Imaging the thick retina: AO-OCT to AO beacon analysis

Donald T. Miller, PhD
Associate Professor
Indiana University, School of Optometry, Bloomington

The paper-thin retina lining the inside of the human eye covers more
than 1,000 mm2. This vast expanse of tissue is readily observed when
viewed through the eye's optics with conventional instruments, yet
little is revealed of the many layers of microstructures that define
its thickness. Recent technological advances such as in adaptive
optics (AO) and optical coherence tomography (OCT) have opened a new
window of opportunity to explore these microstructures in all three
dimensions. Here, I will describe our efforts to develop such an
instrument that combines the strengths of AO (high lateral resolution)
with the strengths of OCT (high axial resolution and sensitivity). For
additional improvement, we also use a customized achromatizing lens to
remove the intrinsic chromatic aberrations of the eye and in doing so
provide near-perfect optics through which the OCT can image the
retina. Our ultrahigh resolution (UHR) AO-OCT instrument has recently
achieved an isotropic 3D resolution approaching 3x3x3 #m3 in retinal
tissue. In addition to the roughly 3X improvement in lateral
resolution over non-AO OCT, AO increases OCT sensitivity (~7 dB) and
reduces the lateral size of speckle noise (3X), an unwanted byproduct
of the interferometric nature of OCT. Paradoxically, the current
implementation of AO to the eye hinges on the assumption that the
retina is thin, i.e., the AO beacon reflects from a single retinal
layer. This assumption fails to account for errors potentially
introduced by reflections from many depths in the thick retina and
underlying choroid. To predict this error, I will present early
results derived from an optical model of the retinal reflection in
conjunction with the physical principles that govern light propagation
through the eye and wavefront sensor.