Image resolution, tissue penetration, and scan speed are among the most important parameters when designing an OCT system for ophthalmic use. Human retinal tissue is highly reflective in the near infrared spectrum range. A SLD at 820nm with 25nm FWHM spectral bandwidth provides 10μm coherence length in retinal tissue. Its appropriate power level, simplicity of use, high resolution, and relatively low cost, make the 820nm SLD the best choice light source for retinal OCT. A 1300nm SLD can penetrate deeper into the sclera tissue and since the 1300nm wavelength is highly absorbed in the vitreous, the ANSI laser safety standard allows higher maximum permissible power to the human eye. Higher scan speed can also be achieved. In this paper, we report two OCT systems that are designed specifically for retinal and anterior segment imaging of the human eye. Retinal OCT scans 400 A-scans per second, 2mm depth in tissue, and 10 μm image resolution with an 820nm SLD. Anterior segment OCT (AC-OCT) scans 2000 A-scans per second, 6mm depth in tissue, and 16μm image resolution with a 1300nm SLD. Benefits of suitable wavelength selection in scanning different tissue are clearly seen in the OCT images. Retinal OCT (OCT3) demonstrates significant improvement over the previous generation (OCT1/OCT2) from both a technical and cost point of view. AC-OCT performs 8 frames of 256 A-scans per second and is capable of imaging the human eye in vivo with minimum eye motion artifacts. It has potential use in refractive surgery, angle-closure glaucoma, and cataract surgery.
The early identification of glaucomatous development is extremely important for treatment of glaucoma. Analysis of optic-nerve-head features may play a crucial role for early glaucoma diagnostics. Here we propose a critical parameter, viz., nerve tissue area, which may prove to be extremely useful for detection of glaucoma in early stages. We report a novel and robust algorithm for OCT-based automatic, objective extraction of critical optic-nerve-head features such as optic disc, nerve tissue area, and optic cup for the first time.
A recently developed modality for blood flow measurement holds high promise in the management of bleeding ulcers. Color Doppler optical coherence tomography (CDOCT) uses low- coherence interferometry and digital signal processing to obtain precise localization of tissue microstructure simultaneous with bi-directional quantitation of blood flow. We discuss CDOCT as a diagnostic tool in the management of bleeding gastrointestinal lesions. Common treatments for bleeding ulcers include local injection of a vasoconstrictor, coagulation of blood via thermal contact or laser treatment, and necrosis of surrounding tissue with a sclerosant. We implemented these procedures in a rat dorsal skin flap model, and acquired CDOCT images before and after treatment. In these studies, CDOCT succeeded in identifying cessation of flow before it could be determined visually. Hence, we demonstrate the diagnostic capabilities of CDOCT in the regulation of bleeding in micron-scale vessels.
Color Doppler optical coherence tomography (CDOCT) is a recent innovation which allows spatially localized flow velocity mapping simultaneous with micro-structural imaging. We present a theoretical model for velocity image formation in CDOCT. The equivalence between the heterodyne detector current power spectrum in CDOCT and the optical source power spectrum is established. We show that stochastic modifications of the detected electronic spectrum by fluctuating scatterer distributions in the flow field give rise to unavoidable velocity estimation inaccuracies, as well as to the fundamental trade-off between the maximum velocity image acquisition rate and the estimated velocity precision. Novel algorithms which permit high fidelity depth-resolved measurements of velocities in turbid media with optimal frame rate are also reported.
We performed imaging and reconstruction of dermal and subdermal blood vessels in a rat skin flap window model. The window model consists of a double thickness of dorsal skin which is sutured to a holding fixture. A 1 cm circle of skin is removed from one thickness, exposing the dermal blood vessels of the opposing side. An optical coherence tomography system operating at 1310 nm was used to image the blood vessels. A series of transverse images of the window model characterized sections of tissue. Off-the-shelf software for desktop and workstation computers was used to preprocess the images, identify and reconstruct blood vessels, and to extract parameters such depth, diameter, and percent volume of blood vessels. Such parameters may be of interest in developing improved treatments for vascular disorders such as port wine stains.
Optical coherence tomography (OCT) is a novel medical imaging modality which utilizes coherence ranging to perform high resolution (approximately 10 micrometer) non-invasive sub- surface imaging of biostructures. We have developed an OCT system consisting of a low-coherence interferometer and a calibration interferometer allowing sub-micron interferogram acquisition accuracy. We propose some digital signal processing strategies for image enhancement in optical coherence tomography. A linear shift invariant system model is presented for describing coherent light-tissue interactions in optical coherence tomography. In this model, the electric field backscattered from a target specimen is treated as a convolution of the incident field and a postulated tissue impulse response which describes the profile of scattering sites within the specimen. Based on this model, a novel technique for enhancing the sharpness of optical coherence tomographic images of biological structures using digital deconvolution is demonstrated. Using this approach, resolution improvement by a factor of greater than 2.2 is achieved in the longitudinal direction.
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