Optical coherence tomography (OCT) is a noninvasive imaging technique that provides microscopic tomographic sectioning of biological samples. By measuring singly backscattered light as a function of depth, OCT fills a valuable niche in imaging of tissue ultrastructure, providing subsurface imaging with high spatial resolution (˜2.0-10.0 μm) in three dimensions and high sensitivity (>110 dB) in vivo with no contact needed between the probe and the tissue.
In biological and biomedical imaging applications, OCT allows for micrometer-scale imaging non invasively in transparent, translucent, and/or highly-scattering biological tissues. The longitudinal ranging capability of OCT is generally based on low-coherence interferometry, in which light from a broadband source is split between illuminating the sample of interest and a reference path. The interference pattern of light reflected or backscattered from the sample and light from the reference delay contains information about the location and scattering amplitude of the scatterers in the sample. In time-domain OCT (TDOCT), this information is typically extracted by scanning the reference path delay and detecting the resulting interferogram pattern as a function of that delay. The envelope of the interferogram pattern thus detected represents a map of the reflectivity of the sample versus depth, generally called an A-scan, with depth resolution given by the coherence length of the source. In OCT systems, multiple A-scans are typically acquired while the sample beam is scanned laterally across the tissue surface, building up a two-dimensional map of reflectivity versus depth and lateral extent typically called a B-scan. The lateral resolution of the B-scan is approximated by the confocal resolving power of the sample arm optical system, which is usually given by the size of the focused optical spot in the tissue.
The time-domain approach used in conventional OCT, including commercial instruments, such as Carl Zeiss Meditec's StratusOCT® and Visante® products, has been successful in supporting biological and medical applications, and numerous in vivo human clinical trials of OCT reported to date have utilized this approach.
An alternate approach to data collection in OCT has been shown to have significant advantages both in reduced system complexity and in increased signal-to-noise ratio (SNR). This approach involves acquiring the interferometric signal generated by mixing sample light with reference light at a fixed group delay as a function of optical wavenumber. Two distinct methods have been developed which use this Fourier domain OCT (FDOCT) approach. The first, generally termed Spectral-domain or spectrometer-based OCT (SDOCT), uses a broadband light source and achieves spectral discrimination with a dispersive spectrometer in the detector arm. The second, generally termed swept-source OCT (SSOCT) or optical frequency-domain imaging (OFDI), time-encodes wavenumber by rapidly tuning a narrowband source through a broad optical bandwidth. Both of these techniques may allow for a dramatic improvement in SNR of up to 15.0-20.0 dB over time-domain OCT, because they typically capture the A-scan data in parallel. This is in contrast to previous-generation time-domain OCT, where destructive interference is typically used to isolate the interferometric signal from only one depth at a time as the reference delay is scanned.
Spectrometer-based implementations of FDOCT have the potential of advantaged phase stability, as the source and detection modules are passive. However, in practice spectrometer based designs for high resolution imaging have been shown to have some shortcomings. High resolution spectrometers rely on a highly dispersive element with optics that provide approximately constant magnification imaging across a broad focal plane. Optical designs using transmission volume phase holograms or reflective Eschelle gratings for achieving desired results are well known and applied in many laboratory set-ups.
A difficulty arises in developing a spectrometer that is readily manufacturable and passively stable in the face of environmental perturbation. One solution to environmental stability has been proposed by Wei et al. utilizing an actively controlled fold mirror as discussed in U.S. Pat. No. 7,480,058. This system has the capability to continually adjust alignment to optimize attributes of the spectrograph.
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