Relative blood velocity measurement in individual microvessels using the self-mixing effect in a fiber-coupled helium-neon laser

A system has been developed for the measurement of relative blood velocity in microvessels by using the self-mixing effect of a laser. A helium-neon laser was coupled to a single-mode optical fiber and the pulled fiber tip (∼30 μm diameter) was positioned on a single microvessel. The backscattered Doppler-shifted laser light from moving red blood cells enters the laser cavity and modulates the laser output by influencing internal laser parameters. The signal of the laser output intensity change was produced using a fiber-coupled photodiode and processed by a signal processor. This processor yields an output signal proportional to the first moment of the power spectral density, i.e., the mean frequency of the Doppler shift, corresponding to the blood flow velocity on an arbitrary instrument scale. Results of the in vitro experiment demonstrated that the current method can detect moving particles in fluid and moving red blood cells in a small plastic tube. Data from the in vivo study showed that this system is capable of measuring relative blood velocity in arterioles and venules and can easily follow the cardiac cycle up to 360 beats/min. Primary data suggest that, in addition to high sensitivity, good spatial and temporal resolution, and convenience of use, the self-mixing technique may have an even greater capacity for analysis of blood flow in microvessels than explored in this study, since information on the absolute velocity and velocity distribution of red blood cells is available in self-mixing signal. Further study on its hematocrit dependence and particle bias effect is needed.