A custom-developed ultrahigh resolution optical coherence tomography with an axial resolution of 1.1 μm in corneal tissue was used to characterize thickness and light scatter of the epithelium and Bowman's layer in keratoconic (KC) cornea noninvasively. A 4-mm wide vertical corneal section around the apex in nine KC and eight normal eyes was imaged in vivo. The epithelium and Bowman's layer were visualized and their thickness profiles were quantified. Scatter was quantified based on the sensitivity normalized mean signal intensity distribution. Average mean thickness of the epithelium and Bowman's layer in KC eyes was significantly smaller (p<0.05) than the normal eyes. The epithelium thickness variation across a central 3-mm cornea was significantly larger in KC eyes than in normal eyes. The scatter in KC eyes was significantly increased only for Bowman's layer. The changes observed in this study could improve our understanding of the underlying disease mechanism of KC and can provide new indications for early disease diagnosis.
A novel optical scanning method for an anterior segment optical coherence tomography (AS-OCT) system has been
described. This method has been designed for imaging the entire anterior segment of the eye (from cornea to posterior
surface of the crystalline lens) at a time. The ability to image the entire anterior segment is crucial in understanding the
mechanism of human accommodation and the efficacy of accommodative intraocular lenses.
In a conventional scanning system the beam is shined straight into the eye parallel to the optical axis. For anterior
segment imaging, large lateral scan area leads to an increase in the angle of incidence on each of the four ocular surfaces.
This causes significant reduction in signal reflected from regions further from the optical axis. This reduction combined
with loss in signal due to coherence makes it difficult to image the entire anterior segment of the eye, where optical
depth penetration of 10mm is required.
To overcome this limitation, we have designed a new OCT scanning system, which achieves close to normal incidences
across all the lateral locations on the ocular surfaces within a 6 mm clear aperture. This provides an increase in the
amount of light scattered back to the system resulting in higher signal-to-noise ratio (SNR). The scanning system
consists of two different custom designed lenses, one of them optimized for cornea and the anterior surface of the
crystalline lens, while the other for the posterior surface of the crystalline lens. Two semicircular halves of each lens
were glued together to form a single optical system.
To evaluate the performance of our design we constructed and imaged a model eye and compared it with images
obtained by conventional telecentric scanning method. SNR improvement by a factor of 3.71 was observed for the front
surface of the lens and 18.83 for the back surface of the lens.
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