Measurement of blood flow velocity in retinal vessels utilizing laser speckle phenomenon

The laser speckle phenomenon was applied to the measurement of the blood flow velocity in glass capillary tubes and human retinal vessels. The instrument consists of a fundus camera, a He-Ne laser, a photomultiplier, a photon-counting unit, a digital correlator, and a microcomputer system. The scattered laser beam from the retina was detected by the photomultiplier and these signals were processed by the photon-counting unit. The digital correlator and the microcomputer calculated the autocorrelation function of these processed signals. The power of the laser beam was 18 mW/cm2, which was sufficiently below the minimum level considered hazardous to the retina. This technique did not require fine optical alignment and the measurement could be done in 1.05 seconds. A linear relationship between the blood flow velocities in glass capillary tubes and the reciprocal of the correlation time was proved, and the pulsatile flow of a human retinal artery was demonstrated. The reproducibility of the method was 12.4 +/- 6.5% in arteries (Mean +/- SD, n = 4) and 12.4 +/- 5.1% in veins (Mean +/- SD, n = 16). The mean blood flow rate in retinal vessels obtained in this experiment agreed with the values of previous studies. This technique utilizing the laser speckle phenomenon is useful in clinical application to measure the retinal blood flow velocity.