In vivo velocity distribution of red blood cells in microvessels measured by a laser Doppler microscope

Velocity distributions were measured in arterioles 25-128 m in diameter in the everted, transilluminated cheek pouch of the anesthetized hamster Mesocricetus auratus with a Laser Doppler Anemometry (LDA) microscope system. The LDA system consisted of a He-Ne laser (0.5mW, 632.8 nm), microscope, sending and receiving optics, photomultiplier tube (PMT), power supply and output electronics. The hamster was placed on the microscope stage and an arteriole selected for measurement. The laser beam was split into two parallel beams and focused from below onto a small region in the mid-plane of the arteriole. The wavefronts in the focal region where the two beams intersect, interfere and form alternate regions of high and low light intensity. As an RBC passes through these regions, it causes variations in the intensity of light. The light is reflected to the PMT by a beam splitter mounted above the objective lens. There it is converted into an electrical signal whose frequency is proportional to the rate at which the RBC crosses the interference fringes. The PMT output is amplified, filtered and displayed on an oscilloscope and frequency counter. With the spatial resolution of the LDA system up to 28 local velocity measurements could be made across the 128 m arteriole. The distribution was parabolic with slight blunting about the center.