In vivo measurement of blood velocity in human major retinal vessels using the laser speckle method.

PURPOSE: To develop a simple, noninvasive method of determining blood velocity and flow through human retinal vessels (RVs), by using the laser speckle method and validating the results by bidirectional laser Doppler velocimetry (LDV).
METHOD: The square blur rate (SBR), a quantitative blurring index of the laser speckle pattern that parallels the velocity of moving substances, obtained from blood flowing through glass capillary tubes (RV analogues), correlated with tube diameter, background reflectance and absorption, flow velocity, and the SBR obtained from blood flowing through underlying glass capillary tubes (choroidal vessel analogues). A nomogram was constructed to calculate the blood velocity in human RVs from the SBR values obtained in vivo. Blood velocities in RVs were determined in 12 normal eyes by using the laser speckle method and bidirectional LDV. Measurements were performed twice at the same site at 1-hour intervals.
RESULTS: Measurements from a temporal superior artery (n = 12; mean ± SD) were blood velocity (V(mean)), 41.7 ± 4.2 mm/s; flow, 13.0 ± 3.2 μL/min; and diameter, 119.5 ±15.7 μm and time to complete one measurement, 65 ± 18 seconds, with the laser speckle apparatus; and V(mean), 37.7 ± 6.7 mm/s; flow, 11.7 ±3.0 μL/min; diameter, 111.1 ±16.6 μm; and measurement time, 112 ± 25 seconds, with the bidirectional LDV apparatus. The results obtained by the two methods correlated with each other (V(mean), r = 0.59, P = 0.023; flow, r = 0.83, P = 0.005; and diameter, r = 0.56, P = 0.032). The coefficients of reproducibility for V(mean), blood flow, and diameter measurement were 9.5% ± 2.5%, 10.5% ± 3.2%, and 5.3% ± 2.7% for the former and 15.3% ± 4.2%, 18.5% ± 4.1%, and 6.2% ± 2.2% for the latter, respectively.
CONCLUSIONS: The laser speckle method accurately and reproducibly determines blood velocity in human RVs in less time than the LDV apparatus requires.