Velocity profile of red blood cells in human retinal vessels using confocal scanning laser Doppler velocimetry

A new method to measure red blood cell velocity profiles in retinal vessels is presented. It is based on the well-established laser Doppler velocimetry with lasers of long coherence length. A confocal arrangement is used for the delivery of a narrow probing beam (diameter at the fundus of about 1/10 the vessel diameter) and the detection of the light scattered by the red blood cells (RBCs). The velocity profile of the RBCs at arterial and venous sites of the major retinal vessels is derived from scans of the probing laser across these vessels. In vitro experiments demonstrate the performance of the instrument. Measurements of arterial velocity profiles obtained at both the systolic and diastolic phases of the cardiac cycle, as well as nonsymmetric profiles determined at different distances downstream from a venous junction, are reported for the human organism for the first time. The wall shear rates calculated from these profiles are in good agreement with data derived from experiments in animals.