THE measurement of blood flow in human tissues is important for the diagnosis and assessment of many diseases. Current ultrasound techniques use the Doppler shift of the frequency of sound waves to measure the motion of blood cells. But the effect is undetectable for blood moving slower than about 10 millimetres per second. Now, researchers in the U.S. have demonstrated an ultrasound-based technique that can measure speeds as low as a quarter of a millimetre per second. The scheme uses laser pulses to track the sound waves generated by locally heating the blood with ultrasound.
The method may find use in biomedical applications ranging from functional brain imaging to the detection of cancer and atherosclerotic plaques. The work of Lidai Wang and others was recently published in the journal Physical Review Letters.