Now there’s a better way to spy on the blood in your veins. Doctors already have two techniques to monitor obstructions in blood vessels, but they both have limitations. The first, Doppler ultrasound imaging, involves irradiating tissue with ultrasound waves; the waves that reflect off flowing blood acquire a Doppler shift, which can be used to pick out blood and calculate its speed. Doppler can't distinguish flowing blood from surrounding tissue unless it's moving quickly, however, which makes minor blood vessels invisible. The second technique, photoacoustic imaging, uses an infrared laser that, when absorbed by blood, heats it. The resulting sudden expansion creates a pressure wave that can be detected outside the body. Photoacoustic imaging picks out blood vessels better, but it can't see flow in a continuous stream. In a study published today in Physical Review Letters, researchers combined the two techniques, utilizing the fact that ultrasound also has a slight heating effect; pulsed ultrasound creates periodic hot spots in blood vessels. By tracking the movement of these hot spots (shown in yellow above) using photoacoustic imaging, the team could calculate the flow rate of the blood, even when it moved slowly through small vessels like capillaries. The researchers hope their technique may aid functional brain imaging, help cancer screening and treatment monitoring, and let doctors detect atherosclerosis before a patient has a heart attack.