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Laser Doppler Theory

Laser Doppler measures the total local microcirculatory blood perfusion including the perfusion in capillaries (nutritive flow), arterioles, venules and shunting vessels. The technique is based on the emission of a beam of laser light carried by a fiber-optic probe.

The light is then scattered and partly absorbed by the tissue being studied. Light hitting moving blood cells undergoes a change in wavelength (Doppler shift) while light hitting static objects is unchanged. The magnitude and frequency distribution of these changes in wavelength are directly related to the number and velocity of the blood cells in the sample volume. The information is picked up by a returning fiber, converted into an electronic signal and analyzed.

laser Doppler monitoring principle
laser Doppler measuring depth diagram |Perimed

The measuring depth depends on tissue properties such as the structure and density of the capillary beds, pigmentation, oxygenation, etc. It also depends on the wavelength of the laser light, and on the distance between the sending and receiving fibers in the laser Doppler probe.In normal skin, using Perimed laser Doppler instruments, a probe with standard fiber separation (0.25 mm), and a 780 nm wavelength laser, the measuring depth will be in the order of 0.5-1 millimeter. In blood-rich organs such as the kidney or the liver, the measuring depth is considerably less than one millimeter. When the blood supply to a region is occluded, the "apparent" measuring depth will increase since the lack of blood will permit more passage of light.

No current laser Doppler instrument can provide absolute perfusion values (e.g. ml/min/100 gram tissue). Measurements are expressed as Perfusion Units (PU), which are arbitrary. To enable comparison of results, it is absolutely essential to calibrate the laser Doppler. Perimed was the first company to develop a special Motility Standard for this purpose.

Several different types of signals can be obtained from the PeriFlux System 5000: Perfusion, Concentration of Moving Blood Cells (CMBC), Total Backscatter (TB) and Velocity. Perfusion is the product of the Velocity and the CMBC signals within the measuring volume.

PeriFlux System 5000 permits evaluation of microvascular perfusion in real time. The technique has the advantage of being non-invasive or invasive, depending on the choice of probe. Special probes are available for invasive procedures in for example muscle, kidney and brain. With a multi-channel PeriFlux System 5000, perfusion measurements can be made at several sites- simultaneously.