Quantitative Optical Imaging and Spectroscopy of Biologi

Child Health And Human Development

Investigators

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Abstract

Scientists in the Section on Biomedical Stochastic Physics (SBSP) devise quantitative theories, develop methodologies, and design instrumentation to study biological phenomena whose properties are characterized by elements of randomness in both space and time. The research focuses on developing quantitative theories applicable to quantitative optical spectroscopy and tomographic imaging of tissues. This requires analyzing different optical sources of contrast such as endogenous or exogenous fluorescent labels, absorption (e.g., hemoglobin or chromophore concentration), and/or scattering. SBSP researchers design and conduct experiments and computer simulations to validate theoretical findings. In addition, collaborations formed with other scientists at the NIH and researchers around the country and world investigate physiological sites where optical techniques might be clinically practical and offer new diagnostic knowledge and/or less morbidity than existing diagnostic methods.[unreadable] [unreadable] Biological tissues often exhibit characteristic regular features or ornamental patterns. Transition from normal tissue function to diseased tissue can be detected by quantifying irregular patterns. The degree of statistical similarities in a region of interest can carry valuable comparative information about the structural features of the tissue and can help to characterize tissue, i.e., analyze disease localization and progression. To visualize subsurface structural features of biological tissues, we have developed a user-friendly polarization imaging system that simultaneously images cross- and co-polarized light. We have developed a quantitative statistical tool, based on Pearson correlation coefficient analysis to enhance the image quality and reveal regions of high statistical similarities within the noisy tissue images. We have shown that under certain conditions, such maps of the correlation coefficient are determined by the textural character of tissues and not the choice of the reference image region, providing information on tissue structure. As an example, the subsurface texture of a demineralized tooth sample was enhanced from a noisy polarized light image. Many biological tissues (muscle, skin, white matter in brain, etc.) are known to be anisotropic, i.e., photons tend to migrate preferentially along fibers. To consider the effects of tissue anisotropy on observed characteristics of fluorescent light, we have generalized our random walk analysis of light propagation in the anisotropic turbid media for the case of a deeply embedded small fluorophore or scattering inclusion with special focus on the time-resolved measurement set-up. Our goal is to find an analytical expression for the expected change in the photon mean time of flight due to the presence of such an abnormality.[unreadable] [unreadable] Fluorophore lifetime imaging is a promising tool for studying tissue environment such as tumors. The lifetime (time for an electron to return from excited state to initial state) of a fluorophore can vary in response to changes in the immediate environment such as temperature, pH, tissue oxygen content, nutrient supply, and bioenergetic status. Mapping the lifetime and location of a fluorophore in tissue at different depths can be used to monitor such parameters. Toward this goal, we have developed a time-resolved lifetime imaging system for in vivo small animal studies that maps fluorophores lifetimes. The system consists of a single source-multiple detector array that scans the surface of the tissue. Using several source-detector separations, one is able to probe different depths of the medium. In collaboration with Dr. Capala in the Radiation Oncology Program of NCI who has developed a pH sensitive dye in the near-infrared region, we have studied the tumor environment below the skin. We have demonstrated that by using simplified back projections we are able to map near surface fluorescent lifetime in vivo. Combining this with the pre-calibrated lifetime response to pH, we have shown that biologically plausible, non-invasive, quantification of pH in mouse tumors can be determined.[unreadable] [unreadable] The oncology community is testing a number of novel targeted approaches for use against a variety of cancers. With regard to monitoring vasculature, it is desirable to develop and assess noninvasive and quantitative techniques that can not only monitor structural changes, but can also assess the functional characteristics or the metabolic status of the tumor. We are testing three potential noninvasive imaging techniques to monitor patients undergoing an experimental therapy: infrared thermal imaging (thermography), laser Doppler imaging (LDI) and multi-spectral imaging. These imaging techniques are being tested on subjects with Kaposi?s sarcoma (KS), a highly vascular tumor that occurs frequently among people infected with acquired immunodeficiency syndrome (AIDS). Cutaneous KS lesions are easily accessible for noninvasive techniques that involve imaging of tumor vasculature, and they thus represent a tumor model in which to assess certain parameters of angiogenesis. The KS studies are ongoing clinical trials under four different NCI protocols.[unreadable] [unreadable] Thermography graphically depicts temperature gradients over a given body surface area at a given time. LDI can more directly measure the net blood velocity of small blood vessels in tissue, which generally increases as blood supply increases during angiogenesis. NIRS is most closely related to visual assessment. In collaboration with Dr. Demos at the Lawrence Livermore National Laboratory, a portable spectral imaging system was designed that captures images with a high-resolution CCD camera at six near-infrared wavelengths (700, 750, 800, 850, 900, and 1000 nm). Collected intensity images are used in a mathematical optical model of skin containing two layers: an epidermis and much thicker, highly scattering dermis. Each layer contains major chromophores that determine absorption in the corresponding layer and the layers together determine the total reflectance of the skin. Local variations in melanin, oxygenated hemoglobin (HbO2), and blood volume are reconstructed through a multivariate analysis. [unreadable] [unreadable] High-resolution confocal laser microscopy is an intensively active field in modern bioimaging technologies because this technique provides sharp, high-magnification, three dimensional imaging with submicron resolution by non-invasive optical sectioning and rejection of out-of-focus information. We have developed a simple fiber-optic confocal microscope with nanoscale depth resolution beyond the diffraction barrier. It is based on combining the advanced properties of a simple apertureless single-mode-fiber confocal microscope design that provides highly sensitive diffraction-free Gaussian point light source/receiver, and a differential confocal microscope approach in which the sharp diffraction free slope of the axial confocal response curve is exploited. [unreadable] [unreadable] We have also developed an algorithm to enhance diffraction-limited images and obtain information on features smaller than the diffraction limit. Our algorithm tries to infer the best estimate of an object based on the diffraction-limited input image. Imaging an object with a diffraction-limited lens introduces in a blurred image, where neighboring pixels on the camera are correlated. The correlations between pixels are determined by the point spread function (PSF) of a diffraction-limited lens.