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We have demonstrated a novel Fourier-domain optical coherence tomography system and signal-processing algorithm for full-range, real-time, artifact-free quantitative imaging of the anterior chamber. Cross-sectional full-range images comprising 1024×800 pixels (axial×lateral) were acquired and displayed at 6.7 images/s. Volumetric data comprising 1024×400×60 pixels (axial×lateral×elevation) were acquired in 4.5 seconds with real-time visualization of individual slices and 3-dimensional reconstruction performed in postprocessing. Details of the cornea, limbus, iris, anterior lens capsule, trabecular meshwork, and Schlemm’s canal were visualized. Quantitative surface height maps of the corneal epithelium and endothelium were obtained from the volumetric data and used to generate corneal thickness maps.
Currently, ophthalmology can benefit from detailed imaging of the ocular anterior segment for a variety of applications. Precise measurement of the dimensions and profiles of the interior structures of the anterior chamber are relevant for imaging abnormalities in the cornea, sclera, iris, and anterior chamber angle. Visualization of the trabecular meshwork and Schlemm’s canal could be a valuable tool for glaucoma diagnosis and treatment and requires resolution in the range of tens of microns. Refractive surgeries, such as laser in situ keratomileusis and photorefractive keratectomy, would also benefit from accurate corneal mapping, especially if it could be performed noninvasively without compromising surgical-field sterility.
Several techniques are currently available for imaging the anterior segment of the eye, each suited to a specialized application. High-frequency ultrasound biomicroscopy provides resolution of up to 20 μm with a depth penetration of up to 5 mm in tissue at up to 8 frames/s, allowing in vivo imaging of the anterior chamber angle.1,2 Ultrasound biomicroscopy requires immersion of the eye in a water bath with the patient in a supine position and is incompatible with concurrent slitlamp observation and high-volume patient screening. Ultrasonic pachymetry is regularly used for corneal thickness measurement but is a point-by-point technique. Automated scanning slit-based technologies, such as OrbScan (Bausch & Lomb, Rochester, New York), optically measure corneal topography and thickness,3,4 which are required for refractive procedures, such as laser in situ keratomileusis and refractive power calculation for intraocular lens (IOL) implantations. The OrbScan, however, does not image structures internal to the cornea or perform measurements of anterior chamber width, required, in addition to the depth, for the sizing and evaluation of phakic IOLs.
Optical coherence tomography (OCT) is an emerging technique for noninvasive cross-sectional imaging of biological tissue.5 Time-domain OCT systems have been commercialized for retinal imaging (400-Hz, A-scan rate, Stratus OCT; Carl Zeiss Meditec, Dublin, California) and for ocular anterior segment imaging (2-kHz, A-scan rate, Visante; Carl Zeiss Meditec). In contrast to retinal OCT systems that operate at 830 nm,6 anterior segment systems operate at 1310 nm owing to decreased attenuation in highly scattering tissues, such as the sclera.7
In recent years, OCT technology has been revolutionized by the development of Fourier-domain techniques,8 which provide a significantly increased signal-to-noise ratio and increased robustness compared with traditional time-domain OCT.9-11 The signal-to-noise ratio advantage of Fourier-domain OCT techniques may be employed for faster image acquisition, enabling practical 3-dimensional OCT imaging in patients for the first time. Two specific implementations of Fourier-domain OCT have been developed: spectrometer-based, or spectral-domain, OCT and swept-source OCT.8 The increased sensitivity of both Fourier-domain OCT system designs comes at the price of limited usable imaging depth,12 partially owing to the presence of a symmetric overlapping image artifact in Fourier-domain OCT images (Figure 1A). This artifact, referred to in the literature as the complex conjugate or mirror image artifact, occurs whenever the sample imaging depth spans both positive and negative distances compared with the length of the set reference path in the Fourier-domain OCT interferometer.
For retinal imaging at 830 nm, practical high-speed Fourier-domain OCT systems have been demonstrated with A-scan rates up to 30 kHz but with maximum imaging depths limited to approximately 2 mm.13-17 Because healthy retinas are thinner than this, the complex conjugate artifact may be avoided by carefully positioning the patient’s retina to keep it completely on one side of the reference position.
Fourier-domain OCT system designs at 1310 nm have been demonstrated with both spectral-domain OCT and swept-source OCT implementations, providing up to 20 kHz (spectral-domain OCT)18 and 15.7 kHz19 or 232 kHz20 (swept-source OCT) A-scan rates. However, all 1310-nm Fourier-domain OCT systems operating at higher than a few kilohertz A-scan rate have been limited in maximum imaging depths to no deeper than 3 to 4 mm. In contrast to retinal imaging, full-depth imaging of the anterior chamber cannot be accommodated by the single-sided sample depth of previously demonstrated Fourier-domain OCT systems. Thus, a solution for the complex conjugate artifact is a prerequisite for bringing the advantages of the Fourier-domain OCT approach to anterior segment imaging.
