We describe a visible-light spectroscopic OCT system designed to obtain localized measurements of hemoglobin oxygenation in the superficial microcirculation and also to obtain localized measurements of optical properties at visible wavelengths. The device is based on a supercontinuum source emitting in the 450-600 nm spectral range, which overlaps the visible absorption band of hemoglobin. The OCT detection system uses a spectral domain set-up using a linear CCD and home-built spectrometer and is implemented in single-mode fiber. Multi-spectral OCT images can be acquired at eight wavelengths simultaneously each with 256 axial pixels, and multi-linear regression processing can be applied at a line-rate of 1 kHz. The dynamic range of the system is characterized and found to be limited by excess noise in the supercontinuum source. The wavelength-dependence of scatter is determined for Intralipid using a single-scatter model. Coherent back-scatter from whole blood is detected in the visible spectrum and used to infer the total attenuation coefficient at 470 nm. The feasibility of obtaining oximetry data over volumes of blood as thin as 20 microns is demonstrated. The work describes first steps towards assessment of hemoglobin oxygenation in the superficial microcirculation with picoliter resolution.
Polarization-sensitive optical coherence tomography has been used to spatially map the birefringence of equine articular cartilage. The polar orientation of the collagen fibers relative to the plane of the joint surface must be taken into account if a quantitative measurement of true birefringence is required. Using a series of images taken at different angles of illumination, we determine the fiber polar angle and true birefringence at one site on a sample of equine cartilage, on the assumption that the fibers lie within the plane of imaging. We propose a more general method based on the extended Jones matrix formalism to determine both the polar and azimuthal orientation of the collagen fibers as well as the true
birefringence as functions of depth.
Polarization-sensitive optical coherence tomography has been used to spatially map the birefringence of equine articular cartilage. Images obtained in the vicinity of visible osteoarthritic lesions display a characteristic disruption of the regular birefringence bands shown by normal cartilage. We also note that significant (e.g. ×2) variations in the apparent birefringence of samples taken from young (18 month) animals that otherwise appear visually homogeneous are found over spatial scales of a few millimeters. We suggest that whilst some of this variation may be due to changes in the intrinsic birefringence of the tissue, the 3-D orientation of the collagen fibers relative to the plane of the joint surface should also be taken into account. We propose a method based on multiple angles of illumination to determine the polar angle of the collagen fibers.
Polarization-sensitive optical coherence tomography (PSOCT) is an emerging optical imaging technique that is sensitive to the birefringence properties of tissues. It thus has applications in studying the large-scale ordering of collagen fibers within connective tissues. This ordering not only provides useful insights into the relationship between structure and function for various anatomical structures but also is an indicator of pathology. Intervertebral disk is an elastic tissue of the spine and possesses a 3-D collagen structure well suited to study using PSOCT. Since the outer layer of the disk has a lamellar structure with collagen fibers oriented in a trellis-like arrangement between lamellae, the birefringence fast-axis shows pronounced variations with depth, on a spatial scale of about 100 μm. The lamellar thickness varies with age and possibly with disease. We have used a polarisation-sensitive optical coherence tomography system to measure the birefringence properties of freshly excised, hydrated bovine caudal intervertebral disk and compared this with equine flexor tendon. Our results clearly demonstrate the ability of PSOCT to detect the outer three lamellae, down to a depth of at least 700 μm, via discontinuities in the depth-resolved retardance. We have applied a simple semi-empirical model based on Jones calculus to quantify the variation in the fast-axis orientation with depth. Our data and modeling is in broad agreement with previous studies using x-ray diffraction and polarization microscopy applied to histological sections of dehydrated disk. Our results imply that PSOCT may prove a useful tool to study collagen organisation within intervertebral disk in vitro and possibly in vivo and its variation with age and disease.
