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.

Optical coherence tomography (OCT) is an emerging alterative imaging tool to confocal microcopy for diagnosing turbid tissue. In layers beyond about 200 to 300 micrometer depth, an increasing fraction of multiple scattered photons begins to deteriorate diffraction limited axial and lateral resolution curves, which otherwise can only be obtained in very superficial layers where the single scattering regime prevails. At greater depths, OCT images suffer contrast (and resolution) degradation due to multiple scattering. Recently, we have developed an analytical model to describe spatial point-spread function (PSF) curves in homogeneous turbid media base on the interferometric principle. It is shown that the parameter mean scattering angle can be derived with reasonable accuracy under the small-angle approximation (SAA) at a given (average) scattering coefficient. Axial PSF curves were acquired with our OCT interferometer in reflection mode to characterize skin tissue in vivo by fitting simulated curves to the experimental data. Mechanical through-focus translation of the focusing objective (around particular penetration depth) generated a single contrast arising from the single and multiple scattered photons. We made two assumptions: (1) the tissue is homogeneous on average and (2) this particular contrast is independent of the type of backscattering (on average). The latter assumption was approximately validated by simulations. Skin tissue probed at 300 and 400 micrometer penetration depth yielded a mean scattering angle (theta) _RMS approximately equals 4 degrees at an average scattering coefficient of (mu) _s approximately equals 11 mm^-1. The small angle value indicates strong forward scattering from large particles.

A new near-infrared coherent imaging technique that can reveal scattering bodies embedded in highly scattering media is presented. Its underlying principle is extended from frequency modulated continuous wave radar systems. This technique has advantages over low coherence tomography as it does not require the reference mirror to be scanned. The tunable laser is characterized and the system's performance is demonstrated on images recorded from solid scattering phantoms. Furthermore a combination of our chirp-tomography (C-OCT) and laser Doppler perfusion imaging (LDPI) is demonstrated. The influence of moving scatterers on the tomographic images are discussed.

The resolution of partial coherence interferometry and optical coherence tomography depends on the spectral properties of the light source used. The minimum distance that can be resolved by these techniques is inversely proportional to the spectral width of the light source. Therefore strong efforts towards using light sources with larger spectral width are presently in progress. However, if the tissue under investigation is dispersive, the interferogram broadens and the resolution decreases. Based on properties of existing light sources, we calculate this signal broadening for trials with different dispersion and compare the results with measurements. On the other hand, light sources with different central wavelengths can be used to measure the group dispersion of a tissue. In this case, the interferograms obtained by partial coherence interferometry are shifted with respect to each other by an amount determined by the group dispersion. Using two superluminescent diodes with wavelengths of 814 and 855 nm, we present results of group dispersion measured in different media of human eyes in vivo. Based on these results we recommend optimum light sources for intraocular ranging.

A high speed technique for performing 'optical biopsies,' or optical diagnostic imaging of in vivo tissue architectural morphology, would greatly enhance the diagnosis and clinical management of many diseases. Optical coherence tomography (OCT) is a novel optical imaging technique that uses low coherence interferometry to obtain micron scale, cross- sectional images of biological systems. OCT was initially applied in ophthalmology to provide high resolution, cross sectional, tomographic images of the transparent structures in the eye and clinical studies show that OCT has considerable promise for the diagnosis of a wide range of retinal macular diseases. OCT imaging in other human tissues is more difficult due to optical scattering. However, recent in vitro studies have shown that OCT can image architectural morphology in highly optically scattering tissues. One of the key technological issues for OCT in optical biopsy is the development of low coherence laser sources. Essential attributes of a clinically viable light source for OCT include high single-transverse-mode power, short coherence length, and a central wavelength optimal for deep penetration within human tissue. Passively mode locked solid state lasers based on Ti:Al₂O₃ and Cr:Mg₂SiO₄ are capable of providing hundreds of milliwatts of single-transverse mode light with coherence lengths as short as 1.8 microns. We present recent developments in the optimization of mode locked solid state lasers for application to OCT and demonstrate the resulting capability to enable fast acquisition of high resolution tomographic images.

Subsurface images of biological tissue obtained by optical coherence tomography (OCT) lack of contrast and are corrupted by coherent noise. In this study we investigated model-based deconvolution methods designed for improving the quality of optical-coherence tomograms of living skin. The methods incorporate a priori information about the point-spread function of the imaging optics, as well as optical properties of the tissue. Deconvolution of the aberrated point-spread function was carried out by using CLEAN, an iterative point reconstruction method. A modification of the standard CLEAN algorithm based on a Wiener filter was made to reduce corrugation artifacts in images of densely packed clusters of scatterers. The algorithms were evaluated first on simulated one-dimensional data arrays and then applied to two- dimensional optical coherence tomograms of skin. Our results suggest that significant improvement in image contrast and resolution can be achieved with the deconvolution algorithm.

