# 2011 Laboratory B: Optical Dosimetry

**GOAL: Examine the impact of source configuration and optical properties on the magnitude and spatial distribution of fluence within tissue**

Select **"Fluence/Interrogation Solver Panel"**. Note that this panel lists various types of source configuration and solver options.

**I. Embedded Isotropic Point Source**

Goal: This portion of the GUI Interaction is to examine the fluence distribution generated by an embedded isotropic point source located at a depth of *l*^{ * }in the medium

- In the dropdown (Forward Model) select Standard Diffusion (Analytic - Isotropic Point Source).
- In
**Solution Domain:**Φ(ρ,*z*). - Use default values for Rho and Z Range.
- In
**Optical Properties:**enter μ_{a}= 0.01 mm^{-1}, μ'_{s}= 1 mm^{-1}. Note that μ'_{s}/ μ_{a}= 100. - Click the
**"Generate Fluence/Interrogation Map"**button at the bottom of the panel. - Examine the shape and magnitude with depth of the fluence distribution along the centerline.
- Replot the fluence distribution for μ
_{a}= 0.1 and 1 mm^{-1}and examine the effect of the increased absorption on the magnitude, axial and lateral penetration depths. - Now keep absorption constant at μ
_{a}= 0.01 mm^{-1}and examine the effect of varying the reduced scattering coefficient μ'_{s}= 0.5, 1.0, and 1.5 mm^{-1}on the magnitude, axial and lateral penetration depths.

**II. Distributed Point Source ("Pencil Beam")**

Goal: This portion of the GUI Interaction is to examine how the fluence distribution is altered when replacing the embedded isotropic point source with a distributed line source that falls exponentially with depth with a constant decay rate of μ'_{t} which is equivalent to 1/*l*^{ * }.

- In the dropdown (Forward Model) select Standard Diffusion (Analytic - Distributed Point Source).
- In
**Solution Domain:**Φ(ρ,*z*). - Use default values for Rho and Z Range.
- In
**Optical Properties:**enter μ_{a}= 0.01 mm^{-1}, μ'_{s}= 1 mm^{-1}. Note that μ'_{s}/ μ_{a}= 100. - Click the
**"Generate Fluence/Interrogation Map"**button at the bottom of the panel. - Examine the shape and magnitude with depth of the fluence distribution along the centerline and also compare with results in Section I.
- Replot the fluence distribution for μ
_{a}= 0.1 and 1 mm^{-1}and examine the effect of the increased absorption on the magnitude, axial and lateral penetration depths. - Now keep absorption constant at μ
_{a}= 0.01 mm^{-1}and examine the effect of varying the reduced scattering coefficient μ'_{s}= 0.5, 1.0, and 1.5 mm^{-1}on the magnitude, axial and lateral penetration depths.

**III. Distributed Gaussian Beam Source**

Goal: This portion of the GUI Interaction is to examine the impact of beam diameter on the amplitude and axial/lateral dispersion of the light in turbid tissues.

- In the dropdown (Forward Model) select Standard Diffusion (Analytic - Distributed Gaussian Source).
- In
**Solution Domain:**Φ(ρ,*z*). - Use default values for Rho and Z Range.
- In
**Optical Properties:**enter μ_{a}= 0.01 mm^{-1}, μ'_{s}= 1 mm^{-1}. Note that μ'_{s}/ μ_{a}= 100. - Select a Gaussian Beam Diameter of 0.2mm.
- Click the
**"Generate Fluence/Interrogation Map"**button at the bottom of the panel. - Keeping the μ'
_{s}value fixed, plot the fluence distribution for μ_{a}= 0.1 and 1 mm^{-1}and examine the effect of the increased absorption on the magnitude, axial and lateral penetration depths of the fluence distribution. - Now set the absorption coefficient, μ
_{a}= 0.01 mm^{-1}and examine the effect of varying the reduced scattering coefficient μ'_{s}= 0.5, 1.0, and 1.5 mm^{-1}on the magnitude, axial and lateral penetration depths. - Click the
**"Generate Fluence/Interrogation Map"**button at the bottom of the panel. - Repeat the above for Gaussian beam Diameter of 0.5mm and 2mm.

**IV. Collimated Point Source Monte Carlo solutions**

Goal: This portion of the GUI Interaction is to examine the impact of optical properties on the amplitude and axial/lateral disperson of the light in turbid tissues.

- Select the
**"Monte Carlo Solver Panel"**. - In the Input File Specification, click the
**"Load Input File"**button. Select the file one_layer_ROfRho_FluenceOfRhoAndZ.xml. This is a Monte Carlo simulation input file that specifies a Discrete Absorption Weighting (DAW) simulation to produce reflectance as a function of source-detector (ρ) separation,*R*(ρ). - Set the optical properties of the tissue layer to be μ
_{a}= 1e-10mm^{-1}, μ'_{s}= 1mm^{-1},*g*= 0.8. - Set Number of Photons to 1000.
- Click the
**"Run Simulation"**button. - Note the "Mean Depth" magnitude, axial and lateral penetration depths of the fluence distribution.
- Increase the absorption coefficient to μ
_{a}= 1mm^{-1}, keeping all other properties constant. - Click the
**"Run Monte Carlo Simulation"**button. - Note the "Mean Depth" magnitude, axial and lateral penetration depths of the fluence distribution.
- Now repeat this exercise keeping μ
_{a}constant at 0.1mm^{-1}and μ'_{s}values of 0 and 1mm^{-1}, noting the mean depth of penetration.

**Questions:**

- How does increasing absorption change the magnitude, axial and lateral penetration depths?
- How does increasing scattering change the magnitude, axial and lateral penetration depths?

**V. Impact of visualizing Radiance versus Fluence distributions using the Monte Carlo CommandLine Application**

- Go to the MC1.0 download folder.
- Execute the Monte Carlo Command Line Application with the input file newinfile_one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle.xml. The command is "mc infile=newinfile_one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle.xml". This creates a folder called "one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle" in this directory with fluence and radiance detector results.
- Click on "load_results_script". This will bring up matlab and also a window to edit load_results_script.m. Change "datanames" to be "one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle". Change "show.FluenceOfRhoAndZ" to be 1, "show.RadianceOfRhoAndZAndAngle" to be 1 and the rest 0. Enter "load_results_script" at MATLAB prompt.
- Three figures should appear, one plot of Fluence, and two plots of radiance in the lower and upper hemisphere.

**Questions:**

- In what way can the angular information shown in the radiance plots be useful?
- Which plot of radiance looks most like Fluence and why?

**Additional Questions:** Consider the situation where you using a laser-based therapy to treat an embedded tumor. For this application, it is critical that you maximize the axial penetration of the light field. However during the treatment, the tissue absorption may increase due to increased blood flow and scattering may decrease due to morphological changes in the tissue. In this context comment on the following:

- How does an increase in absorption impact (a) the axial penetration of the light field and (b) the lateral dispersion of the light field?
- How does a decrease in scattering impact (a) the axial penetration of the light field and (b) the lateral dispersion of the light field?
- Would increasing the beam diameter improve (a) the axial penetration of the light field or (b) the lateral dispersion of the light field? Is this impact the same regardless of the optical properties?