Phase-sensitive imaging of the outer retina using optical coherence tomography and adaptive optics

Abstract

The cone photoreceptor’s outer segment (OS) experiences changes in optical path length, both in response to visible stimuli and as a matter of its daily course of renewal and shedding. These changes are of interest, to quantify function in healthy cells and assess dysfunction in diseased ones. While optical coherence tomography (OCT), combined with adaptive optics (AO), has permitted unprecedented three-dimensional resolution in the living retina, it has not generally been able to measure these OS dynamics, whose scale is smaller than OCT’s axial resolution of a few microns. A possible solution is to take advantage of the phase information encoded in the OCT signal. Phase-sensitive implementations of spectral-domain optical coherence tomography (SD-OCT) have been demonstrated, capable of resolving sample axial displacements much smaller than the imaging wavelength, but these have been limited to ex vivo samples. In this paper we present a novel technique for retrieving phase information from OCT volumes of the outer retina. The key component of our technique is quantification of phase differences within the retina. We provide a quantitative analysis of such phase information and show that–when combined with appropriate methods for filtering and unwrapping–it can improve the sensitivity to OS length change by more than an order of magnitude, down to 45 nm, slightly thicker than a single OS disc. We further show that phase sensitivity drops off with retinal eccentricity, and that the best location for phase imaging is close to the fovea. We apply the technique to the measurement of sub-resolution changes in the OS over matters of hours. Using custom software for registration and tracking, these microscopic changes are monitored in hundreds of cones over time. In two subjects, the OS was found to have average elongation rates of 150 nm/hr, values which agree with our previous findings.

Review of adaptive optics OCT (AO-OCT): principles and applications for retinal imaging [Invited]

“…Currently most AO-OCT systems are based on the Basler camera, which allows for A-scan rates between 100 and 250 kHz [98][99][100][101]. Using four independent spectrometers (and 4 separate cameras) in combination with an optical switch, the duty cycle of the system can be optimized to 100% and A-scan rates of 1MHz have been realized [82].…”

Understanding the changes of cone reflectance in adaptive optics flood illumination retinal images over three years

“…The use of AO Optical Coherence Tomography (OCT) has also provided information on the dependence of the reflectance on depth in the inner cone interfaces, leading for the first time to a 3D characterisation of the cone reflectance [14]. Phase-sensitive implementations of spectral-domain OCT were also developed, leading to a further improvement in OCT axial resolution [25].…”

Adaptive optics optical coherence tomography with dynamic retinal tracking

“…At the core of the temporal analysis was a registration algorithm based on strip-wise cross correlation, an earlier version of which was described by Jonnal et al (2012) [9]. En face images were divided into thin strips oriented parallel to the fast scan direction and consisted of about 7 contiguous fast B-scans, the width being approximately the cone row spacing at the retinal location imaged.…”

“…Spectral-domain optical coherence tomography with adaptive optics (AO-OCT) is a highly sensitive in vivo method for three dimensional imaging of the living microscopic retina [1][2][3][4][5][6][7][8][9][10][11][12]. OCT provides ultrahigh axial resolution using broadband light sources and exquisite sensitivity for detection of faint reflections from essentially any layer in the retina.…”

“…Registration and dewarping algorithms can help further decrease the effects of motion artifacts in the AO-OCT volume videos, but the motion must remain small enough so that the structure of interest stays in the field of view for the entire acquisition [9,18]. An alternative that has gained interest is active image stabilization in which eye motion is measured and corrected in real time.…”