In numerous medical and scientific fields, knowledge of the optical properties of tissues can be applied. Among many different ways of determining the optical properties of turbid media; integrating sphere measurements are widely used. However, this technique is associated with bulky equipment, complicated measuring techniques, interference compensation techniques, and inconvenient sample handling. This paper describes measurements of the optical properties of porcine brain tissue using novel instrumentation for simultaneous absorption and scattering characterization of small turbid samples. The system used measures both angularly and spatially resolved transmission and reflection and is called Combined Angular and Spatially-resolved Head (CASH) sensor. The results compare very well with data obtained with an integrating sphere for well-defined samples. The instrument was shown to be accurate to within 12% for μa, and 1% for μs' in measurements of intralipid-ink samples. The corresponding variations of data were 17%, and 2%, respectively. The reduced scattering coefficient for porcine white matter was measured to be 100 cm-1, while the value for coagulated brain tissue was 65 cm-1. The corresponding absorption coefficients were 2 and 3 cm-1, respectively.
KEYWORDS: Calibration, Optical properties, Sensors, Monte Carlo methods, Diffuse reflectance spectroscopy, Tissue optics, Absorption, Scattering, Semiconductor lasers, Digital signal processing
We present a versatile and compact fiber optic probe for real- time determination of the absorption and the reduced scattering coefficients from spatially resolved continuous wave diffuse reflectance measurements. The probe collects the diffuse reflectance at six distances in the range 0.6 - 7.8 mm at four arbitrary wavelengths, which were 660, 785, 805, and 974 nm in these experiments. The maximum sampling rate for one cycle of measurements including all four wavelengths is about 100 Hz. The absorption and the reduced scattering coefficients are extracted real-time from the probe measurements using multivariate calibration methods based on multiple polynomial regression and Newton-Raphson algorithms. The system was calibrated on a 6 X 7 matrix of Intralipid/ink phantoms with optical properties within typical biological ranges, e.g. at 785 nm, the ranges of the absorption and the reduced scattering coefficients, were 0 - 0.3/cm and 6 - 16/cm, respectively. Cross-validation tests showed that the mean prediction error, relative to the ranges of absorption and the reduced scattering coefficients were 2.8% and 1.3%, respectively.
We consider two different skin structure models. The first structure consists of epidermis, dermis/blood, and subcutaneous tissue. The second structure consists of epidermis/dermis, adipose tissue and muscle tissue. A new solution based on diffusion theory of the cw local diffuse reflectance from a three-layered skin tissue structure is presented. Comparisons with Monte Carlo simulations are carried out favorably. It is shown that the functional form of the radial dependence of the diffuse reflectance from multilayer and single layer models are identical. We use a modified expression originating from diffusion theory to fit the diffuse reflectance. We discuss the sensitivity of the local diffuse reflectance as a function of the optical properties of separate layers in both structures. Moreover, we investigate the influence on the local diffuse reflectance with changes in the optical properties corresponding to normal changes in tissue glucose concentration and blood volume. The necessity of multilayer models lies within their ability to provide a detailed description of the light-tissue interaction rate than their applicability to practical data analysis of the local diffuse reflectance measurements.
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