Laser Doppler fluxmetry is a diagnostic procedure that provides information about skin microcirculation and is based on the Doppler effect. A helium laser emits light that is reflected by moving erythrocytes in the blood. The amount of reflected light allows conclusions to be made about flow velocity.
What is laser Doppler fluxmetry?
Laser Doppler fluxmetry is a diagnostic procedure that allows statements to be made about the microcirculation of the skin and is based on the Doppler effect. In other words, it allows the physician to determine blood flow within the smallest vessels and end-stream pathways. The so-called fluid flow measurement technique encompasses various methods for determining the physical quantities of fluid flows. One flow measurement technique used in medicine is laser Doppler fluxmetry. This is a non-invasive method based on the Doppler effect that measures skin microcirculation. A helium-neon laser is at the heart of the procedure. The laser emits light that reflects off moving structures such as erythrocytes. In this way, the physician determines laser Doppler flux as a relative measure of blood flow within the smallest vessels and end-stream pathways. The quantities are expressed in arbitrary units. The Laser Doppler system is suitable, for example, for suspected occlusive diseases within phlebology. Other possible applications are for dermatologists, who can use the procedure to gather information about malignant changes in the skin. Dysplastic nevi or malignant melanomas have certain reflected light microscopic criteria and are associated with morphological and functional changes within the vascular architecture. For this reason, measurement of flow characteristics by laser Doppler fluxmetry in this context can provide information to assess the malignancy of any skin lesions. The method is sometimes referred to as laser Doppler anemometry or laser Doppler flowmetry.
Function, effect, and goals
Laser Doppler anemometry is used for non-contact optical measurement of point velocity components within fluid flows. Inform medical fluxmetry involves the measurement of blood flow. In this method, a laser beam is split into two different beams with the aid of beam splitters, which cross at the measuring point. This creates an interference fringe pattern in the crossing area. Particles such as erythrocytes generate a scattered light signal in the photodetector as they move through the fringe pattern. The laser Doppler technique is thus based on determining the Doppler shift of scattered light from moving and laser illuminated objects. The frequency of light cannot be measured directly and is therefore determined by superposition with reference beams in the range of a few megahertz. Different models are available in this context. The interference fringe model is extremely descriptive and especially favorable for small particles such as erythrocytes. However, the Doppler model describes the signal generation more comprehensively and includes the interference fringe model at the same time. A helium laser is used in laser Doppler fluxmetry. The light is scattered and partially absorbed by the tissue under examination. Once the light hits moving blood cells, its wavelength changes, which is known as a Doppler shift. The light on static objects remains unchanged in its wavelength. The magnitude of the changes in wavelength is thus directly related to the speed of blood cells. This information is converted and analyzed and recorded as an electronic signal by the measuring device. The measurement depth depends on tissue properties such as structure and density in the capillary bed, pigmentation or oxygenation. The measuring device is equipped with a transmitting and a receiving electrode, and the distance between the transmitting and receiving elements within the laser Doppler probe also has an influence on the measurement depth. To determine microcirculation in normal skin, a probe with a standard distance of 0.25 mm and laser wavelengths of around 780 nm is usually used. When examining blood-rich organs such as the kidney or liver, the measurement depth is usually much less than one millimeter. The measurement is performed in perfusion units.Meanwhile, variations of fluxmetry are also used in patients with osteoporosis to determine bone fragility. Laser Doppler fluxmetry is also frequently used today to monitor the progress of phlebological therapies, especially drug therapies. Another area of application for the procedure is in ophthalmology, where fluxmetry is used, for example, to assess glaucoma damage.
Risks, side effects, and hazards
Laser Doppler fluxmetry offers several features and advantages. First, it is a noncontact, noninvasive procedure. Especially when skin lesions with suspected malignancy are involved, its performance is associated with advantages for the patient. Thanks to fluxmetry, the patient does not necessarily have to undergo an invasive procedure to clarify the initial suspicion. Since malignant skin changes alter flow velocity and vessel architecture, non-invasive fluxmetry can already provide extensive information in this context and allow the physician to decide whether a biopsy and thus an invasive procedure appears necessary at all. Laser Doppler fluxmetry can be performed on an outpatient basis and is not associated with any risks or side effects for the patient. Studies have carefully investigated whether laser irradiation of malignant skin lesions, for example, could result in scattering. Such a risk is now considered to be excluded without exception. Laser Doppler fluxmetry also offers the physician various advantages. On the one hand, the procedure is comparatively inexpensive compared to other diagnostic methods, and on the other hand, the time required is also estimated to be rather low. The use of this non-invasive method reduces the burden on both the patient and the physician. However, after fluxmetry, minimally invasive or invasive procedures may be required if the findings are appropriate.
