Analysis of contrast and motion signals generated by human blood constituents in capillary flow

The flow of individual corpuscles through retinal capillaries may now be observed noninvasively by using adaptive optics (AO). To explore their imaging properties, we imaged retinal capillary flow in two healthy subjects at 593 nm with a flood-based AO ophthalmoscope, at a variety of retinal locations and levels of defocus. The image intensity of red cells and plasma depends upon capillary depth relative to focus: red cells appear brighter than background, and plasma darker, for capillaries posterior to focus. The reverse is true for capillaries anterior to focus. Contrast reversals were obtained over 0.05 D (∼14 μm), which are well within the typical undulations in depth of retinal capillaries. We relate these observations to phase-contrast defocusing microscopy. This defocusing effect confounds flow measurements, which rely on correlation of image intensity between successive locations along the same capillary, a requirement made further difficult by high physiological variability in flow. Peak correlation was maintained >0.25 over a distance of 22±15 μm (roughly the spacing between red cells) and over a duration of 154±49 ms (roughly eight times the temporal period between red cells). We provide a 2D correlogram approach that significantly improves robustness in the face of optical and physiological variability, compared to the traditional spatiotemporal plot, without requiring additional data.