Optics & Photonics News

octors often use laser Doppler flowmetry to measure microcirculation—the flow of blood through small vessels in organs to the surrounding tissue.

The technique is used to assess burns and wounds, diabetes, inflammatory responses and other conditions that affect blood flow. It works via scattering: When laser light irradiates tissue, it is scattered along many random paths; the light scattered by moving red blood cells is frequency-shifted by an amount proportional to the velocity of the cells. This Doppler-shifted light interferes with light that has not interacted with moving cells to produce beat frequencies that are proportional to the velocity of the cells (typically in the frequency range from a few Hz up to 20 kHz).

Researchers first observed this effect in the microcirculation of the skin more than 30 years ago. They used a laser to illuminate the skin at a single point and then analyze the frequency spectrum of the backscattered light. They noticed that the frequency spectrum changed when the blood flow was occluded and released. Such early experiments formed the basis of commercial single-point laser Doppler blood flow monitors.

Subsequently, scanning mirrors that imaged blood flow were introduced into clinical practice. With these systems, images were built up point by point. Such imagers provide useful spatial information to clinicians, but they take a long time to produce an image (often as long as five minutes for a 256 3 256 pixel image). The slow scanning speed means that motion artifacts can be a problem, and the technique can be uncomfortable and inconvenient for patients. Moreover, the imagers cannot detect rapid changes in blood flow—which are useful for studying inflammatory responses and other conditions.

Laser Doppler devices have progressed from single-point sensors to imaging systems that build up images point by point through scanning. Recent advances in laser Doppler blood flow imaging are moving toward imaging at higher frame rates—an advance that will make imaging easier by reducing the likelihood of motion artifacts. It will also open up new applications in Doppler imaging—for example, the imaging of blood flow transient in inflammatory responses or of brain activity during surgery.

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Full-field imaging methods

Recent investigations into laser Doppler blood flow have focused on reducing the image acquisition time. One approach is called laser speckle contrast imaging. It involves acquiring full-field images of tissue using a conventional charge-coupled device (CCD) camera. The contrast of the detected speckle pattern is calculated over a sub-array within the image ( typically, 7 3 7 pixels). Over the integration time of the camera, the speckle becomes blurred as blood cells move, reducing the contrast. Provided there is an accurate model relating contrast to flow, this provides a measurement of blood flow.

Another possibility involves introducing photodetector arrays that can rapidly take the data from the chip. Line

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Results from a typical occlusion and release test that was done using a single pixel from a CMOS sensor.

[ Blood flow in hand and forearm ]

We implement a 1,024-point FFT to obtain flow values across the array in 1. 6 ms, with a 64 3 64 pixel image acquired in approximately 4 s. These are typical images of veins on the (left) hand and (right) forearm of a healthy volunteer. These were obtained with the modified moorLDLS system.

illumination and a linear photo-detector array can be used so that scanning in one dimension produces a 2-D image.

Improvements in commercial complementary-metal-oxide-semiconductor (CMOS) camera technology have allowed researchers to perform full-field laser Doppler blood flow imaging. Images are read out at a high frame rate and transferred to an external processor to extract blood flow information. Systems developed by both the University of Twente (Netherlands) and the École Polytechnique Fédérale de Lausanne (Switzerland) have demonstrated the exciting potential of this technology.