Principle, Validity, and Reliability of Scanning Laser Doppler Flowmetry

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Abstract
Purpose The objective of this study is to present the reliability and validity of scanning laser Doppler flowmetry (SLDF) performing a high-definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow. Methods The examination of blood flow by SLDF is based on the optical Doppler effect. The data aquisition and evaluation system is a modified laser scanning device; the wavelength of the laser source is 670 nm, with a power of 100 μW (Heidelberg Engineering, HRF). The reliability of SLDF was estimated by performing five seperate measurements in 10 eyes on 5 days. The validity of the method was tested by two experiments. First, in an experimental set-up, the capability of SLDF to measure the velocity of a moving plane in absolute units was estimated. Second, comparative measurements were performed of retinal blood flow in 16 normal eyes and in 33 glaucomatous eyes with SLDF and a commercially available single-point laser Doppler flowmeter (Oculix). Results We found SLDF to produce a high reliability. The reliability coefficients rx of flow, volume, and velocity were 0.82, 0.81, and 0.83, respectively. Comparative measurements of the retinal blood flow by SLDF and a single-point laser Doppler flowmeter of corresponding retinal points showed a linear and significant relationship between flow (r = 0.83, p < 0.0001), volume (r = 0.51, p < 0.0001), and velocity (r = 0.59, p < 0.0001). In the experimental set-up, SLDF was able to quantitatively measure velocity in absolute units. Conclusions SLDF enables the visualization of perfused vessels of the jux-tapapillary retina and the optic nerve head in high resolution by two-dimensional mapping of the optical Doppler shift and a reproducible evaluation of capillary blood flow.

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