GOAL: This GUI Interaction aims to examine (a) the sensitivity of diffuse reflectance to optical absorption and scattering as a function of source-detector separation and time; and (b) the influence of source-detector separation and optical properties on the spatial region probed by the detected photons.

I. Sensitivity of Spatially-Resolved Diffuse Reflectance to Optical Properties

Goal: This portion of the GUI Interaction is to examine the impact of optical absorption and scattering on R(ρ).

1. On the "Forward/Analysis" Panel select: (a) Forward Model: Scaled Monte Carlo (g=0.8) and (b) Solution Domain: R(ρ).
2. Select start and stop locations to 0.5 and 9.5 mm, respectively with 19 points (every 0.5 mm).
3. In Optical Properties: enter μ_a = 0.01mm^-1, μ'_s=1mm^-1.
4. Press "Plot Reflectance" button.
5. Press "Hold On".
6. Press ∂R/∂μ_a radio button.
7. Press "Plot Reflectance" button.
8. Examine the shape, magnitude and sign of this curve.
9. Fix μ_a=0.01mm^-1. Repeat the above steps for μ'_s = 0.5 and 1.5 mm^-1.

   Question: Can you explain these results intuitively? Do the results of the ∂R/∂μ_a plots at these three different μ'_s values conform to your results in GUI Interaction C?

10. Press "Clear All".
11. Now begin again with "Optical Properties" μ_a = 0.01mm^-1, μ'_s=1mm^-1.
12. Press "Plot Reflectance" button.
13. Press "Hold On".
14. Press ∂R/∂μ'_s radio button.
15. Press "Plot Reflectance" button.
16. Fix μ'_s=1mm^-1. Repeat the above steps for μ_a = 0.1 and 1 mm^-1.

    Question: Do the results of the ∂R/∂μ'_s plots at these three different μ_a values conform to your results in GUI Interaction C?

II. Sensitivity of Temporally-Resolved Diffuse Reflectance to Optical Properties

1. On the "Forward/Analysis" Panel select: (a) Forward Model: Scaled Monte Carlo (g=0.8) and (b) Solution Domain: R(ρ,t).
2. For the Independent axis choose t and set ρ = 10 mm.
3. Choose "Start" = 0 ns and "Stop" = 1.0 ns with 101 time points (1 point every 5 ps).
4. In Optical Properties: enter μ_a = 0.01mm^-1, μ'_s = 1mm^-1.
5. Plot time-resolved reflectance, R(t).
6. Deselect "Hold On".
7. Press ∂R/∂μ_a radio button.
8. Press "Plot Reflectance" button.
9. Repeat the above for μ'_s = 0.5 and 1.5 mm^-1.

   Question: Can you explain these results intuitively? Do the results of the ∂R/∂μ_a plots at these three different μ'_s values conform to your results in GUI Interaction C?

10. Now select "Clear All" and return and start again with Optical Properties: enter μ_a = 0.01mm^-1, μ'_s = 1mm^-1.
11. Plot time-resolved reflectance, R(t).
12. Deselect "Hold On".
13. Press ∂R/∂μ'_s radio button.
14. Press "Plot Reflectance" button.
15. Repeat the above for μ_a = 0.1 and 1 mm^-1.

    Question: Can you explain these results intuitively? Do the results of the ∂R/∂μ'_s plots at these three different μ_a values conform to your results in GUI Interaction C?

III. Impact of Optical Properties and Source-Detector Separation on Photon Hitting Density

Goal: This portion of the GUI Interaction is to examine how source detector separation and optical properties affect the region of tissue that is sampled in a spatially-resolved diffuse reflectance measurement R(ρ).

1. Select "Fluence/Interrogation Solver Panel".
2. Switch to Map View.
3. In Forward Model: select Standard Diffusion (Analytic - Distributed Line Source).
4. In Map Type select phd (Photon Hitting Density).
5. In Solution Domain: Φ(ρ,z).
6. Use default values for Rho and Z Ranges.
7. In Optical Properties: enter μ_a = 0.01 mm^-1, μ'_s = 1 mm^-1. Note that μ'_s / μ_a = 100.
8. Set the source-detector separation ρ = 10 mm.
9. Press "Generate Interrogation Map" button at the bottom of the panel.
10. Examine the shape and magnitude of the photon hitting density.
11. Determine the mean sampling depth <z> of all the detected photons.
12. Repeat for source-detector separations of ρ = 3 mm and 1 mm.
13. What is the variation in <z> with ρ?
14. Repeat III.5-III.12 for fixed μ'_s = 1 mm^-1 with μ_a values of 0.1 and 1 mm^-1.

Problem Based Learning Exercise:

1. You are designing a dual wavelength fiber optic probe with 3 source-detector pairs to detect the formation of new blood vessels during the integration of a skin graft. The neovascular layer is expect to form 2mm below the surface in a highly scattering tissue with μ'_s = 2mm^-1. The initial blood volume fraction (BVF) in the graft is 0.5% at 60% oxygenation and you wish to detect changes in the BVF up to 3% along with an increase in blood oxygenation up to 85%. Do you think this is possible? If not, why not?If so, what 3 source-detector separation distances and 2 wavelengths would you choose.