C2. Clinical & Scientific Applications
The four instruments that are now operational are addressing a diverse set of questions about both normal and diseased retina described below:
C2.1. AOSLO Imaging of the Vasculature and Vascular Disease
We have made considerable progress toward the original aim of measuring hemodynamics. AOSLO can directly measure static and dynamic properties of the retinal microvasculature in living eyes (Martin 2005) (Fig C8). The main advantage AOSLO offers is that it is non-invasive (no fluorescent dyes) which allows for long-term measurements in a single session as well as frequent repeated measures if necessary. Such measurements in the human retina have no precedent, as current non-invasive measures are limited to larger vessels (Grunwald 2005, Sullivan 1999, Yazdanfar 2003). The only non-invasive methods to measure flow and properties of the smallest capillaries in the retina have relied on subjective, entoptic phenomena (Applegate 1990, Riva 1980). The goal of the UCB-AR study was to characterize the dynamic and structural properties of the parafoveal capillaries in healthy human eyes. The full analysis is still in progress but the following summarizes the velocity, pulsatility, size of the foveal avascular zone (FAZ) and the capillary density near the fovea to date.
Fig. C8: AOSLO image of the macular region. Capillaries are labeled with flow direction and leukocyte velocity in mm/sec. Scale bar is 1 deg.
Leukocyte Velocity: The velocity of single leukocytes coursing through the smallest capillaries was measured directly from the videos. 6 subjects had a mean parafoveal leukocyte velocity of 1.37 mm/second, ranging from 0.77 to 2.10 mm/second (Martin 2005).
Leukocyte Pulsatility: We determined the extent to which flow in the smallest retinal capillaries was pulsatile. Extended imaging times with the AOSLO allowed measurement of many leukocytes in a single video. At the same time the pulse was measured and its timing was indicated directly on the AOSLO video. Velocities were computed as a function of their timing within the pulse cycle. For eight subjects, the mean leukocyte velocity ranged from 0.631–1.94 mm/sec, with an overall mean of 1.29 mm/sec. The pulsatility, (Vmax-Vmin)/Vmean, ranged from 0.308-0.654 (mean of 0.457). Given that pulsatility is observed in the smallest capillaries, it will be important to control for this in future studies of the hemodynamics in retinal disease, such as diabetes.
Parafoveal Capillaries and the Foveal Avascular Zone: Along with dynamic measures, AOSLO was used to measure the foveal avascular zone (FAZ) and density of the surrounding capillaries. In 20 subjects we found a mean FAZ diameter: 0.5562 mm (range 0.2624 - 1.0519 mm); mean FAZ area: 0.2885 mm2 (range: 0.0775 - 0.5379 mm2). There was a small but significant tendency for the FAZ to be slightly elongated (8%) in the horizontal direction.
C2.2. In vivo Fluorescence Imaging of the Radial Peripapillary Vessels in Macaque Retina
Fig. C9 A) the retinal vasculature near the disc with deep focus; B) the radial peripapillary vasculature with superficial focus at the same location.
The peripapillary vasculature provides oxygenation and nutrients to the thickest portion of the primate and human nerve fiber layer. Imaging of these vessels is of great clinical importance because a prominent theory of the etiology of glaucoma (Chung 1999), implicates these vessels in early changes, but it has not been possible to image this vasculature, possibly because the peripapillary vessels are masked by the brighter underlying retinal vessels. UR has imaged the peripapillary vasculature in macaque monkeys using fluorescein angiography by taking advantage of the confocal nature of the AOSLO (Fig. C9). As shown in C8 above, AOSLO imaging can also permit imaging of the retinal microvasculature without the use of angiographic agents, encouraging the possibility of imaging the radial peripapillary capillaries without fluorescein, a goal of a collaborative project with Steve Russell (University of Iowa) in the next funding period.
→ C3. Adaptive Optics In Vivo Cellular Imaging← C1. Instrument Development