A custom-built dynamic-focus swept-source optical coherence tomography (SS-OCT) system with a central wavelength of 1310 nm was used to image the anterior eye from the cornea to the lens. An electrically tunable lens was utilized to dynamically control the positions of focusing planes over the imaging range of 10 mm. The B-scan images were acquired consecutively at the same position but with different focus settings. The B-scan images were then registered and averaged after filtering the out-of-focus regions using a Gaussian window. By fusing images obtained at different depth focus locations, high-resolution and high signal-strength images were obtained over the entire imaging depth. In vivo imaging of human anterior segment was demonstrated. The performance of the system was compared with two commercial OCT systems. The human eye ciliary body was better visualized with the dynamic-focusing SS-OCT system than using the commercial 840 and 1310 nm OCT systems. The sulcus-to-sulcus distance was measured, and the result agreed with that acquired with ultrasound biomicroscopy.
The effectiveness of speckle reduction using traditional frame averaging technique was limited in ultrahigh speed optical
coherence tomography (OCT). As the motion between repeated frames was very small, the speckle pattern of the frames
might be identical. This problem could be solved by averaging frames acquired at slightly different locations. The
optimized scan range depended on the spot size of the laser beam, the smoothness of the boundary, and the homogeneity
of the tissue. In this study we presented a method to average frames obtained within a narrow range along the slow-axis.
A swept-source OCT with 100,000 Hz axial scan rate was used to scan the retina in vivo. A series of narrow raster scans
(0-50 micron along the slow axis) were evaluated. Each scan contained 20 image frames evenly distributed in the scan
range. The imaging frame rate was 417 HZ. Only frames with high correlation after rigid registration were used in
averaging. The result showed that the contrast-to-noise ratio (CNR) increased with the scan range. But the best edge
reservation was obtained with 15 micron scan range. Thus, for ultrahigh speed OCT systems, averaging frames from a
narrow band along the slow-axis could achieve better speckle reduction than traditional frame averaging techniques.
The detection of early-stage keratoconus is one of the most important safety issues in screening candidates for corneal
refractive surgeries. We propose to use epithelial thickness maps to assist the diagnosis of keratoconus. The corneal
epithelial thickness in normal and keratoconic eyes was mapped with optical coherence tomography (OCT). A Fourier-domain
OCT system capable of acquiring 26,000 axial-scans per second was used. It has an axial resolution of 5μm in
cornea. A pachymetry scan pattern (8 radials, 1024 axial-scans each, 6mm diameter, repeat 3 times) centered at the pupil
center was used to image the cornea. The 3 repeated radial scans on each meridian were registered and averaged. Then
the anterior corneal, posterior corneal and epithelial boundaries were segmented automatically with a computer
algorithm by increased signal intensity at corresponding boundaries. The epithelial thickness map was generated by
interpolating epithelial thickness profile calculated from each meridian. Normal and keratoconic eyes (24 eyes each)
were scanned 3 times. The central epithelial thickness in normal eyes was thicker than those of keratoconic eyes (mean
difference 2.1 μm, t-test p=0.05). The epithelium was thinner superiorly than inferiorly in normal eyes (mean difference
-1.4±1.1μm, p<0.001) while thicker superiorly than inferiorly in keratoconic eyes (2.0±4.1 μm, p=0.02).
Optical coherence tomography (OCT) provides a non-contact and non-invasive means to visualize the corneal anatomy at micron scale resolution. We obtained corneal images from an arc-scanning (converging) OCT system operating at a wavelength of 830nm and a fan-shaped-scanning high-speed OCT system with an operating wavelength of 1310nm. Different scan protocols (arc/fan) and data acquisition rates, as well as wavelength dependent bio-tissue backscatter contrast and optical absorption, make the images acquired using the two systems different. We developed image-processing algorithms to automatically detect the air-tear interface, epithelium-Bowman's layer interface, laser in-situ keratomileusis (LASIK) flap interface, and the cornea-aqueous interface in both kinds of images. The overall segmentation scheme for 830nm and 1310nm OCT images was similar, although different strategies were adopted for specific processing approaches. Ultrasound pachymetry measurements of the corneal thickness and Placido-ring based corneal topography measurements of the corneal curvature were made on the same day as the OCT examination. Anterior/posterior corneal surface curvature measurement with OCT was also investigated. Results showed that automated segmentation of OCT images could evaluate anatomic outcome of LASIK surgery.