Ultrahigh-resolution ophthalmic optical coherence tomography.

Abstract

Here we present new technology for optical coherence tomography (OCT) that enables ultrahigh-resolution, non-invasive in vivo ophthalmologic imaging of retinal and corneal morphology with an axial resolution of 2–3 μm. This resolution represents a significant advance in performance over the 10–15-μm resolution currently available in ophthalmic OCT systems and, to our knowledge, is the highest resolution for in vivo ophthalmologic imaging achieved to date. This resolution enables in vivo visualization of intraretinal and intra-corneal architectural morphology that had previously only been possible with histopathology. We demonstrate image processing and segmentation techniques for automatic identification and quantification of retinal morphology. Ultrahigh-resolution OCT promises to enhance early diagnosis and objective measurement for tracking progression of ocular diseases, as well as monitoring the efficacy of therapy. Current clinical practice emphasizes the development of techniques to diagnose disease in its early stages, when treatment is most effective and irreversible damage can be prevented or delayed. In ophthalmology, the precise visualization of pathology is especially critical for the diagnosis and staging of ocular diseases. Therefore, new imaging techniques have been developed to augment conventional fundoscopy and slit-lamp biomicroscopy. Ultrasonography is routinely used in ophthalmology, but requires physical contact with the eye and has axial resolutions of approximately 200 μm (ref. 1). High-frequency ultrasound enables approximately 20 μm axial resolutions, but due to limited penetration, only anterior eye structures can be imaged2. Confocal microscopy has been used to image the cornea with sub-micrometer transverse resolution3. Scanning laser ophthalmoscopy enables en face fundus imaging with micron-scale transverse and approximately 300-μm axial resolution4,5. None of these techniques, however, permits high-resolution, cross-sectional imaging of the retina in vivo. Recently, optical coherence tomography (OCT) has emerged as a promising new technique for high-resolution, cross-sectional imaging6,7. OCT is attractive for ophthalmic imaging because image resolutions are 1–2 orders of magnitude higher than conventional ultrasound, imaging can be performed non-invasively and in real time, and quantitative morphometric information can be obtained. OCT is somewhat analogous to ultrasound imaging except that it uses light instead of sound. High-resolution, cross-sectional images are obtained by measuring the echo time delay of reflected infrared light using a technique known as low coherence interferometry8,9. OCT imaging was first demonstrated in the human retina in vitro6 and in vivo10,11. Recently, it has been extended to a wide range of other non-transparent tissues to function as a type of optical biopsy12–15. To date, however, the most important clinical applications of OCT have been retinal imaging in ophthalmic diagnosis7,16–22. Current ophthalmic OCT systems have 10–15-μm axial resolution and provide more detailed structural information than any other non-invasive ophthalmic imaging technique6,7,10,11. However, the resolution of current clinical ophthalmic OCT technology is significantly below what is theoretically possible. Improving the resolution of OCT ophthalmic imaging would enable structural imaging of retinal pathology at an intraretinal level, as well as improve the accuracy of morphometric quantification. The axial resolution of conventional ophthalmic OCT systems is limited to 10–15 μm by the bandwidth of light sources used for imaging. Short-pulse, solid-state lasers can generate ultrabroad bandwidth, low-coherence light13. A broadband Cr:Forsterite laser operating in the near infrared (1,300 nm) has permitted cellular-level OCT imaging in developmental biology specimens with 6-μm axial resolution13. For ophthalmic imaging, light sources operating at 800 nm are necessary to avoid absorption in the ocular media. Recently, a state-of-the-art broadband Ti:Al2O3 laser has been developed for ultrahigh (~1 μm) axial resolution, spectroscopic OCT imaging in non-transparent tissue at 800-nm center wavelength23,24. We describe here ultrahigh-resolution ophthalmic OCT based on this state-of-the-art optical technology and demonstrate its potential to provide enhanced structural and quantitative information for ophthalmologic imaging.