions: This in vitro study demonstrates the feasibility of using a positive bolus tracer, induced by image-guided ultrasound excitation, to measure flow. Potential applications include measurement of the portal vein to hepatic artery flow ratio, known as the hepatic perfusion index.