The reliability of optical techniques for non-invasive monitoring of glucose can be
significantly improved by the deployment of a subcutaneous implantable sensor that can closely
track the changes in the local concentration of glucose in skin. We have developed a novel
implantable sensor that can track glucose-induced changes in the optical turbidity of the implant.
In this sensor, optical turbidity decreases significantly with increased glucose concentrations.
We performed comparative measurements by optical coherence tomography (OCT) used to
monitor backscattering or specular reflection originated from specific structures within the sensor
and by collimated light transmission measurement technique to measure the changes in light
attenuation as function of glucose concentration within the sensor as well as when the sensor was
implanted in phantom media or in tissue samples. These measurements showed that glucose-induced
changes in the transmission values derived from OCT monitoring of the sensor turbidity differed up
two times from those obtained by collimated transparency measurement (CTM) technique. These
results were used to determine the values for scattering coefficients of tissue and the sensor and to
estimate the relative loss in sensor sensitivity as a function of implantation depth in tissue. The
results suggest that the implantable sensor can be placed in turbid medium such as skin up to an
optical depth of 12 mean free paths (mfp), one could expect. For a turbid medium such as skin with a
scattering coefficient (µs ) of 10mm-1, this would result in geometrical depth of implantation at 1.2
mm beneath the tissue where sensor sensitivity of 50% or higher is expected. The study
demonstrates that it could be feasible to engineer a novel optical sensor for glucose monitoring that
can be implanted under the skin while providing a high degree of sensitivity and specificity for non-invasive
glucose monitoring.
Results of comparative ex vivo measurements of backscattering from rabbit cornea made by optical coherency tomography and confocal microscopy strongly suggests the existence of small angle backscattering from the cornea. Apparently, this is associated with a zero order peak of diffraction on the corneal fibril "lattice" structure, which was first suggested by D. Maurice in 1957 as a physical basis of corneal transparency. By using water suspension of polystyrene spheres as a scattering standard, absolute values of backscattering coefficients of normally hydrated cornea for wide and small angle backscattering and degree of fibrils arrangement could be estimated.
Recently we reported measurements of the corneal in-depth light backscattering distribution (CDLBD) by optical coherence tomography (OCT) as a possible tool for investigation of the corneal hydration and estimation of water gradients inside the cornea.
In this paper, we present additional results demonstrating a strong correlation (R = 0.99) between the amplitude of light backscattering as measured by OCT and the corneal thickness. In contrary to the well-known effect of the immediate increase observed in corneal opacity during corneal swelling, we observed an initial decrease of the amplitude of light backscattering as measured by OCT in the anterior part of the stroma during the swelling process. The possible explanation for this observation is discussed. Also methodological improvements for accurate measurements of the CDLBD in vivo are outlined.
The knowledge of water content of the cornea (hydration level H) can provide crucial information for the assessment of corneal function. The correlation between the corneal thickness and its hydration enables us to estimate H indirectly by measuring changes in corneal thickness and scattering using OCT. The magnitude and axial distribution of the backscattering signal from the cornea yields additional information about the hydration gradient across the cornea. We present data on the effect of corneal hydration on its thickness and scattering in natural processes of de- and rehydration, as well as in stress tests with the use of glycerol-based dehydrating agent Ophthalgan. Our data demonstrate that scattering signal changes up to 50 times when corneal thickness varies from 60% to 200% of its normal state. The distribution of scattering intensity across the cornea also depends on the hydration level and gradient of the water distribution. Thus, OCT can provide a noninvasive and non-contact method for safe and fast measurement of thickness and optical properties of the cornea, and therefore, for estimation of corneal hydration level and corneal function.
Noninvasive monitoring of analytes can be performed with optical coherence tomography (OCT) technique. This technique may allow measurement of optical properties of tissue (attenuation, scattering, optical thickness, etc.) that may be dependent on analyte concentration. Accurate monitoring of analyte concentration requires measurement of the optical properties with high accuracy. The accuracy of measurements depends on OCT technical characteristics and the level of speckle noise. In this paper, we report the results of the calibration of OCT system sensitivity for absolute and relative measurements of the backscattering and total attenuation coefficients in scattering standard, tissue phantoms (suspensions of polystyrene microspheres in water solutions of glucose), and human skin. We measured the OCT sensitivity as a function of depth and used this dependence for correction of signals. The amplitude and spatial period of backscattered signal modulation resulted from speckle noise were measured for the scattering standard and human skin. The dependence of speckle and electronic noise on the range of spatial and temporal averaging of OCT signals was determined. Our studies show that the accuracy of measurement of changes in optical properties of tissue with OCT technique can be significantly improved by reducing of speckle noise and by using the signal correction algorithm.
A totally computerized multifunctional laser gas analyzer operating within the 3-μm spectral region is described. A technique and the computer program for laser gas analysis experimental data processing are proposed for the case of multicomponent gas mixtures with complex overlapping spectra. The application of this device for IR absorption high resolution spectroscopy, environmental control (long-path gas analysis), and disease diagnosis is discussed.
KEYWORDS: Gas lasers, Spectroscopy, Absorption, Dye lasers, Control systems, Chemical analysis, Data processing, Absorption spectroscopy, Spectroscopes, Nonlinear crystals
A totally computerized multi-function laser gas analyzer operating within 3 micrometers spectral region is described. The application of this device for IR absorption high resolution spectroscopy, for environmental control (long-path gas analysis), and for diseases diagnosis is discussed.
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