IU School of Optometry: Dr. Don Miller's Lab

Introduction to High-Resolution Retinal Imaging
Donald T. Miller, PhD Laboratory
Associate Professor, School of Optometry

The human retina is extremely thin and delicate, much like a piece of wet tissue paper. Yet its 1/100th of an inch thickness supports a microcosm of diverse cells organized in discrete layers, each playing a critical role in how we see. Human vision starts when photoreceptors at the back of the retina collect and respond to light. Normal photoreceptor function as well as overall retinal function is essential for normal vision, yet techniques to assess these processes in vivo are limited. Current optical and electrophysiological techniques have limited spatial resolution and sensitivity and target only specific functional processes. New optical modalities that are rapid, specific, and non-invasive hold the promise of greatly expanding our capability to monitor more accurately and completely retinal function and health.

Observing retinal cells through the natural pupil of the eye with a camera, however, is challenging because the inherent low contrast of most cells in the retina, the superimposed reflections from cells at different depths in the thick retina, and optical defects in the cornea and crystalline lens that blur the retinal image. At Indiana University we have developed two complimentary cameras that are based on adaptive optics (AO) and spectral-domain optical coherence tomography (SD-OCT) to achieve unprecedented 3D resolution (3x3x5 microns³), sensitivity, and speed. AO is used to remove the blur and SD-OCT to optically section micron-scale slices from the thick retina.

The need for very high 3D spatial resolution is illustrated by Figure 1 below that shows a series of point spreads for various combinations of AO and major camera architectures as well as two commercial instruments (without AO). A scaled histological cross section of the human retina is shown on the left for comparison. The AO-OCT point spread is exceedingly small in size (3x3x5 microns³) and is significantly smaller than those of the other camera architectures. More importantly, the AO-OCT point spread is at least as small as the cell nuclei shown in the retina cross-section suggesting that these cells can be resolved in all three dimensions.

The long-term goal of our research is to establish high resolution, high specificity optical techniques as valid tools for probing physiologic processes of the retina at the cellular scale. The resulting ability to study individual retinal cells, in vivo, promises improvements in early detection and treatment monitoring for diseases of the retina.

3D Resolution of Current Retina Cameras

Figure 1. Resolution of current AO and non-AO ophthalmic imaging devices relative to the human retina. The left shows a histological cross section of human retina at 4.17 degrees eccentricity and a 100-micron scale bar. (from Boycott and Dowling). To the right are point spread functions that are drawn to scale for various combinations of AO and ophthalmoscope architectures (conventional flood illumination, cSLO, and OCT). For simplicity, the PSFs are displayed as 2D projections with their width and height representing the ophthalmoscope's lateral and axial resolutions, respectively. Note that the displayed PSF for the AO flood-illuminated ophthalmoscope represents an effective PSF rather than the true PSF, since out-of-focus light reduces image contrast, not resolution. Two commercial ophthalmoscopes are also shown.

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	URL: http://www.opt.indiana.edu/people/faculty/miller/retimage.htm Revised: March 16, 2007
	IU Optometry home page: http://www.opt.indiana.edu/ Comments: Web Administrator