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
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.
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
3 107 2. 8
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.
However, data bottlenecks are a problem, as Serov and co-workers found when they demonstrated full-field laser Doppler