In this report we present the results on optical coherence tomography (OCT) imaging of human skin with pigmented lesions and compare these results with the postexperimental histological analysis. We demonstrate that beside the proposed limitation of the image depth due to a strong absorption of the probing radiation, the melanin-containing layers of tissues are characterized by an increased backscatter. It is especially interesting to note that after a proper contrasting of OCT pictures, fine spatial distributions in the scattering cell conglomerations become distinguishable. As an example we show the case of the border-line nevus where, in accordance with the histological data, we have succeeded in localizing a melanin containing cell distribution over the all basal membrane on the in vivo recorded tomographs. The peculiarities of the studied OCT images can be explained by characteristics of light scattering by a single living cell. We have performed a computer simulation of electromagnetic wave propagation in a single cell using the full set of the Maxwell equations with known dielectric constants for various intracellular components. A particular result of this modeling is that due to a noticeable difference in dielectric constants between the cytoplasm and the melanin granules, a substantial increase of the cell backscattering cross-section takes place.

We discuss different modifications of white light interferometry, for the acquisition of human skin morphology. In a first experiment we display the diffusion of light within tissue, versus time. Light is focused onto the surface of the sample, penetrates the sample, is scattered and partly emerges from the surface again. For each point of the surface we can measure a certain run time profile of the emerging photons, via the speckle contrast. The local scattering behavior of the skin is encoded in the run time profile. Further we present a sensor for the acquisition of cross-sectional images of volume scatterers, we call it 'spectral radar.' The scattering amplitude a(z) along one vertical axis from the surface into the bulk can be measured within one exposure. No reference arm scanning is necessary, hence a short measurement time is possible. The depth uncertainty within a range of 1000 micrometer is about 10 micrometer. In first measurements we distinguished a melanoma maligna from healthy skin, in vitro and we measured the thickness of a fingernail in vivo. We further demonstrate a third method, the 'coherence radar' for in vivo measurements of skin surface topology, with an accuracy of a few micrometers, and a field of 512 by 512 pixels.

Optical low coherence tomography (OCT) is a promising new method for non invasive, in vivo measurements of biological high scattering tissue. Crossectional images with microscale resolution in a range of about 1 5tm can be produced. A scanning point detection system is realized which combines an interferometric method with an endoscope. A superluminescence diode with a center wavelength of 830nm and a coherence length of l5jtm is coupled into the interferometrical setup. The backscattered light from a tissue sample and the reflected light from the scanning reference mirror is recombined at the detector. Interference occurs only if the pathlength difference is within the coherence length of the light source. Heterodyne detection is used to obtain high speed and high dynamic range measurements of the interferometric signals. We present in vivo OCT skin measurements, where we analyze the borders between the upper skin layers. We describe the influence to the light attenuation inside the skin after treating the skin with oil. We compare in vivo OCT skin measurements with in vitro skin measurements. Additionally first in vivo skin measurements of malignant melanoma show that OCT is a promising diagnostic method in dermatology. Keywords: skin cancer, optical coherence tomography, in vivo skin measurements, endoscope

In the past ten years, the dual beam version of partial coherence interferometry has been developed for measuring intraocular distances in vivo with a precision on the order of 0.3 to 3 micrometer. This technique has now been further improved by using diffractive optics. A special diffractive optical element focuses part of the laser beam on the vertex of the cornea and lets the other collimated parallel part of the beam pass through. The beams remitted from the eye will thereby be converted into parallel beams. The light power oscillations in the corresponding interferograms are much stronger than those of the narrow interference fringes obtained without that technique what significantly improves the signal to noise ratio. This makes it possible to clearly differentiate signals from different fundus layers. High precision in vivo fundus measurements have been performed at various positions on the human retina in order to obtain fundus profiles. These measurements have been synthesized to tomographic images of the human eye fundus. In order to localize the exact measurement point on the retina simultaneously to the fundus scans, a fundus camera has been implemented into the partial coherence interferometry system that allows a clear identification of the individual A-scan positions.

Optical coherent tomography (OCT) enables one to follow the pulse-to-pulse kinetics of laser interactions with turbid biological tissues. In experiments we investigate the effect of free running mid infrared laser radiation of different wavelengths on a cataract-suffered human lens in vitro. Different regimes of laser ablation and preablation surface transformations are monitored in situ.

