Measuring intravascular blood flow is currently only possible with invasive methods. Swiss researchers from École polytechnique fédérale de Lausanne (EPFL) and Geneva University Hospital decided to see whether color Doppler ultrasound data holds enough information to derive the velocity of blood flowing through a vessel. They created a mathematical model of the vessels and blood within, and how ultrasound interacts with them. The results are pretty impressive, though not accurate enough for clinical use, and point to the possibility of one day getting accurate blood flow gauging from existing hardware.
Here’s a video of the software processing color Doppler data from a conventional clinical ultrasound transducer: