 |
|
|
Description of the Adaptive Optics Spectral-Domain OCT Camera
Figure 1 below shows schematic and photographs of the Indiana AO SD-OCT camera. The camera consists of three sub-systems:
A bite bar stage, identical to that described for the flood illumination camera, is used to stabilize the head. The combination of AO and SD-OCT provides a 3D resolution of 3x3x4.8 µm3 (width x length x depth) in the retina.
SD-OCT Imaging: SD-OCT is based on a fiber-based Michelson interferometer. Its illumination channel contains a 10 mW broadband SLD (λMEAN = 842 nm, Δλ = 50 nm). The sample channel contains the AO system and XY scanners. The reference channel includes a water vial that partially compensates for chromatic dispersion induced by the eye. The overall path length of sample and reference channels is 4.7 m. The detection channel contains a transmission grating and a line-scan CCD detector (12 bit, 512 pixels, Atmel) that operates at 75,000 A-scans/s.
AO Wavefront Correction: The centerpiece of the AO system is a two-mirror configuration consisting of a 37 actuator AOptix bimorph and a 144 actuator Boston deformable mirror (not shown in figure). The Shack-Hartmann wavefront sensor consists of a 20 x 20 lenslet array and a Dalsa 1M60 CCD camera. The AO system can perform up to 30 wavefront measurements and corrections per second, although the typical rate during experiments is 20 Hz. Measurement and correction is across a 6.6 mm pupil.
   |
| Figure 1. (top) Detailed layout of the fiber-based AO SD-OCT retina camera. The camera consists of three channels: (1) sample channel, (2) reference channel, and (3) detection channel. The AO system is integrated into the sample channel. BS, DM, and P refer to the fiber beam splitter, AOptix deformable mirror, and planes that are conjugate to the pupil of the eye, respectively. (middle and bottom) Photographs of the AO SD-OCT camera. |
AO SD-OCT Results
Figure 2 below shows video B-scans acquired with the Indiana AO SD-OCT camera on one subject. Adjustment of focus was realized by generating +0.5 diopters of sphere on the AOptix deformable mirror, which also compensated for the higher-order wave aberrations. In the left video, note the two rows of punctuated bright spots at the photoreceptor layer that correspond to individual cone cells. These spots are not evident when focus is shifted to the nerve fiber layer (right), which itself becomes brighter. Figure 3 shows video B-scans at two different retinal eccentricities, revealing bright spots (cones) that are larger and further spaced at the larger eccentricity. A 7 to 8.2 dB increase in SNR of the photoreceptor layer is observed with AO correction.
 |
|
Figure 2. AO SD-OCT B-scans of the same patch of retina with focus at the photoreceptors (left) and at the nerve fibers (right). Camera sensitivity is sufficient to detect reflections from all major retinal layers. B-scans were acquired at a rate of 25 per second during dynamic correction. Retinal location is at 2 degrees eccentricity (nasal).
Download Figure 2 (left) video: |
 |
|
Figure 3. AO SD-OCT B-scans at 2 degrees (left) and 7 degrees (right) retinal eccentricity (nasal). Focus is at the photoreceptors. B-scans were acquired at a rate of 25 per second during dynamic correction.
Download Figure 3 video: |
The AO SD-OCT camera also acquired small volume images at high speed. The short acquisition time per volume minimized retina motion artifacts. Figure 4 shows a representative volume image and C-scans extracted from different depths in the volume. The volume image is 38 x 150 x 1100 µm3 (width x length x depth) and was acquired in <80 ms with a sampling density of 1 x 1 x 2.2 µm3. C-scans outside the depth of focus (which coarsely straddles the photoreceptor layer) are dominated by high spatial frequency irregular structure that is suggestive of speckle. In-focus C-scans (at IS/OS junction and posterior tip of the OS) reveal a regular array of bright spots. While one might expect a similar regular pattern to occur at the ELM as the inner segments of the photoreceptors terminate there, none was discerned.
 |
|
Figure 4. C-scans (right) extracted from several depths in AO SD-OCT volume images (left) that were acquired at 2 degrees eccentricity on two subjects, (a) and (b). C-scans correspond to the outer plexiform layer (OPL), external limiting membrane (ELM), IS/OS junction, OS posterior tip, and RPE. Focus is approximately at the plane of the photoreceptors. C-scans are displayed using a linear intensity scale and are 38 µm x 150µm. Volume images were acquired in <80 ms.
Download Figure 4 (left) video: |
Volume imaging with AO SD-OCT provides quantitative information about the photoreceptors. Qualitatively, the pattern of bright spots (Figure 4 above) is similar to that of cone photoreceptors imaged with the AO flood illumination camera. This similarity was tested by comparing spot spacing (in Figure 4) to known cone spacing estimated from histology and psychophysical observations. Figure 5 below shows the results. The strong agreement at 1 degree, 2 degrees, and 3 degrees eccentricity and the consistent trend up through 7 degrees supports the view that the observed bright spot pattern at the OS posterior tip (e.g., that in Figure 4) is indeed reflections from individual cone cells.
 |
Figure 5. Row spacing of bright spots at the posterior tips of the cone outer segments in AO SD-OCT C-scans as a function of retinal eccentricity. The spacing measurements strongly agree with cone estimates from histology (superior and inferior retina) (Curcio, et al., 1990) and from psychophysical observations of aliasing with interference fringes (Williams, 1988). |
|
URL: http://www.opt.indiana.edu/people/faculty/miller/aooct.htm Revised: March 20, 2007 | |
 |
IU Optometry home page: http://www.opt.indiana.edu/ Comments: Web Administrator |
 |
Copyright © 2007 The Trustees of Indiana University | Copyright Complaints |