Techniques borrowed from phase-shift interferometry have been demonstrated to suppress the complex conjugate artifact in Fourier-domain OCT, resolving positive and negative distances for low-speed, full-range imaging of the ocular anterior segment.21-24 These techniques experienced image corruption from miscalibrated phase steps and sample motion, since they depended on sequential acquisition of phase-separated spectral interferograms. A polarization-encoding technique that simultaneously acquired two 90°-phase stepped interferograms has been presented in the literature25 but was subject to corruption from birefringence. Instantaneous frequency-shifting techniques for full-range imaging have also been presented,26-28 but are not compatible with spectrometer-based spectral-domain OCT systems.
We have recently described a novel approach using interferometers constructed from 3×3 optical couplers for simultaneous acquisition of phase-separated spectral interferometric data29,30 as well as a high-speed compatible quadrature projection algorithm to correct for imperfect or miscalibrated phase shifts, including non-90°-phase shifts.31 In this article, we present a pilot study of the combination of these technologies to provide high-resolution, quantitative imaging of the ocular anterior segment in vivo using a quadrature projection 3×3 swept-source OCT system at an A-scan rate more than 3 times higher than that attainable with time-domain OCT systems.7,32-34
This study was approved by the institutional review board of Duke University Medical Center. Six healthy volunteers underwent imaging with the novel Fourier-domain OCT system. The 3×3 Fourier-domain OCT system (Figure 2) was integrated into a standard slitlamp biomicroscope to assist in patient alignment, which required only a few seconds. A rapidly tunable laser source (Micron Optics Inc, Atlanta, Georgia) having a central wavelength of 1310 nm and an 84-nm full-width at half-maximum spectral width (corresponding to an OCT axial resolution of 9 μm in air) provided a sweep rate of 6.7 kHz. The mean optical power at the eye was 3.75 mW, which is well below the American National Standards Institute limit for maximum permissible exposure of 15.4 mW at this wavelength.36 The lateral image resolution, defined as the full-width at half-maximum of the sample arm beam focus at the cornea, was 19 μm.
Full-range complex conjugate-resolved axial scans were generated by processing the 3 phase-separated detector outputs using the quadrature projection algorithm, described in detail elsewhere.31 The full-range images were acquired, processed, and displayed in real time using custom software (modified from a base package provided by Bioptigen Inc, Durham, North Carolina) on a standard desktop computer. Each A-scan consisted of 1024 pixels. High-resolution images consisted of 800 A-scans and were displayed at 6.7 frames/s, and high-speed images consisted of 400 A-scans and were displayed at 13.4 frames/s. The volumes presented here consisted of 60 image elevations acquired in 4.5 seconds. During volume acquisition, “fly-through” images were displayed in real time. Volume rendering was performed in postprocessing using public-domain software (RMR Systems, Suffolk, England). The 3-dimensional images of the volumetric data appeared visually smooth and did not require additional processing for image registration owing to eye motion.
As described in previous reports of anterior segment imaging using time-domain OCT,7,33,34,37 image processing was required to correct for light refraction at the air-cornea interface. To obtain quantitative corneal thickness measurements, we used a method for refraction correction based on Snell’s law (Figure 3).38 The corneal epithelium and endothelium were manually segmented and fit by a fourth-order polynomial. The vertical difference between these lines represented the optical distance through the cornea, dopt(x). The angle at which the beam was refracted at the epithelium was calculated using the slope, mepi(x), given by the first derivative of the polynomial fit to the epithelial surface. Assuming a telecentric scan, the angle of incidence relative to the surface normal was given by θi=tan-1(mepi[x]). The angle of the refracted beam was given by θr=sin-1(sinθi/nc), in which nc=1.38, the assumed mean refractive index of the cornea.39 The true location of the endothelial surface was calculated relative to the epithelial coordinates (xepi, zepi) using the relations xcor=xepi-([dopt/nc]sinθr) and zcor=zepi-([dopt/nc]cosθr). The corneal thickness was measured perpendicular to the slope of the epithelium at each lateral location. The corneal thickness map and the topographic maps of the corneal epithelium and endothelium resulting from these processing steps were smoothed by a 2-dimensional low-pass image filter.
We compared a complex conjugate artifact-resolved Fourier-domain OCT image of the healthy ocular anterior segment using quadrature projection processing with an unprocessed image (Figure 1). In the complex conjugate-resolved image, the corneal epithelium, iris, anterior lens capsule, and anterior chamber angle were clearly observed. A faint image of the ciliary body was also observed. The dim artifactual line at the vertical center of the image (corresponding to the 0 path-length position) was due to imperfect subtraction of the reference spectrum in image processing. Higher-resolution images of the angle region were obtained by reducing the lateral scan length while preserving the number of A-scans per image (Figure 4). The trabecular meshwork and Schlemm’s canal were visible, but the outline of the ciliary body was only dimly observed as a result of signal loss due to scattering in the tissue.
Volumetric Fourier-domain OCT imaging of the anterior segment is demonstrated in Figure 5. Cross-sectional planes of the angle (Figure 6) were acquired with the scan length reduced to 7 mm. The iris, limbus, and a faint outline of the ciliary body are visible. The images consisting of 1024×400×60 pixels (axial×lateral×elevation) were cropped to 512 pixels axially for volume rendering.
