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Ryan P. McNabb, Anthony N. Kuo, Joseph A. Izatt; Novel Optical Design for Circumferential SDOCT Gonioscopy of the Ocular Angle. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3618.
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The iridocorneal angle is optically inaccessible for direct viewing due to total internal reflection at the air-cornea interface. Clinically, gonioscopy using a contact lens is used to visualize the angle from inside the anterior chamber, but provides only qualitative, en face views at discrete angular orientations. Optical coherence tomography (OCT) has been used to image the angle region from outside the eye, however this external approach suffers from reduced resolution/contrast due to strong scattering in the overlying sclera, as well as shadowing artifacts. To provide for high resolution, circumferential, internal OCT imaging of the angle, we introduce a novel gonioscopic OCT (GOCT) system design.
The GOCT optics were designed to provide continuous, 360 degree optical access to the ocular angle from within the anterior segment. This was achieved using a novel gonioscopic contact lens incorporating a conical mirror and aspheric annular axial lens operating together to correct azimuthally dependent astigmatism. The GOCT optics were implemented as an attachment to a commercial SDOCT hand-held scanner operating with =840nm, Δ=50nm, 5 kHz A-scan rate (Bioptigen, RTP, NC). A custom OCT annular radial scan pattern was developed for efficient cross-sectional imaging of the angle region in a continuous 360 degree sweep around the ocular meridian. With this system, volumetric circumferential datasets consisting of 360 cross-sectional meridian B-scans composed of 800 A scans.To test the optical design, a cadaver eye was perfused with PBS via a 23 gauge butterfly needle inserted through the limbus and terminating underneath the iris. The column height was adjusted until the anterior chamber appeared formed. The GOCT scanner was then aligned with the sample using mechanical translators and used to image the ocular angle.
The optical design approach achieved a focused spot size < 12µm across a 500µm scan range in numerical ray-trace modeling. In a cadaver eye, the ocular irido-corneal angle was visualized in its full circumferential extent in one continuous tomogram. The posterior cornea and iris surface were visualized around the entire circumference, as was the needle entry location.
We describe a novel optical design for continuous circumferential OCT gonioscopy with potential applications for high-resolution quantitative tomography of angle structures without artifacts seen in current external imaging approaches.
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