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Al-Hafeez Dhalla, Anthony N. Kuo, Ryan P. McNabb, Joseph A. Izatt; Full-range, High Speed Anterior Segment Imaging At 1040nm With Complete Complex Conjugate Artifact Removal. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1740.
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© ARVO (1962-2015); The Authors (2016-present)
In vivo imaging of the anterior segment has potential clinical applications in ocular biometry and refractive surgery. Current generation commercial spectral domain optical coherence tomography (SDOCT) systems provide only 2-3mm of useful imaging depth range, which limits imaging to the central cornea. Next generation swept source optical coherence tomography (SSOCT) systems provide 5-6mm depth range, which is still insufficient to image the entire anterior segment. Both SDOCT and SSOCT suffer from the complex conjugate ambiguity, which allows for only half of the potential depth range to be displayed and is responsible for bothersome image wrapping artifacts. Here, we describe a novel SSOCT system which completely resolves the complex ambiguity and doubles the imaging range of SSOCT. The extended imaging depth makes possible visualization of the entire anterior segment, from the anterior surface of the cornea to the posterior surface of the lens.
An SSOCT system was constructed using an Axsun Technologies laser (1040nm, 100 kHz sweep rate). Complex conjugate removal was achieved using optical heterodyning by employing a grating-based optical delay line (ODL) in the reference arm. This ODL imparts a phase modulation on the reference light, which up-converts the spectral interferogram in the optical domain. As this technique shifts, rather than suppresses, the complex conjugate, complete removal of the artifact may be achieved. System performance including SNR, falloff, and imaging performance was tested using an attenuated reflector and fresh excised porcine eyes.
The system SNR was 96dB with 3mW on the sample. Transverse and axial resolutions were ~30µm and ~10µm, respectively. The total imaging range was 20mm lateral and 12.4mm axial in air (9.3mm in tissue). The depth range over which the signal amplitude was at least half of its maximum (6dB range) was 10.5mm. The complex conjugate artifact was suppressed below the noise floor of the system, implying a suppression ratio of at least 63.1dB. High quality imaging of the complete anterior segment of porcine eyes was achieved at 50Hz frame rate for 2000 (lateral) x 2304 (axial) pixel images.
We developed a novel SSOCT system that completely resolves the complex conjugate artifact, thus doubling the imaging range. This technology enables visualization of all of the refractive components of the anterior segment in a single acquisition, which may eventually support the capability to model the entire optical system of the eye in real time.
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