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Abstract:
Background and objective: This paper is focused on the analysis of a method of estimation of the absorption and scattering coefficients of nonhomogeneous two layered structures in time resolved near infrared spectroscopy using method based on cumulative distributions of time of flight of photons. Methods: The research is based on the analysis of the superposition of cumulative distributions of time of flight of photons. This approach allows for detailed analysis of small variations in characteristics of time of flight of photons caused by an electromagnetic propagation in highly scattering non-homogeneous media. The method presented, based on the variation of statistical minimum distance estimation, is compared to the method of standard curve fitting. It is analyzed by fitting the results obtained from Monte-Carlo simulations of light propagation in the turbid medium to the data from the simulated measurements. Results: The analysis is carried out for a vast range of optical properties of two layered medium in reflectance geometry. Conclusions: The method allows the estimation of the optical parameters despite the noise in the measured signal, with higher accuracy and generally with smaller number of error function evaluations.

Keywords:
optical properties, approximation, light propagation, time of flight of photons, Monte-Carlo simulations, cumulative distributions

Abstract:
A multi-center study has been set up to accurately characterize the optical properties of diffusive liquid phantoms based on Intralipid and India ink at near-infrared (NIR) wavelengths. Nine research laboratories from six countries adopting different measurement techniques, instrumental set-ups, and data analysis methods determined at their best the optical properties and relative uncertainties of diffusive dilutions prepared with common samples of the two compounds. By exploiting a suitable statistical model, comprehensive reference values at three NIR wavelengths for the intrinsic absorption coefficient of India ink and the intrinsic reduced scattering coefficient of Intralipid-20% were determined with an uncertainty of about 2% or better, depending on the wavelength considered, and 1%, respectively. Even if in this study we focused on particular batches of India ink and Intralipid, the reference values determined here represent a solid and useful starting point for preparing diffusive liquid phantoms with accurately defined optical properties. Furthermore, due to the ready availability, low cost, long-term stability and batch-to-batch reproducibility of these compounds, they provide a unique fundamental tool for the calibration and performance assessment of diffuse optical spectroscopy instrumentation intended to be used in laboratory or clinical environment. Finally, the collaborative work presented here demonstrates that the accuracy level attained in this work for optical properties of diffusive phantoms is reliable.

Abstract:
Optical technique based on diffuse reflectance measurement combined with indocyanine green (ICG) bolus tracking is extensively tested as a method for clinical assessment of brain perfusion in adults at the bedside. Methodology of multiwavelength and time-resolved detection of fluorescence light excited in the ICG is presented and advantages of measurements at multiple wavelengths are discussed. Measurements were carried out: 1. on a physical homogeneous phantom to study the concentration dependence of the fluorescence signal, 2. on the phantom to simulate the dynamic inflow of ICG at different depths, and 3. in vivo on surface of the human head. Pattern of inflow and washout of ICG in the head of healthy volunteers after intravenous injection of the dye was observed for the first time with time-resolved instrumentation at multiple emission wavelengths. The multiwavelength detection of fluorescence signal confirms that at longer emission wavelengths, probability of reabsorption of the fluorescence light by the dye itself is reduced. Considering different light penetration depths at different wavelengths, and the pronounced reabsorption at longer wavelengths, the time-resolved multiwavelength technique may be useful in signal decomposition, leading to evaluation of extra- and intracerebral components of the measured signals

Keywords:
indocyanine green bolus, near-infrared spectroscopy, fluorescence, time-resolved spectra, multiwavelength detection