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Y. K. Tao, M. Zhao, C. A. Toth, J. A. Izatt; High-Speed Complex Conjugate Resolved Retinal Spectral Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2007;48(13):154.
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© ARVO (1962-2015); The Authors (2016-present)
Recent advances in Spectral Domain Optical Coherence Tomography (SDOCT) have shown strong potential in clinical retinal imaging, enabling high-resolution, motion-artifact-free cross-sectional imaging and rapid accumulation of volumetric macular datasets. SDOCT suffers from complex conjugate image artifact, in which positive and negative distances in the sample cannot be uniquely resolved. Current retinal imaging practice limits the sample entirely within the positive or negative displacement range, thus avoiding this artifact but effectively halving the potential imaging depth. While tolerable for normal retina, imaging of extended pathologies (such as vitreous strands, deep optic nerve head cups, and choroidal structures) is limited by the characteristic roll-off in sensitivity associated with SDOCT systems, and would benefit from full-depth SDOCT imaging. Previous approaches for resolving this artifact involve using multiple spectrometers, other expensive components, or have limited response time. Here we demonstrate a solution for simple, inexpensive, high-speed complex conjugate artifact resolved SDOCT imaging of human retina.
Integrating-bucket phase acquisition was implemented on a high-speed SDOCT retinal system. Sinusoidal signals corresponding to 4-step quadrature interferometric signals were used to continuously shift the reference mirror mounted on a PZT stack. Interferometric signals were captured on a 1024-pixel line-scan CCD at an A-Scan rate of 51.9kHz. A quadrature projection algorithm was used to complex conjugate resolve sets of four phase-stepped A-scans into vivo images of fovea and optic nerve head.
At the A-Scan rate of 51.9kHz, the algorithm obtained DC suppression of 67dB and complex conjugate artifact suppression of 33dB. In vivo normal human retina and optic nerve-head images showed complex conjugate artifact removal down to the noise floor for most regions, although some artifact remained from strong reflectors in the nerve-head region.
High-speed, full-depth, unambiguous images (1000 A-Scans/image acquired at 12.9 images/sec) were demonstrated using a single line-scan CCD. While several approaches for removing the complex conjugate artifact in SDOCT have been demonstrated, integrating-bucket phase stepping is a simple and inexpensive method suitable for high-speed imaging that is only limited by the read-time of the CCD.
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