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Ultrahigh-resolution high-speed retinal imaging using spectral-domain optical coherence tomography

Barry Cense, Nader A. Nassif, Teresa C. Chen, Mark C. Pierce, Seok-Hyun Yun, B. Hyle Park, Brett E. Bouma, Guillermo J. Tearney, and Johannes F. de Boer

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Abstract

We present the first ultrahigh-resolution optical coherence tomography (OCT) structural intensity images and movies of the human retina in vivo at 29.3 frames per second with 500 A-lines per frame. Data was acquired at a continuous rate of 29,300 spectra per second with a 98% duty cycle. Two consecutive spectra were coherently summed to improve sensitivity, resulting in an effective rate of 14,600 A-lines per second at an effective integration time of 68 µs. The turn-key source was a combination of two super luminescent diodes with a combined spectral width of more than 150 nm providing 4.5 mW of power. The spectrometer of the spectral-domain OCT (SD-OCT) setup was centered around 885 nm with a bandwidth of 145 nm. The effective bandwidth in the eye was limited to approximately 100 nm due to increased absorption of wavelengths above 920 nm in the vitreous. Comparing the performance of our ultrahighresolution SD-OCT system with a conventional high-resolution time domain OCT system, the A-line rate of the spectral-domain OCT system was 59 times higher at a 5.4 dB lower sensitivity. With use of a software based dispersion compensation scheme, coherence length broadening due to dispersion mismatch between sample and reference arms was minimized. The coherence length measured from a mirror in air was equal to 4.0 µm (n=1). The coherence length determined from the specular reflection of the foveal umbo in vivo in a healthy human eye was equal to 3.5 µm (n=1.38). With this new system, two layers at the location of the retinal pigmented epithelium seem to be present, as well as small features in the inner and outer plexiform layers, which are believed to be small blood vessels.

©2004 Optical Society of America

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Media 3: AVI (2312 KB)     

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Figures (8)
Fig. 1.
Fig. 1. Source spectrum of the BroadLighter (black); spectrum returning from the reference arm (red). Both spectra were measured with a commercial optical spectrum analyzer. The reference spectrum in blue was recorded with our high-speed spectrometer, and by comparing the blue and red line it demonstrates the decrease in sensitivity of the line scan camera above 850 nm. Spectrum amplitudes were adjusted so that all three curves fit within the same graph.

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Fig. 2.
Fig. 2. The phase θ(k) obtained from a mirror in a model eye and from a specular reflection in the fovea (left axis). The residual dispersion not compensated for by the polynomial fit is given as a function of k (right axis).

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Fig. 3.
Fig. 3. Coherence function obtained from a mirror in air. Uncompensated data (red) is compared with a coherence function after dispersion compensation (black). The density of points was increased by a factor of 8 using a zero-padding technique.

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Fig. 4.
Fig. 4. Coherence functions obtained from a mirror at different path length differences z. The coherence function at z=700 µm was dispersion compensated, and the data of all other curves was multiplied with the same phase e -iθ(k) before Fourier transformation. The coherence length for path length differences up to 1200 µm was 4.0 µm in air, 4.1 µm for z=1700 µm and 4.3 µm for z=2200 µm.

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Fig. 5.
Fig. 5. Shot noise measurement using the BroadLighter in an SD-OCT configuration. The shot noise level was determined with illumination of the reference arm only. The measured shot noise curve was fit with a theoretical equation of the shot noise, demonstrating that the system was shot noise limited. [20]

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Fig. 6.
Fig. 6. Structural image of the fovea. The dimensions of each image are 3.1×0.61 mm. The image is expanded in vertical direction by a factor of 2 for clarity. Layers are labeled as follows: RNFL – retinal nerve fiber layer; GCL – ganglion cell layer; IPL – inner plexiform layer; INL – inner nuclear layer; OPL – outer plexiform layer; ONL -outer nuclear layer; ELM – external limiting membrane; IPRL – interface between the inner and outer segments of the photoreceptor layer; RPE – retinal pigmented epithelium; C – choriocapillaris and choroid. A highly reflective spot in the center of the fovea is marked with an R. A blood vessel is marked with a large circle (BV) and structures in the outer plexiform layer are marked with smaller circles. In the movie, these structures can also be seen in the IPL. Two layers at the location of the RPE at the left and right are marked with arrows and an asterisk (*). Click on the image to view the movie (29.3 frames per second and 500 A-lines per frame, aspect ratio 1:3.2, short version 45 frames (1.5 s, 2.4 MB), long version 90 frames (3.1 s, 5.3 MB). In the movie, a floater can be seen in the vitreous at the left hand side above the retina. The repositioning of the galvo mirror after each scan creates an artifact in the image on the right hand side.

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Fig. 7.
Fig. 7. Coherence function obtained from a reflective spot in the fovea. The coherence length is equal to 4.8µm in air.

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Fig. 8.
Fig. 8. Structural image of the fovea. The dimensions of each image are 6.2×1.2 mm. The slow axis in the movie scans over 3.1 mm. Click on the image to view the movie (2.4 s, 45 frames, with 29.3 frames per second and 500 A-lines per frame, 2.3 MB).

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Equations (2)

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SNR SD − OCT = η · P sample · τ i E v
θ ( k ) = θ ( k 0 ) + ∂ θ ( k ) ∂ k ∣ k 0 ( k 0 − k ) + 1 2 · ∂ 2 θ ( k ) ∂ k 2 ∣ k 0 ( k 0 − k ) 2 + … + 1 n ! · ∂ n θ ( k ) ∂ k n ∣ k 0 ( k 0 − k ) n
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