Imaging the foveal cone mosaic with a MEMS-based adaptive optics scanning laser ophthalmoscope
- Author(s): Li, Yiang
- Advisor(s): Roorda, Austin J
- et al.
Our knowledge of the structure of the human photoreceptor mosaic is mostly based on histological data. Imaging microscopic structure in intact eyes has traditionally been difficult due to structural imperfections in the eye's optics called aberrations. The introduction of adaptive optics (AO) into vision science has allowed us to access the living human retina at microscopic levels, opening up new possibilities for both basic and clinical research. This dissertation concerns the advancement of AO technology for retinal imaging while emphasizing its application to imaging the foveal cone photoreceptor mosaic in living human eyes. Foveal cones provide a fundamental challenge for today's AO systems due to their small size (2 µm diameter). As a result, much of my effort has been put towards improving AO system performance to resolve these small cells consistently. I have improved the wavefront correction capabilities of an adaptive optics scanning laser ophthalmoscope (AOSLO) using a single MEMS deformable mirror, so that the smallest foveal cones in some eyes can now be resolved. Specifically, many of the nonlinear characteristics of the particular MEMS device used have been negated in the new wavefront controller, and the wavefront reconstructor has been optimized by incorporating measurement noise and aberration (Kolmogorov) statistics. This contribution is significant because, prior to this research, the capability to image the entire foveal cone mosaic in vivo had never been demonstrated using this imaging modality.
Some basic scientific investigations were carried in parallel with the technical developments. Specifically, I used this MEMS-based AOSLO to investigate how foveal fixation is related to the cone density distribution and to determine the inter-subject variability of foveal cone density in relation to eye length. The foveae of 18 healthy eyes (18 subjects) with axial lengths from 22.86 mm to 28.31 mm were imaged and analyzed. The entire foveal cone mosaic was resolved in four eyes, but cones within 0.03 mm (≈ 0.1°) from the foveal center remained unresolved in most eyes. The preferred retinal locus of fixation deviated significantly (P < 0.001) from the location of peak cone density for all but one eye. Retinal cone density decreased significantly (P < 0.05) with increasing axial length 0.30 mm away from the foveal center but not closer, so we can conclude that the axial myopia progression causes retinal stretch. However, how axial length affects cone density within the central fovea, or foveola, is swamped by other factors besides just cone density due to high levels of inter-subject variability observed there.