Abstract
Accurate determination of in-vivo light fluence rate is critical for preclinical and clinical studies involving photodynamic therapy (PDT). The light fluence distribution in tissue depends on both the tissue optical properties and the incident field size. This study compares the longitudinal light fluence distribution inside biological tissue in the central axis of circular uniform light field with different radii for a range of in-vivo tissue optical properties (absorption coefficients (µa) between 0.01 and 1 cm-1 and reduced scattering coefficients (µs') between 2 and 40 cm-1). This was done using Monte-Carlo simulations for a semi-infinite turbid medium in an air-tissue interface. The end goal is to develop simple analytical expressions that would fit the results from the Monte Carlo simulation for circular beams with different radii. A 6-parameter model (ϕ/ϕair=(1-b⋅e-λ1d)(C2e-λ2d+C3e-λ3d)) can be used to fit MC simulation. Each of these parameters (b, C2, C3, λ1, λ2, and λ3) is expressed as a function of tissue optical properties and beam radius. These results can then be compared against the existing expressions in the literature for broad beam for analysis in both accuracy and applicable range. The analytical function can be used as rapid guide in PDT to calculate in vivo light fluence distribution for known tissue optical properties.
Highlights
Light propagation in tissue has been extensively studied for the past three decades to understand how light energy is distributed and absorbed within the target tissue
For light fluence rate of broad beams, we have found that all four fitting parameters in Eq (4) are one-dimensional function of Rd as used by Jacques [6], where Rd is the diffuse reflectance of broad beam
Simple analytical expressions are provided to determine the parameters as a one dimensional function of diffuse reflectance, Rd, for broad beam and as at least two dimensional functions of tissue optical properties, μa and μs’, or diffuse reflectance and beam radius, r
Summary
Light propagation in tissue has been extensively studied for the past three decades to understand how light energy is distributed and absorbed within the target tissue. This technique is limited to measurement of light fluence rate at selected points or along a catheter rather than in a full three-dimensional volume. Model calculations such as Monte Carlo simulation [2] and diffusion approximation theory [3] are more practical and have been widely employed to estimate the light distribution in tissue noninvasively, given that the 3D distribution of the tissue optical properties is known. Diffusion theory is attractive due to its simplicity in calculating light fluence rate quickly and accurately in scattering media It has been found useful in various tissue optics applications as its validity condition of μa
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