Employing Positron Emission Tomography to Study the Dynamics of Droplets in Fluid Flows

Positron emission tomography (PET) is a method used to diagnose and map the spread of cancer in human body using radioactive agents. In this method, the radioactive material spontaneously emits positrons that undergo annihilation, thus generating a pair of photons that are fired back-to-back to conserve momentum. The high-energy photons penetrate nearly any medium (including most metals) and are sensed on a grid of small detectors (scanner), allowing for triangulation of the annihilation position and ultimately for the reconstruction of the three-dimensional radioactivity distribution in the entire body. In the present work, we leverage on the strengths of PET and exploit it to study the dynamics of droplets suspended in fluid. We produce droplets from Fluorine-18 (¹⁸F) water and release them in a vessel filled with an Ethanol-Glycerol binary mixture, positioned inside a PET scanner. By changing the mixture ratio, we tune its density and the diffusivity of water in it, allowing us full control over the experiment pace which in turn ensures the dynamics is captured with the radioactivity levels we used. Using a unique probabilistic model we developed, we infer the droplet trajectories and their time-dependent activities, that is their rates of positron emissions, which can accurately estimate their size evolution. Our inference is compared against results from image analysis to video footage of the experiment, showing good quantitative agreement. The use of PET to study the fluid dynamics of suspended droplets can potentially be used in a variety of applications, including drug delivery or emulsion stability in flows.