ARLINGTON, Va., Sept. 20, 2002 - Like weather forecasters mapping cloud movements or police tracking a speeding car, biomedical engineers can now gauge the speed of blood cells rushing through human veins.

In the March issue of the journal Optics Express, the team of researchers describes a technique called optical Doppler tomography (ODT). The technique can produce instant, high-resolution images of 1 to 2 millimeters through tissue, deep enough to penetrate just below human skin.

If perfected, it might used to guide delicate brain surgery, diagnose certain types of cancer---both on the skin and within the body---and monitor blood flow in the eye, an early way of detecting diseases such as macular degeneration and glaucoma.

The new technique combines the Doppler effect with optical coherence tomography (OCT). OCT is analogous to ultrasound imaging except that infrared light waves are used instead of high-frequency sound waves, giving the technique much finer resolution than ultrasound.

The Doppler effect changes our perception of sound or light coming from a moving object. For example, a train's wailing horn shifts to a higher pitch as the train approaches, then to a lower pitch as it passes. The Doppler effect works the same for light, but light travels too quickly for us to notice.

When an OCT signal bounces off a moving object, it undergoes a Doppler shift just as if the signal originated from the object. Comparing the original signal with the bounced signal reveals the speed of objects in an image, much like radar waves bouncing off a moving vehicle tell police its speed or a meteorologist the speed of a cloud. But tracking the minute velocities of blood in the tiny environment of blood vessels requires much greater sensitivity and accuracy than that of tracking a car or a cloud.

In previous work, the researchers developed a similar method that used a photodetector to read the bounced infrared light signal directly, then computers calculated the complex analytic signal as it related to the original signal.

Although essential, this computation made the imaging process too slow for practical purposes, said lead researcher Zhongping Chen, Ph.D., of the University of California, Irvine. "The previous system had high [blood cell] velocity sensitivity but was relatively slow, about 2 seconds for one frame."

In guiding brain surgery, for example, surgeons need instant images to avoid damaging crucial brain tissue or tiny blood capillaries they can't see because they're either too small or obscured by other tissue. To get around this roadblock, the researchers designed a system to measure and calculate the signals optically instead of by computer.

The system combines two infrared light signals: the original one and a polarized interference signal. By shifting the polarized light wave 90 degrees out of phase with the original light signal, the researchers obtained the velocity of blood cells by reading the combined signals directly, avoiding the delay of digital signal processing.

"The real-time phase-resolved optical coherence tomography can image tissue structure and microcirculation simultaneously at the rate of 20 frames per second," Chen said.

Other possible clinical applications include diagnosing and managing second-degree burns. Knowing the exact amount of circulation in any small blood vessels underneath a burn could diminish the guesswork for doctors considering a skin graft. A wrong choice could require more surgery later on.

The technique could also be used to monitor cancer therapies designed to kill tumors by starving them of blood.

Chen said his team is pursuing clinical trials for some of these potential applications.