Optical Doppler velocimetry at various retinal vessel depths by variation of the source coherence length

We report on what we believe is a novel approach to measuring the velocity of red blood cells (RBC's) at different depths of retinal vessels by use of low-coherence sources. The technique, variable coherence optical Doppler velocimetry (VCODV), performs Doppler shift measurements through autodyne mixing between the light scattered by the RBC's and by the vessel front wall (reference). Only the light from RBC's moving at a depth less than half the coherence length (CL) mixes efficiently with the reference. Measurements of the Doppler shifts from RBC's with sources of four different CL's in a 152-microm vein of a volunteer confirmed the feasibility of VCODV. This approach has the potential to monitor in vivo retinal RBC velocity gradient at the vessel wall and the velocity profile within the blood vessel in the condition of symmetric blood flow profiles.