Real-time blood velocity measurements in human retinal vein using the laser speckle phenomenon.

PURPOSE To measure the in vivo blood velocity in human retinal veins using a laser speckle system. METHODS The system consists of a fundus camera, a diode laser, an image sensor, and a personal computer system. The fundus area, including a target retinal vein, is illuminated with a diode laser through a fundus camera and the laser speckle pattern is imaged onto the area sensor. From the time change of the contrast of the speckle pattern, the normalized blur (NB) value, a quantitative index of blood velocity, was calculated using a logic board. RESULTS In an in vitro experiment, the NB obtained from blood flow in 50-300 microm internal diameter glass capillary tubes, used as an analogue of a retinal vein, correlated with the diameter of the tube, the actual blood flow rate, and the background NB value, which was used as an analogue of choroidal circulation. In the in vivo experiment, the blood velocity in human retinal veins of approximately 50 microm in diameter was estimated in 16 normal human eyes using nomograms based on the result of the in vitro experiment. Velocity averaged 11.1+/-0.6 mm/s (mean +/- SD, n = 16) in retinal veins 53+/-6 microm in diameter. The coefficient of reproducibility of 5-minute interval measurements was 2.5+/-0.9%, and it took 63+/-15 seconds for one measurement. CONCLUSIONS The present methodology is clinically valid for measuring blood velocity in retinal veins.