: The speed of blood moving through the body's largest arteries and veins can be measured using Doppler ultrasound. But in narrower arteries, where the flow's speed and volume are lower, the Doppler effect is too weak to yield a significant measurement. To overcome that limitation, Lidai Wang of Washington University in St Louis and his colleagues have developed a new and different method that uses heat as a tracer. In the first step, a focused beam of ultrasound raises the temperature of blood inside the vessel of interest by a few degrees. The warmed volume of blood goes with the flow. Its progress, and therefore its speed, is detected by irradiating the vessel with pulses of infrared. The IR pulses cause the blood to emit ultrasound pulses whose amplitude depends on the blood's temperature. Using the method, Wang's team has measured flows as low as 0.24 mm/s.