We present the results of detailed optical coherence tomographic (OCT) observations of both normal and abnormal rat neocortices obtained in vivo. A compact fast-scanning OCT device employing simultaneously two superluminescent sources of radiation at different wavelengths (0.83 and 1.28 micron) has recorded three dimensional images and performed two-color comparative analysis of the tomograms. We are able to obtain image information at 2.5 mm depths, enabling imaging through the entire rat cerebral cortex. Structures inside and at the surface of the brain have been optically detected, including the dura matter, blood vessels, and the hippocampal/cortical boundary. From these, we have recorded and stored the first OCT album of the cortex. Subsequent histologic analysis shows a good correlation with the OCT tomograms, particularly within the cortical surface layers and the boundary between gray and white matter. We are also able to detect differences between normal and abnormal cerebral structure using OCT.

The overview is summarizing speckle-correlation, speckle- interferometric, and polarimetric methods and instruments designed for tissue structure imaging and their optical parameters monitoring.

We have developed a technique, called laser speckle contrast analysis (LASCA), to monitor capillary blood flow. Like the earlier method 'single-exposure speckle photography' it uses the phenomenon of time-varying speckle, drawing on models that relate the statistics of the speckle pattern, especially the contrast, to the velocity of the scatterers. Temporal variations in the pattern blur the captured image, whether photographic or digital, reducing the contrast. LASCA is a fully-digitized, full-field, non-invasive technique and operates in quasi-real-time: it grabs an image and then produces a 2-dimensional map of contrast variations representing differing velocities. Previously the data had been stretched, as a full range of contrasts could not be achieved. This problem has been addressed and we believe solved, thus giving us a full range of contrasts. In this paper we look at the effects of changing the exposure time. If it is varied it should be possible, according to the models used, to look at different ranges of velocity. Also we hope to show that by using different wavelengths together it is possible to distinguish between flow at different depths in the skin. We illustrate the validity of our technique with some of our results and then compare these to other methods of measuring capillary blood flow.

Depolarization effects accompanying the coherent light multiple scattering by optically inhomogeneous tissues are studied for the certain cases of the light-tissue interaction. Correlation between polarization states of the scattered light fields (speckle patterns) for different stages of the transition from multiple scattering mode to the single scattering one is experimentally analyzed for the case of human sclera enlightenment process. This process can be induced by the application of the special chemical agents (e.g., Trazograph solution). Reversibility of the enlightenment is demonstrated by measuring mean intensities of the scattered light for different polarization states. Some possibilities of the usage of this approach for turbid media structure visualization are discussed.

A depth-resolved imaging system is described for recording three dimensional images of objects embedded in diffuse media. Time-gated holographic imaging, employing photorefractive multiple quantum well devices as the recording media, is used to obtain real-time whole-field depth-resolved two dimensional images. Infra-red radiation has been used which corresponds to the medical imaging window.

We report the development of an optical technique for noninvasive in vivo imaging of tissue structure and blood flow dynamics with high spatial resolution (2 - 15 micrometer) in biological systems. The technique is based on coherence optical Doppler tomography (ODT), which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations. The exceptionally high resolution of ODT allows noninvasive in vivo imaging of both blood microcirculation and tissue structures surrounding the vessel, which has significance for biomedical research and clinical applications. Tomographic velocity imaging of in vivo blood flow in a rat mesentery is demonstrated.

The problem of nondestructive testing of microstructure parameters, both aerosols and water suspension, is actual for biology, medicine, and environmental control. Among the methods of optical investigations and diagnostics of light scattering media the holographic method plays a special role. A hologram of scattering volume allows us to reproduce the optical wave field to obtain information on the parameters of microparticles: size, shape, and spatial position. Usually this is done by analysis of the particle images reconstructed from the hologram. On the basis of calculated and experimental results, characteristics of holographic methods are analyzed in this paper. These estimations demonstrate a possibility to use the above methods for investigation of media in biomedical science and clinical practice. A lot of micro-organisms and other living particles are transparent or semitransparent ones. In this case the reconstructed image of the particle will show a spot formed due to light focusing by the particle in addition to its cross section. This circumstance allowed us to propose a method of determining of refractive index of transparent and semitransparent microparticles, that, in turn, can provide identification of the particles type. The development of this method is presented. To make measurement of the size-distribution of particles one can do this simultaneously with the reconstruction of scattering optical field from the hologram. In this case a small angle optical meter (for example, focusing lens) can be placed just behind the illuminated hologram. The reconstructed field is composed of the initial one and its conjugate. Each of these components as well as interference between them can bear an additional information on the medium. The possibility of extraction of this information is also discussed.

