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In vivo retinal optical coherence tomography at 1040 nm-enhanced penetration into the choroid

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Abstract

For the first time in vivo retinal imaging has been performed with a new compact, low noise Yb-based ASE source operating in the 1 µm range (NP Photonics, λc=1040 nm, Δλ=50 nm, Pout=30 mW) at the dispersion minimum of water with ~7 µm axial resolution. OCT tomograms acquired at 800 nm are compared to those achieved at 1040 nm showing about 200 µm deeper penetration into the choroid below the retinal pigment epithelium. Retinal OCT at longer wavelengths significantly improves the visualization of the retinal pigment epithelium/choriocapillaris/choroid interface and superficial choroidal layers as well as reduces the scattering through turbid media and therefore might provide a better diagnosis tool for early stages of retinal pathologies such as age related macular degeneration which is accompanied by choroidal neovascularization, i.e. extensive growth of new blood vessels in the choroid and retina.

©2005 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. Simple model of the human eye. The allowed power density for collimated laser light for 10 s continuous illumination of the cornea according to ANSI laser safety standard as well as ICRNIRP-guidelines is given in black. The green line shows the maximum power density from the front surface of the retina (internal limiting membrane) after double passing 25 mm of water of the anterior eye segment (AES) and vitreous while the blue line indicates the back surface of the photoreceptor outer segment. Water absorption mainly reduces the maximum power densities after double pass. The respective power densities from beneath the highly absorbing and scattering monocellular retinal pigment epithelium (RPE) are given in brown and from the Bruch’s membrane and choriochapillaris (CC) in orange. Representative backscattering sites inside the highly scattering choroid in a depth of 150 µm are depicted in thick red and at 300 µm in thin red. The relative spreading of the axial point spread function corresponding to a 100 nm wide spectrum at 1000 nm due to the dispersion of water corresponding to a 500 µm thick sample is shown in dash dotted grey.
Fig. 2.
Fig. 2. Comparison of in vivo retinal OCT of the same human fovea using 800 nm (a) with 3 µm and 1040 nm (b) with 7 µm axial resolution. Both OCT systems have comparable sensitivity of ~95 dB. Deeper penetration into the choroid is accomplished by using 1040 nm, enabling the visualization of choroidal vessels and branching (B red asteriks) shown in two times magnification of the corresponding tomograms. Intensity diagrams obtained at 800 nm (blue curve) and 1040 nm (red curve) at similar positions depict a significant higher signal at and below the RPE at 1040 nm. A penetration depth of about 500 µm is achieved at 1040 nm in contrast to a penetration depth of about 300 µm at 800 nm.
Fig. 3.
Fig. 3. Horizontal (a) and vertical (b) OCT cross-section performed with an ASE based ophthalmic OCT system centered at 1040 nm with 7 µm axial resolution. Due to enhanced penetration into the choroid several vessels are clearly visualized in the parapapillary region (arrows).
Fig. 4.
Fig. 4. Comparison of in vivo retinal OCT of a healthy human fovea at 1040 nm (a) with a human fovea with RPE atrophy (b) at 800 nm, where regions with intact RPE and defect RPE can clearly be distinguished. Only the region with defect RPE in the center indicated by the two arrows in figure B has comparable penetration at 800 nm to the tomogram acquired at 1040 nm due to reduced melanin concentration. Towards the margins at regions with intact RPE the penetration is significantly less than at 1040 nm and comparable to the tomogram acquired at 800 nm.
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