Properties of photon density waves in multiple-scattering media

Applied Optics (Impact Factor: 1.78). 02/1993; 32(4):607-16. DOI: 10.1364/AO.32.000607
Source: PubMed


Amplitude-modulated light launched into multiple-scattering media, e.g., tissue, results in the propagation of density waves of diffuse photons. Photon density wave characteristics in turn depend on modulation frequency (omega) and media optical properties. The damped spherical wave solutions to the homogeneous form of the diffusion equation suggest two distinct regimes of behavior: (1) a high-frequency dispersion regime where density wave phase velocity V(p) has a radicalomega dependence and (2) a low-frequency domain where V(p), is frequency independent. Optical properties are determined for various tissue phantoms by fitting the recorded phase (?) and modulation (m) response to simple relations for theappropriate regime. Our results indicate that reliable estimates of tissue like optical properties can be obtained, particularly when multiple modulation frequencies are employed.

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Available from: Lars O. Svaasand, Mar 31, 2015
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    • "Examples of forward models include analytical models using the diffusion approximation to solve the radiative transport equation [8–14], numerical models based on Monte Carlo (MC) simulations [15–20] and empirical models [5,21]. Instruments used to measure diffuse reflectance for the quantification of tissue optical properties can be categorized into time-resolved [22], frequency-domain [23] and steady-state methods [5,8–21]. Time-resolved and frequency-domain approaches are limited to near-infrared wavelengths with relatively large penetration depths (>2 mm) and are not be suitable for detecting optical properties of epithelium and superficial stroma where epithelial precancer and early cancer reside. "
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    ABSTRACT: We applied hyperspectral imaging to measure spatially-resolved diffuse reflectance spectra in the visible range and an iterative inversion method based on forward Monte Carlo modeling to quantify optical properties of two-layered tissue models. We validated the inversion method using spectra experimentally measured from liquid tissue mimicking phantoms with known optical properties. Results of fitting simulated data showed that simultaneously considering the spatial and spectral information in the inversion process improves the accuracies of estimating the optical properties and the top layer thickness in comparison to methods fitting reflectance spectra measured with a single source-detector separation or fitting spatially-resolved reflectance at a single wavelength. Further development of the method could improve noninvasive assessment of physiological status and pathological conditions of stratified squamous epithelium and superficial stroma.
    Full-text · Article · Apr 2011 · Biomedical Optics Express
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    • "In this case, a boundary light source is intensity modulated, leading to propagation of so-called photon density waves in tissue. Photon density waves have been used in many applications to characterize optical properties of various scattering media including biomedical tissues [1,32]. We have previously developed a general mathematical framework for modeling these photon density waves [4]. "
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    ABSTRACT: We present the first algorithm for solving the equation of radiative transfer (ERT) in the frequency domain (FD) on three-dimensional block-structured Cartesian grids (BSG). This algorithm allows for accurate modeling of light propagation in media of arbitrary shape with air-tissue refractive index mismatch at the boundary at increased speed compared to currently available structured grid algorithms. To accurately model arbitrarily shaped geometries the algorithm generates BSGs that are finely discretized only near physical boundaries and therefore less dense than fine grids. We discretize the FD-ERT using a combination of the upwind-step method and the discrete ordinates (S(N)) approximation. The source iteration technique is used to obtain the solution. We implement a first order interpolation scheme when traversing between coarse and fine grid regions. Effects of geometry and optical parameters on algorithm performance are evaluated using numerical phantoms (circular, cylindrical, and arbitrary shape) and varying the absorption and scattering coefficients, modulation frequency, and refractive index. The solution on a 3-level BSG is obtained up to 4.2 times faster than the solution on a single fine grid, with minimal increase in numerical error (less than 5%).
    Full-text · Article · Oct 2010 · Biomedical Optics Express
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    • "Broadband DOS combines multi frequency domain photon migration (FDPM) with broadband steady state (SS) spectroscopy. Multi-frequency FDPM separates the effects of absorption and scattering in tissues (Fishkin and Gratton, 1993; Tromberg et al., 1993). Specifically, we have used a P1 approximation to the transport equation in the semi-infinite geometry using an extrapolated boundary condition (Fishkin et al., 1996; Haskell et al., 1994). "
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    ABSTRACT: Structural changes in water molecules are related to physiological, anatomical and pathological properties of tissues. Near infrared (NIR) optical absorption methods are sensitive to water; however, detailed characterization of water in thick tissues is difficult to achieve because subtle spectral shifts can be obscured by multiple light scattering. In the NIR, a water absorption peak is observed around 975 nm. The precise NIR peak's shape and position are highly sensitive to water molecular disposition. We introduce a bound water index (BWI) that quantifies shifts observed in tissue water absorption spectra measured by broadband diffuse optical spectroscopy (DOS). DOS quantitatively measures light absorption and scattering spectra and therefore reveals bound water spectral shifts. BWI as a water state index was validated by comparing broadband DOS to magnetic resonance spectroscopy, diffusion-weighted MRI and conductivity in bound water tissue phantoms. Non-invasive DOS measurements of malignant and normal breast tissues performed in 18 subjects showed a significantly higher fraction of free water in malignant tissues (p < 0.0001) compared to normal tissues. BWI of breast cancer tissues inversely correlated with Nottingham-Bloom-Richardson histopathology scores. These results highlight broadband DOS sensitivity to molecular disposition of water and demonstrate the potential of BWI as a non-invasive in vivo index that correlates with tissue pathology.
    Full-text · Article · Dec 2008 · Physics in Medicine and Biology
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