Doctors use ultrasound to measure blood flow in the body, but they can't detect flow in small, slow moving vessels where diseases often start. To do that, they are now employing sonic blasts that heat up a tiny drop of blood.

Researchers at Washington University in St. Louis say that the sonic blasts will create a warm sensation because it is heating up your vessels. Meanwhile, infrared laser pulses bounced off the warmed blood would be picked up by the ultrasound.

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The technique detects anything under 10 millimeters per second, and apparently won't hurt at all.[Physical Review Letters via New Scientist]