Polarization-sensitive optical coherence tomography (PSOCT) is a powerful new optical imaging modality that is sensitive to the birefringence properties of tissues. It thus has potential applications in studying the large-scale ordering of collagen fibers within connective tissues and changes related to pathology. As a tissue for study by PSOCT, intervertebral disk represents an interesting system as the collagen organisation is believed to show pronounced variations with depth, on a spatial scale of about 100 microns .We have used a polarisation-sensitive optical coherence tomography system to measure the birefringence properties of bovine caudal intervertebral disk and compared this with equine flexor tendon. The result for equine tendon, Δn = (4.4 ± 0.15) x 10-3 at 1.3μm, is somewhat larger than values reported for bovine tendon. The annulus fibrosus of freshly excised intact bovine intervertebral disk displays an identical value of birefringence, Δn = (4.4 ± 0.4) x 10-3 at 1.3μm. However the retardance does not increase uniformly with depth into the tissue but displays a pronounced discontinuity at a depth of around 300 microns. This is believed to be related to the lamellar structure of this tissue, in which the collagen fiber orientation alternates between successive lamellae as depth into the tissue increases. The nucleus pulposus displays polarization conversion equivalent to a birefringence an order of magnitude smaller than these values i.e. Delta;n = (0.278 ± 0.007) x 10-3. Our measurement protocol cannot distinguish this from the effects of depolarization due to multiple scattering. These results imply that PSOCT could be a useful tool to study collagen organisation within intervertebral disk in vivo and its variation with applied load and disease.
Polarization-sensitive optical coherence tomography (PSOCT) is a powerful new optical imaging modality that is sensitive to the birefringence properties of tissues. It thus has potential applications in studying the large-scale ordering of collagen fibers within connective tisues and changes related to pathology. As a tissue for study by PSOCT, intervertebral disk respresents an interesting system as the collagen organization is believed to show pronounced variations with depth, on a spatial scale of about 100 μm. We have used a polarization-sensitive optical coherence tomography system to measure the birefringence properties of bovine caudal intervertebral disk and compared this with equine flexor tendon. The result for equine tendon, δ = (3.0 ± 0.5)x10-3 at 1.3 μm, is in broad agreement with values reported for bovine tendon, while bovine intervertebral disk displays a birefringence of about half this, δ = 1.2 x 10-3 at 1.3 μm. While tendon appears to show a uniform fast-axis over 0.8 mm depth, intervertebral disk shows image contrast at all orientations relative to a linearly polarized input beam, suggesting a variation in fast-axis orientation with depth. These initial results suggest that PSOCT could be a useful tool to study collagen organization within this tissue and its variation with applied load and disease.
Polarization-sensitive optical coherence tomography is a new important technique in biomedical imaging. To describe PS- OCT we have developed a Monte Carlo model for polarized light propagation in a multiple layered birefringent scattering medium based on the Jones formalism. Our algorithm makes it possible to derive the depth-resolved Stokes vector and Mueller matrix, which provides a compete characterization of the optical polarization properties of a biological tissue.
The application of polarization-sensitive optical coherence tomography (PS-OCT) creates new possibilities for biomedical imaging. In this work we present a numerical simulation of the signal from a PS-OCT interferometer. We explore the possibility to retrieve information concerning the optical birefringence properties of multiple layered tissues from the depth-resolved PS-OCT interferometric signal, in the presence of strong elastic light scattering. Our simulation is based on a Monte Carlo algorithm for the propagation of polarized light in a birefringent multiple scattering medium. Confocal and time-gated detection are also included. To describe the polarization state of light we use the Jones formalism, which reduces the calculation time compared with the full Stokes-Mueller formalism. To analyze the polarization state of the partially polarized backscattered light we applied a standard method using the Stokes vector, which is derived from the Jones vector. In this work we examined the Stokes vector variations with depth for the different tissues types. The oscillations of the Stokes vector are clearly demonstrated in the case of uniform birefringent medium. We also investigated a two-layered tissue, with a different birefringence of each layer. The Stokes vector variation with depth is compared to the uniform case and used to assess the depth-sensitivity of PS-OCT. Our simulation results are also compared with published experimental results of other groups.
The article proves the problem of determining the mechanical vibration amplitudes of complicated movements by optical homodyne interferometry to be incorrect. The possibility of its approximate solution by the least-squares method is presented. The above method usage for the restoration of mechanical system vibration spectrum has been verified.
The possibility of determination of the complicated form of the object motion by the homodyne system has been shown. The method is based on the use of Fourier representation of the function of the object motion. The results of the experimental data processing for a nonharmonic vibrating mirror have been presented.
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