Laser Doppler Velocimetry

Basic Principle. Laser Doppler velocimetry (LDV) is a rela­tively new clinical method for assessing cutaneous blood flow. This real-time measurement technique is based on the Dopp­ler shift of light backscattered from moving red blood cells and is used to provide a continuous measurement of blood flow through the microcirculation in the skin. Although LDV provides a relative rather than an absolute measure of blood flow, empirical observations have shown good correlation be­tween this technique and other independent methods to mea­sure skin blood flow.

According to the fundamental Doppler principle, the fre­quency of sound, or any other type of monochromatic and co­herent electromagnetic radiation such as laser light, that is emitted by a moving object is shifted in proportion to the ve­locity of the moving object relative to a stationary observer. Accordingly, when the object is moving away from an ob­server, the observer will detect a lower wave frequency. Like­wise, when the object moves toward the observer, the fre­quency of the wave will appear higher. By knowing the difference between the frequencies of both the emitted and the detected waves, the Doppler shift, it is possible to calcu­late the velocity of the moving object according to the follow­ing equation:

f = 2vjf cos в/е (1)

where f is the Doppler shift frequency, в is the mean angle that the incident light makes with the moving red blood cells, е is the speed of light, f0 is the frequency of the incident light, and v is the average velocity of the moving red blood cells. Because the red blood cells do not move through the microcir­culation at a constant velocity and light scattering leads to a wide distribution of angles в, the Doppler-shifted light con­tains a spectrum of different frequency components.

The Doppler shift of laser light caused by the average blood velocity in the capillaries (around 103 m/s) is very small and difficult to measure directly. Therefore, the frequency shifted and unshifted backscattered light components from the skin are mixed on the surface of a nonlinear photodiode. The out­put from the photodiode, an average dc offset voltage and a small superimposed ac component, is amplified and band pass filtered to eliminate low-frequency components in the range between 10 and 50 Hz. These frequencies are attributed to noise resulting from motion artifact and high-frequency noise components (typically in the kilohertz range) resulting from nonbiological noise. As the average red blood cells (RBC) ve­locity is increased, the frequency content of the ac signal changes proportionally.

Assuming a constant blood flow geometry, Stern (24) pro­posed the following empirical relationship between the ampli­tude of the Doppler-shifted spectrum and the velocity of the blood flow:

F =(o2P((o)dco (2)

where F is the root-mean-square (rms) bandwidth of the Doppler power spectrum signal, w is the angular frequency, and P(w) is the power spectral density of the Doppler signal. To compensate for laser light intensity, skin pigmentation, and numerous other factors that affect the total amount of light backscattered from the skin, the flow parameter is usu­ally calculated by multiplying the percentage of light reflected from the moving RBCs by the mean photodiode current, which is a function of the average backscattered light in­tensity.

Instrumentation. The original light source used in LDV was a HeNe laser (25,26). Newer systems use a much smaller and less expensive single-mode semiconductor laser diode in the near-infrared region around 750 to 850 nm as a light source. These wavelengths are near the isosbestic wavelength of oxy and deoxyhemoglobin (i. e., 810 nm) so that changes in blood oxygenation have no effect on the measurement. Some LDV systems are equipped with different light sources (e. g., green, red, or near-infrared), which allow measurement from differ­ent tissue layer depths because light penetration depth is wavelength-dependent. Typical output powers used in LDV range from 1 to 15 mW.

In most LDV systems, the laser output is coupled through a small focusing lens into the polished end of a flexible plastic or silica optical fiber (25 to 1000 ^m core diameter), which illuminates the blood directly in invasive measurements or the surface of the skin in noninvasive applications. Light backscattered from the biological media is collected either by the same optical fiber used for illumination or by a separate receiving fiber mounted in close proximity to the illuminating fiber tip. A rigid probe helps to maintain the two optical fiber tips parallel to each other and also perpendicular to the sur­face of the illuminated sample. Depending on the application, a wide selection of probe geometries and sizes are available commercially. In invasive applications, the optical fibers can also be inserted through a catheter for measurement of flow inside a blood vessel. In most noninvasive applications, the flow probes are attached to the surface of the skin by a dou­ble-sided adhesive ring. Because blood perfusion is strongly dependent on skin temperature, some LDV systems also have probes with built-in heaters to control and monitor skin tem­perature. Absolute calibration of an LDV instrument is inherently difficult to obtain because blood flow in the skin is highly com­plex and variable. Because accurate calibration standards or suitable physical models of blood flow through the skin do not exist, instrument calibration is usually accomplished empiri­cally either from an artificial tissue phantom, which is often made out of a colloidal suspension of latex particles, or by comparing the relative output from the laser Doppler instru­ment with other independent methods for measuring blood flow.

In practice, most commercial systems express and display the Doppler-shifted quantity measured by the instrument ei­ther in terms of blood flow (in units of milliliters per minute per 100 g of tissue), blood volume (in milliliters of blood per 100 g of tissue), or blood velocity (in centimeters per second).

The clinical and medical research applications of LDV range from cutaneous studies of ischemia in the legs (27) to general subcutaneous physiological investigations related to the response of various organs to physical (temperature, pres­sure) and chemical (pharmacological agents) perturbations that can alter local blood perfusion. LDV has been used exten­sively in dermatology to assess cutaneous microvascular dis­ease (28,29), arteriosclerosis, or diabetic microangiopathy; in plastic surgery to determine the postoperative survival of skin grafts; in ophthalmology to evaluate retinal blood flow (30,31); and in evaluating skeletal muscles (32). To date, LDV re­mains mainly an experimental method. Although it has been widely used as a research and clinical tool since the mid 1970s, LDV has not reached the stage of routine clinical ap­plication.

Fluorescence Spectroscopy

Many dyes that absorb energy can reemit some of this energy as fluorescence. Laser-induced fluorescence emission is cur­rently being investigated for the early detection, localization, and imaging of normal and abnormal tissues, determining whether a tumor is malignant or benign and identifying ex­cessive areas of atherosclerotic plaque. One of the future goals is to incorporate this technique into special fiber optic based guidance systems used during ablation or laser angioplasty particularly inside the coronary arteries.

Diffuse Reflectance and Transillumination Spectroscopy

Several methods are being developed to measure the absorp­tion spectra of tissues illuminated by laser light. In a rela­tively new technique known as photon time-of-flight spectros­copy, researchers are trying to measure the temporal spreading of very short pulses of laser light as photons un­dergo multiple scattering in the tissue. By measuring the time it takes the light to travel through the tissue it is possi­ble to estimate how much light scattering and absorption oc­curs. Some of these time-resolved or ‘‘photon migration’’ meth­ods are being evaluated clinically as a potential alternative to ionizing radiation used in X-ray mammography for early noninvasive diagnosis of breast cancer.

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