Extended in vivo anterior eye-segment imaging with full-range complex spectral domain optical coherence tomography

Abstract

Abstract. We demonstrate the capability of full-range complex ͑FRC͒ spectral domain optical coherence tomography ͑SD-OCT͒ to image the anterior eye segment from the cornea to the posterior surface of the lens. With an adapted spectrometer design, we developed a SD-OCT system with an extended normal ͑single half-space͒ depth range of 7 mm ͑in air͒. This OCT-intrinsic depth range was doubled with a FRC technique. We demonstrate the performance of our OCT system by imaging the whole anterior segment of a healthy human eye in vivo. Optical coherence tomography ͑OCT͒ is an imaging modality that enables high-resolution cross-sectional imaging of biological tissues and translucent materials.1,2 Spectral domain ͑SD͒ OCT is a high-speed and high-sensitivity variant of OCT that largely replaced the older time domain variant in the recent years.In SD-OCT, the depth-resolved information can be reconstructed by Fourier transform of the cross spectral density measured with a spectrometer located in the detection arm of an interferometer.3,4 However, SD-OCT suffers from two drawbacks that restrict its measurement range.First, the Fourier transform of the real-valued crossspectral density is symmetrical about the zero path difference. Therefore, one cannot distinguish between positive and negative optical path differences with respect to the reference mirror. This effect particularly concerns imaging of objects with larger depth extensions, such as the anterior eye segment, where measurement ranges of the order of 10 mm are needed. In order to suppress the mirror images, the so-called fullrange complex ͑FRC͒ technique was introduced.5 In addition to the amplitude of the spectral interferogram, its phase is measured to reconstruct the full complex spectral interferogram-the analytic function. Inverse Fourier transformation of the analytic function directly yields the true object structures without any mirror terms. Several FRC approaches were developed that differ in the way of generating the / 2 phase-shifted quadrature function of the spectral interferometric signal.6-8 An elegant method uses the phase modulation that is introduced by off-pivot-point illumination of the galvanometer scanner mirror.9-11 A second drawback of SD-OCT is the spectrometer-intrinsic depth range, which is limited by its spectral resolution. Until now, the depth range of standard SD-OCT systems is ϳ3 mm, which can be doubled by applying FRC methods. However, this axial measurement range is not sufficient to cover the entire anterior eye segment.In this letter, we demonstrate an SD-OCT system with a modified spectrometer design combined with a FRC measurement range doubling that achieves an imaging depth of 14 mm, sufficient to cover the human anterior eye segment from the cornea to the posterior surface of the lens. Figure 1 shows a schematic diagram of our system. A superluminescent diode ͑Superlum, Moscow͒ with a center wavelength of 835 nm and a bandwidth ͑FWHM͒ of 18 nm was used as the light source. The round trip coherence length was 17 m. The col...

Extended scan depth optical coherence tomography for evaluating ocular surface shape

“…Some methods can improve the SNR in the scleral regions, such as phase-shift technology. 17,24 Using a camera with higher number of pixels, for example, a COMS (complementary metal oxide semiconductor) camera with 4096 pixels (Basler Sprint camera), could also improve the sensitivity rolloff performance. 25 The other reason for poor repeatability is eye motion during measurement.…”

“…Both SD-OCT and SS-OCT have been widely applied in the ophthalmic imaging of the posterior and anterior segments. 6,16,17,19,20 Compared with SS-OCT, SD-OCT has the advantage of higher resolution and increased phase stability for functional imaging. 23 Despite rapid advances in SD-OCT, the limited imaging range due to finite resolution of spectrometer remains a major drawback.…”

“…Due to the large slope of the ocular surface, this drawback causes a decrease in signal-to-noise ratio (SNR) from the corneal apex to the sclera. Several approaches have been proposed to overcome this problem, such as phase shift techniques, 17,24,25 an optical frequency comb with additional tunable fiber Fabry-Perot interferometer, 26 and a spectrometer with special custom design. 27 Recently, OCT has been used to quantitatively measure the biometry of the anterior ocular segment.…”

“…13 It can be used to capture a cross sectional or three dimensional volume images. 6,[14][15][16][17][18][19][20] Compared with conventional time domain OCT, Fourier domain OCT (FD-OCT) offers significant advantages in imaging speed and sensitivity. 21,22 There are two primary types of FD-OCT implementations, depending on the light source and detector configuration.…”

Off-axis full-field swept-source optical coherence tomography using holographic refocusing

“…Artifacts that are introduced by the setup, such as lines from reflections of optical components, as for example of the protective glass of the image sensor, can be removed. In addition, the measurement depth can be doubled as the off-axis reference beam resolves the complex conjugate ambiguity similar to full-range OCT. 17,18 `fs t 250 µm Figure 3: A typical A-scan of an FD-OCT system. The different terms of the interference signals can be distinguished, but they influence imaging quality and cause artifacts.…”