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R. P. McNabb, A. N. Kuo, J. A. Izatt; Motion Artifact Reduced SDOCT of the Anterior Segment. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5679.
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Corneal topography is currently the primary method for clinically assessing corneal shape. Placido ring-based corneal topography accurately measures the curvature of the anterior surface of the cornea while calculating the total power of the cornea by assuming a constant ratio between the anterior and posterior surfaces of the cornea. This power calculation fails however when the ratio between surfaces can no longer be considered constant, notably in patients who have undergone laser refractive surgery. Spectral domain optical coherence tomography (SDOCT) can overcome this limitation by offering the capability to acquire full tomographic information of both anterior and posterior surfaces. A major limitation in this method is the corruption of data by low spatial frequency patient motion during the multiple seconds required to obtain a volumetric image. We have developed a novel scanning approach based on high-speed distribution of individual A-scans across the cornea, thus encoding patient motion into high spatial frequencies which are then removed by spatial filtering.
Conventional galvanometer based SDOCT scanning systems are satisfactory for scanning sequential A-scans in a line across a sample, however their minimum settling time due to mirror inertia is insufficient for fast distributed scanning. Our design employs an orthogonal acousto-optic deflector (AOD) pair, which provides inertialess scanning. The use of an AOD with a broadband source as in SDOCT has additional requirements to prevent spatial spectral dispersion at the image plane. A double-Amici prism is used to provide negative spatial dispersion prior to the AOD pair. The focusing objective is a novel diffractive-refractive hybrid lens, designed to provide negative chromatic aberration, fully correcting the spatial chromatic dispersion caused by both AODs.
The fast distributed corneal scanning system has been designed, built, and implemented using an SDOCT system with a line scan rate of 20 kHz. The source is a SLD centered at 828nm with 70nm of bandwidth. Spot diagrams of 25 spots rapidly distributed across a 5 mm x 5 mm grid show spot sizes (FWHM) within the designed specification of 40 µm. Transition time between arbitrarily separated spots for the AOD is at 13µs.
We have designed and built a high speed SDOCT system that provides for fast non-sequential cornea scanning through the use of a novel optical design. This system will be used for compensation of patient motion for improved analysis of corneal shape using volumetric SDOCT datasets.
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