OCT has also been proven useful for ophthalmology applications ( 15–17) where structural information provides an indirect measure of physiologically relevant information that is useful for disease diagnosis. For example, OCT is excellent at rendering a depth-resolved profile of macular hole disease progression ( 12), which generally involves a thinning or detachment of the retina and is difficult to assess with en face imaging methods ( 13, 14). The popularity of the method lies in its ability to perform high-resolution cross-sectional imaging and analysis of structural changes in the eye during disease progression. Since the invention of OCT, the method has been extensively applied in ophthalmology settings. ( iii) Additional functionality-while a basic OCT imaging method is able to render depth-resolved structural images of the target, more sophisticated OCT imaging strategies can provide additional functional information, such as flow (through Doppler OCT) ( 6, 7), tissue structural arrangement (through birefringence OCT) ( 8, 9), and the spatial distribution of specific contrast agents (through molecular contrast OCT) ( 10, 11). Further, OCT probes are typically optical fiber-based, and they may be made to be sufficiently small and pliable to operate within the gastrointestinal (GI) tract ( 4) and major blood vessels ( 5). ( ii) Flexibility-the light fluence level required for OCT imaging is low enough that OCT can be used in sensitive tissue locales, such as the eye ( 3). ( i) Quality images-OCT has demonstrated the ability to render images with 0.5 m resolution ( 2). The success of OCT in making such a rapid transition from research and development into the clinical setting is not surprising given the numerous advantages that it offers clinicians. Optical coherence tomography (OCT) was first developed by Fujimoto's group at MIT about 10 years ago ( 1), and the method has since matured into an important clinical imaging modality. The review also discusses OCT enhancements and functional methods based on SDOCT format and concludes with possible directions that this research may take in the near future. The two peer approaches are compared in speed, scan depth range, complexity, spectral regions of operation, and methods of detection.
After a brief introduction and a discussion on sensitivity advantage, methods of implementation of the two SDOCT schemes will be presented. Owing to its high speed and superior sensitivity, SDOCT has become indispensable in biomedical imaging applications. Unlike time domain (TD) OCT, the reference arm is stationary in both SDOCT methods, which allows for ultra high-speed OCT imaging. There are two approaches to SDOCT-one that uses a broadband source and a spectrometer to measure the interference pattern as a function of wavelength and the other that utilizes a narrowband tunable laser that is swept linearly in k ∼ 1/λ space during spectral fringe data acquisition. SDOCT is an interferometric technique that provides depth-resolved tissue structure information encoded in the magnitude and delay of the back-scattered light by spectral analysis of the interference fringe pattern. This paper reviews the current state of research in spectral domain optical coherence tomography (SDOCT).
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