Abstract
Chromatic aberrations are an important design consideration in high resolution, high bandwidth, refractive imaging systems that use visible light. Here, we present a fiber-based spectral/Fourier domain, visible light OCT ophthalmoscope corrected for the average longitudinal chromatic aberration (LCA) of the human eye. Analysis of complex speckles from in vivo retinal images showed that achromatization resulted in a speckle autocorrelation function that was ~20% narrower in the axial direction, but unchanged in the transverse direction. In images from the improved, achromatized system, the separation between Bruch's membrane (BM), the retinal pigment epithelium (RPE), and the outer segment tips clearly emerged across the entire 6.5 mm field-of-view, enabling segmentation and morphometry of BM and the RPE in a human subject. Finally, cross-sectional images depicted distinct inner retinal layers with high resolution. Thus, with chromatic aberration compensation, visible light OCT can achieve volume resolutions and retinal image quality that matches or exceeds ultrahigh resolution near-infrared OCT systems with no monochromatic aberration compensation.
Highlights
Optical coherence tomography (OCT) has advanced the detection, diagnosis and monitoring of retinal diseases by enabling high-resolution volumetric and cross-sectional structural imaging of the retina [1], and more recently, by revealing vascular perfusion [2, 3]
A 1.4 μm axial resolution in tissue was estimated from a mirror reflection without the achromatizing lens in place [11], assuming a tissue refractive index of 1.4
While the improvement in signal intensity with achromatization is difficult to assess in a controlled fashion, we generally observed brighter images after achromatization. These results suggest that achromatization is needed to fully appreciate the benefits of broad visible light OCT bandwidths and optimize axial resolution
Summary
Optical coherence tomography (OCT) has advanced the detection, diagnosis and monitoring of retinal diseases by enabling high-resolution volumetric and cross-sectional structural imaging of the retina [1], and more recently, by revealing vascular perfusion [2, 3]. Recent pilot studies have shown the capability of visible light spectral / Fourier domain (spectrometer-based) OCT to perform structural and functional imaging of the human retina [10,11,12] with improved axial resolution [13, 14] compared to near-infrared OCT. The chromatic aberration of the eye limits the achievable resolution of visible light OCT. Chromatic aberration is substantially larger for visible wavelengths than for near infrared wavelengths [18] Both optical glass used in lenses and ocular media in the human eye exhibit large changes in refractive index with wavelength across the visible spectrum. The resulting chromatic aberrations are especially consequential in ultrahigh axial resolution and spectroscopic visible light OCT, where broad bandwidths are needed. We must consider the impact of chromatic aberrations in both the OCT sample arm (taken to include all elements in the optical path, including the human eye) and the spectrometer
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