Two-fiber laser Doppler velocimetry in blood: Monte Carlo simulation in three dimensions

Laser Doppler velocimetry in blood when fiber-optic catheters are used is limited by the effects of multiple scattering and flow disturbance produced by the catheter. To minimize the effects of flow disturbances, the system should be designed to project the velocity-sensing region as far as possible from the surface of the catheter. Because of multiple scattering, determining the region of sensitivity requires the use of transport theory. We present numerical results of Monte Carlo simulations of the spatial Doppler sensitivity field of a two-fiber velocimeter. The simulations show that a peak of sensitivity can be projected approximately 0.5 mm from the fiber tips (at 685 nm); the use of longer wavelengths would increase this distance somewhat. The generation of such a peak requires overlapping the partially spatially coherent propagation regions of the two fibers, which in turn depends strongly on the relative orientation of the fibers, even though much of the scattered light is diffuse.