Research Highlights

Previously, we reported the design of a new diffusing probe that employs a standard two-layer diffusion model to recover the optical properties of turbid samples. This particular probe had a source-detector separation of 2.5mm and performance was validated with Monte Carlo simulations and homogeneous phantom experiments. The goal of the current study is to characterize the performance of this new method in the context of two-layer phantoms that mimic the optical properties of human skin. We analyze the accuracy of the recovered top layer optical properties and their dependences on the thickness of the top layer of two-layer phantoms. Our results demonstrate that the optical properties of the top layer can be accurately determined with a 1.6mm source-detector separation diffusing probe when this layer thickness is as thin as 1mm. Monte Carlo simulations illustrate that the interrogation depth can be further decreased by shortening the source-detector separation.

Using our new diffusing probes, we have characterized two-layer silicone phantoms having various top layer thicknesses. Measurement results indicate that the recovered top layer optical properties, μ_a and μ_s', deviate from true values within 5% when the top layer thickness is larger than 2mm. As the top layer thicknesses decreases to 1mm, the deviation of recovered optical properties is strongly dependent on the source-detector separation of the diffusing probe and the optical properties of samples under investigation. Our measurement results indicate that the errors of optical properties recovered from a 1mm top layer thickness two-layer phantom are less than 15% when a diffusing probe of 1.6mm source-detector separation is employed. We observed that interrogation depth decreases as the source-detector separation decreases and as the absorption coefficient of the sample increases. Monte Carlo simulations support our experiment results qualitatively. Simpson et al. indicated the dermis sample they used were from human abdomen and breast and had thickness in the range from 1.5mm to 2m. From our measurement and simulation results, it is reasonable to infer that the interrogation region is located primarily in the dermis layer if either the 1.6mm or 3mm source-detector separation diffusing probe were employed to measure skin sites at abdomen and breast. Further reduction in interrogation depth can be achieved by shortening the source-detector separation of diffusing probe. We will employ our diffusing probe to determine optical properties of in-vivo normal skin and abnormal skin, such as melanoma and port wine stain, in the near future.

Figure 1. A diffusing probe on a two-layer phantom.

Figure 2. (a) μ_a and (b) μ_s' recovered from light skin two-layer phantoms, and (c) μ_a and (d) μ_s' recovered from dark skin two-layer phantoms. The thickness of the top layer of the two-layer phantoms varies from 1mm to 8mm. Two diffusing probes having source-detector separations of 3mm (solid squares and solid circles) and 1.6mm (squares and circles) were employed. Dash-dot lines represent optical properties of the substrate of two-layer phantoms.

Figure 3. Interrogation depth versus diffusing probe's source-detector separation determined from Monte Carlo simulations. Samples have homogeneous semi-infinite geometry. Solid triangles and open triangles represent the interrogation depths of the probe measuring samples having optical properties of light skin and dark skin, respectively.

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