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. Select the "Forward/Analysis" Panel.
2. In the dropdown (Forward Model) select Scaled Monte Carlo - NURBS (g=0.8, n=1.4).
3. In Solution Domain: select Steady-State R(ρ).
4. Select start and stop locations to 0.5 and 9.5 mm, respectively with 19 points (every 0.5 mm).
5. In Optical Properties: enter μ_a = 0.01mm^-1, μ'_s=1mm^-1.
6. Click the "Plot Reflectance" button.
7. Confirm the "Hold On" checkbox is checked.
8. Click the ∂R/∂μ_a radio button.
9. Click the "Plot Reflectance" button.
10. Examine the shape, magnitude and sign of this curve.
11. 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?

12. Click the "Clear All" button.
13. Now begin again with "Optical Properties" μ_a = 0.01mm^-1, μ'_s=1mm^-1.
14. Click the "Plot Reflectance" button.
15. Confirm the "Hold On" checkbox is checked.
16. Click the ∂R/∂μ'_s radio button.
17. Click the "Plot Reflectance" button.
18. 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. Select the "Forward/Analysis" Panel.
2. In the dropdown (Forward Model) select Scaled Monte Carlo (g=0.8).
3. In Solution Domain: select Time-Domain R(ρ,t).
4. For the Independent axis choose t and set ρ = 10 mm.
5. Choose "Start" = 0 ns and "Stop" = 1.0 ns with 101 time points (1 point every 5 ps).
6. In Optical Properties: enter μ_a = 0.01mm^-1, μ'_s = 1mm^-1.
7. Click the "Plot Reflectance" button.
8. Uncheck the "Hold On" checkbox.
9. Click the ∂R/∂μ_a radio button.
10. Click the "Plot Reflectance" button.
11. 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?

12. Now Click the "Clear All" button.
13. Start again with Optical Properties: enter μ_a = 0.01mm^-1, μ'_s = 1mm^-1.
14. Click the "Plot Reflectance" button.
15. Uncheck the "Hold On" checkbox.
16. Click the ∂R/∂μ'_s radio button.
17. Click the "Plot Reflectance" button.
18. 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 the "Fluence/Interrogation Solver Panel".
2. In the dropdown (Forward Model) select Standard Diffusion (Analytic - Distributed Line Source).
3. In Map Type select phd (Photon Hitting Density).
4. In Solution Domain: select phd(ρ,z).
5. Use default values for Rho and Z Ranges.
6. In Optical Properties: enter μ_a = 0.01 mm^-1, μ'_s = 1 mm^-1. Note that μ'_s / μ_a = 100.
7. Set the "Source-Detector Separation" (ρ = ) to 10 mm.
8. Click the "Generate Fluence/Interrogation Map" button at the bottom of the panel.
9. Examine the shape and magnitude of the photon hitting density.
10. Determine the mean sampling depth <z> of all the detected photons.
11. Repeat for source-detector separations of ρ = 3 mm and 1 mm.
12. What is the variation in <z> with ρ?
13. 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.