Adaptive Optics for Vision Science

Chapter 17 - Indiana University AO-OCT System

17.1   INTRODUCTION

The Indiana adaptive optics (AO) retinal camera was originally developed to
perform en face time-domain optical coherence tomography (OCT) using a
novel flood illumination approach and a two-dimensional charge-coupled
device (CCD) [1]. Its core AO design evolved out of the original Rochester
AO conventional flood illumination camera by Liang et al. [2]. With the discovery
of the significant speed and sensitivity advantages of spectral-domain
OCT (SD-OCT)
over time-domain approaches, the camera was redesigned
for parallel SD-OCT using a novel line illumination scheme [3]. The three-
dimensional (3D) optical resolution of the resulting AO SD-OCT camera
(3.0 μm × 3.0 μm × 5.7 μm in the horizontal, vertical, and axial directions,
respectively) is the highest to date in the living human eye and was sufficient
to observe the interface between the inner and outer segments of individual
photoreceptor cells, resolved in both the lateral and axial dimensions. This
subcellular structure in all three dimensions has not been previously reported
due to the poor axial resolution of conventional AO cameras, such as flood
illumination and scanning laser ophthalmoscope (SLO) systems, and the
poor lateral resolution of OCT systems. A current limitation of the AO
SD-OCT camera is speckle noise that results from the coherent nature of
OCT detection. This noise makes it difficult to correlate retinal reflections
with cellular features, though the application of speckle reduction techniques
and the implementation of real-time 3D imaging in the future will certainly
lead to improved performance.

Parallel to the AO-OCT development, we discovered an elegant approach
for rapidly flood illuminating the retina with incoherent light. The high
imaging rates (60 Hz continuous: 500 Hz short burst) coupled with the high
transverse resolution afforded by the AO system permitted quick navigation
through the retina and imaging of individual cells that were of relatively high
contrast, such as photoreceptors and moving blood cells, without image warp
and motion blur. These high speeds are two to three orders of magnitude
faster than other AO flood-illuminated retinal cameras and can be more than
an order faster than AO scanning laser ophthalmoscopes.

The aim of this chapter is to provide a detailed overview of the Indiana
camera’s technical features, its optical performance, and finally, example
results. General background information on OCT imaging systems can be
found in Chapter 10.

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