Clinical efficacy of the low power laser (LPL) in medical treatments is still not well established. In a double blind, placebo controlled study, we tried to find out first which type of LPL is more efficient, and second if coherence is an important character for clinical efficacy. We treated 1228 patients having different rheumatic diseases, with low power diode, used as follows: A group: IR coherent diode, continuous emission, 3 mW power; B group: IR coherent diode, pulsed emission, output power about 3 mW; C group: IR noncoherent diode continuous emission 9 mW power; D group: both IR diode lasers (continuous or pulsed) and HeNe laser, continuous emission, 2 mW power; E group: placebo laser as control group. The energy dose used for every group was the same, as well as the clinical protocols. The positive results were: 66.16% for A group; 64.06% for B group; 48.87% for C group; 76.66% for D group, and 39.07% for E group. Finally, we showed that LPL is really efficient in the treatment of some rheumatic diseases, especially when red and IR diode laser were used in combination. The type of emission (continuous or pulsed) is not important, but coherence is obviously necessary for clinical efficacy.

At different pathological stages, the changes both of blood and lymph microcirculation parameters are observed. These parameters are of great importance in diagnostics. The type of these changes may indicate both the kind and the degree of disease. Investigation of the behavior of dynamic characteristics of these flows at different stages is of special interest. In this paper the peculiarities both of blood and lymph motion have been considered. The further development of speckle-interferometrical method has been carried out for the investigation of the dynamic characteristics of blood and lymph flows in microvessels. Analysis of two dynamic parameters which had been introduced in previous papers concerning this problem, is made in this paper. The influence of lymphotropic agent both on lymph flow and its dynamic characteristics is also discussed.

In the work, behavior of the elements of light scattering matrix (LSM) is investigated during the process of aggregation of proteins of the eye lens. We consider a system of spherical scatterers as a model of the lens. The Monte-Carlo method is used to trace migration and scattering of photons in a medium. Light scattering on the particles is described by the Mie formulas. We investigate influence of the multiple scattering on spatial and polarization characteristics of the scattered laser light accounting for the given geometry of the experiment and obtain angular dependencies of the LSM elements. Also the angular dependencies of weights of different orders of scattering are obtained. It is shown, that for small particle systems the influence of multiple scattering has a maximum for the scattering angle equal to 90 degrees. Finally, we discuss light depolarization in the lens.

The structural state of radioulnary bones of European mooses living in different ecological conditions has been studied by x-ray diffraction methods. The x-ray diffraction patterns were fixed at the wide angular interval including a small-angle part. Parameters of microporosity were calculated from angular distributions of small-angle scattering intensities. A width and an intensity of collagen reflections at the middle angles allowed us to define the texture perfection of structural components. Finally the spectrum of observed reflections and their angular positions permitted us to find the phase composition. It has been established that unfavorable ecological conditions cause the loosening of bone tissues and the disorientation of the structure elements. Bedsides the phase composition and the relative content of amorphous and crystalline components change. The experiments have shown that observed diffractive effects allow us to elaborate methods controlling structure state of bones.

Holographic microinterferometry method and its results of the evaluation of diffusion coefficients in microvolumes of liquids in particular those in the homologous series of saturated hydrocarbons in organic solvents are presented.

Using developed technique for acquisition of collimated transmittance spectra (over 450 - 600 nm range, collection of transmitted light within 10^-4 srad) we defined the optimum composition of the phantom (physical model reproducing optical parameters of the object) of bloodless dermis. Then the series of phantoms of different thickness was employed in testing of feasibility of gelatin gel-milk (19%) phantom for reproducing polarization properties of human skin. It was fond that the polarization degree decays two order of magnitude less intensively than intensity does, and at the thickness of the layer up to 0.4 - 0.5 mm we could expect a negligible depolarization of linearly polarized light (over 450 - 600 nm range) propagating through the tissue. This counts in favor of perspectivity of the application of polarization techniques (e.g. fluorescence) in the study of the processes occurring at fraction of millimeter depths in biotissue. But very weak decay of polarization with depth does not agree with literature data for skin slabs, this fact is attributable to the presence of significant fraction of too large scatterers in whole milk.

Holographic microscopy with conjugate reconstruction for the interferometric determination of three-dimensional displacement is described. Utilizing the advantages of conjugate reconstruction the holographic microscope has been optimized in optical and numerical parameters. Phase shifting and carrier fringe techniques have been applied for interferogram evaluation and are compared regarding measurement range, accuracy, and handling.