Confocal scanning laser Doppler flowmetry: Experiments in a model flow system

Purpose: We conducted this study toward validating confocal scanning laser Doppler flowmetry (SLDF), a new noninvasive technique for measuring retinal and optic nerve head hemodynamics. Methods: We designed a model flow system using a glass capillary coupled to a microlitre syringe driven by an infusion pump. Eleven capillaries with parallel walls (internal diameters ranging 705 to 25 μm) were used. The capillaries were perfused with skim milk over a range of pump flow rates. At each flow rate, measurements were made with the Heidelberg Retina Flowmeter (HRF) to study the relationship between HRF-measured flow and actual flow. The initial experiments (n = 2) were conducted to establish the approximate velocity operating range of the HRF with single HRF measurements across a wide range of flow rates, whereas the subsequent experiments (n = 9) were concentrated within this operating range with five HRF measurements at each flow rate. Results: When pump flow rates were converted to actual velocity at the measurement point for the initial experiments, the velocity operating range of the HRF was ~0.08 to 1.0 mm/s. For velocities of > 1 mm/s, HRF measured velocity was not linearly related to actual velocity. Within the operating range, there was a highly significant linear relationship between HRF-measured flow and actual flow (0.935 ≤ r ≤ 0.990, p < 0.001). When the curves of HRF-measured velocity versus actual velocity for the different experiments were plotted, they largely superimposed. The variability of the HRF measurements was between 3.57% and 4.05% and was independent of flow rate. Conclusions: SLDF measures reliably and linearly within a given operating range.