Topographic maps of the cornea were generated by postprocessing the volume image of the anterior chamber, providing a grid of 24 000 depth measurements across the cornea. A surface height map of the corneal epithelium is shown in Figure 7A. The endothelial surface height map in Figure 7B was corrected for refraction as described previously. The corneal thickness map obtained using the prototype 3×3 swept-source OCT system was compared against data from a commercial Pentacam system (Oculus, Dudenhofen, Germany) acquired from an adult male volunteer with normal vision. The results are shown in Figure 8 with the color and lateral dimension scales for the 3×3 swept-source OCT data matched to the graphical report from the commercial Pentacam system. The minimum corneal thickness measured with the swept-source OCT system was 580 μm, comparable with the 575-μm measurement provided by the Pentacam system.
The full-range, high-speed Fourier-domain OCT system presented here facilitated ocular anterior chamber imaging owing to an increased A-scan rate and denser pixel sampling compared with previous time-domain OCT systems.7,32-34 The prototypical Fourier-domain OCT system used in this study operated at an A-scan rate of 6.7 kHz, which was limited by the acquisition speed of the digitization electronics. The ocular anterior segment images consisted of 1024 axial points, nearly double that of earlier time-domain systems.7 Because of the increased sensitivity of Fourier-domain OCT techniques, the optical exposure was maintained at 3.75 mW (compared with 4.9 mW used in recent studies7,33), despite the higher system speed.
Schlemm’s canal is nominally 100 to 200 μm in diameter and was not easily observed using earlier time-domain systems.7,33 The Fourier-domain OCT system provided improved visualization of Schlemm’s canal and the trabecular meshwork (Figure 4).
The increased A-scan rate of the Fourier-domain OCT system also facilitated volume acquisition, which was demonstrated here for the first time in the entire anterior segment. Volume rendering of 3-dimensional anterior segment Fourier-domain OCT data (Figure 5) permits convenient assessment of the cornea, iris, and pupil. Interactive manipulation of volumetric images (Figure 6) is very similar to techniques well known in diagnostic radiology and could become important for pathologic diagnosis, for example, facilitating assessment of the extent of tumors on the iris or ciliary body.
The corneal surface maps (Figure 8) were generated from large data sets (24 000 voxels), providing improved precision compared with point-by-point pachymetry, potentially allowing reliable fitting of aberrations with higher order than sphere and cylinder. Accurate surface topographical and thickness measurements of the cornea, particularly in a noncontact imaging modality, are potentially useful for surgical monitoring and post-operative measurements. The increased signal-to-noise ratio of Fourier-domain OCT may also facilitate thickness mapping of hazy corneas after refractive surgeries for which disagreement has been demonstrated between measurements acquired using ultrasonic pachymetry and OrbScan.3,4 Calculation of required refractive power is also beneficial for phakic IOL implantations, as is measurement of chamber depth and width, particularly for iris-anchored IOLs.
We have presented a full-range, quadrature projection 3×3 swept-source Fourier-domain OCT system, providing high-resolution, real-time display of the ocular anterior segment. The system was used to quantitatively measure corneal thickness and it provided images of the angle region. The high axial resolution of the Fourier-domain OCT system and increased lateral sampling facilitated in vivo imaging of Schlemm’s canal.
Funding/Support: This study was supported by grants EY013516 and EY017393 from the National Eye Institute. Dr Asrani is a recipient of the Career Development Award from Research to Prevent Blindness Inc, New York, New York.
Tarsiers, some of the world’s smallest primates, have the largest eyes relative to body size among mammals (Figure).2 Human eyes would have to be the size of apples to match tarsier eye proportions. Surprisingly, they are named for their extremely long feet and not for their enormous and fascinating eyes, which take up about half of the relatively tiny face. Each eye is larger than the animal’s brain or its stomach. Tarsiers originate from the Philippines and nearby islands and are mostly active at night. However, they lack a reflective tapetum lucidum characteristic of most nocturnal animals so having very large eyes helps to maximize their sensitivity in low light. Unlike many other nocturnal animals who have slit-shaped pupils, tarsiers have humanlike round pupils that are able to constrict very effectively to about half a millimeter.3 Their eyes have retinae with both rods and cones and a fovea, which is unusual for nocturnal animals. The large crystalline lens aids in dim light.4 Tarsiers cannot rotate their huge eyes; instead, they are able to turn their heads nearly 360°.
You can see the tarsier in action in an amusing 1939 black-and-white movie describing scientists at Yale University School of Medicine studying one to “better understand the human eyes” and to help in “designing everyday things.”5
Ilya Rozenbaum, MD
Christoph Faschinger, MD, PhD
Robert Ritch, MD, Section Editor
Financial Disclosure: Drs Sarunic and Izatt are inventors of intellectual property, assigned to Duke University, related to the subject of this article. Dr Izatt is a founder and part-time employee of Bioptigen Inc.