[Optical Doppler velocimetry of red blood cells at different depths in retinal vessels by varying the coherence length of the source: feasibility study]. / Vélocimétrie par effet Doppler optique des globules rouges à différentes profondeurs dans les vaisseaux rétiniens en variant la longueur de cohérence temporelle de la source: etude de faisabilité.

PURPOSE: To report on a novel approach to measure the velocity of red blood cells (RBCs) at different retinal vessel depths. METHOD: The technique is an extension of conventional laser Doppler velocimetry using light sources of various coherence lengths (CL). Light scattered by the moving RBCs interferes with that reflected from the anterior vessel wall only if the optical path difference is shorter than CL. Therefore, using low coherence light sources, localized measurements of RBCs velocity can be performed. RESULTS: Measurements of RBCs velocity at different depths in a main retinal vein (diameter: 152 microns) of a volunteer has been performed using 4 different light sources with CLs of 14 microns, 21 microns, 32 microns and > m. Measured values are in good agreement with theoretically predicted values. CONCLUSION: This new approach permits to measure RBCs velocity at different depths of retinal vessels in the human retina.

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.