Virtual Photonics Technology Initiative

2017 Laboratory D: Monte Carlo Simulations: Spatial and Angular Distributions

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GOALS: Gain familiarity with the use of Monte Carlo Simulations to provide fluence and radiance predictions

Bring up the GUI

I. Monte Carlo Simulations of Narrow Collimated Beam Irradiation

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

  1. Select the Monte Carlo Solver Panel.
  2. Click Download Prototype Input Files button. This download sample infiles in a zip file. Extract the contents of the zip file.
  3. In the Input File Specification, click the Load Input File button. Select the file: infile_one_layer_ROfRho_FluenceOfRhoAndZ.txt. 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) and fluence in cylindrical coordinates (ρ,z).
  4. Set the optical properties of the tissue layer to be μa = 0.01mm-1, μ's = 1mm-1, g=0.8.
  5. Set Number of Photons to 1000.
  6. Click the Run Simulation button.
  7. On the Map View plot, note the Mean Depth magnitude, axial and lateral penetration depths of the fluence distribution.
  8. Increase the absorption coefficient to μa = 1mm-1, keeping all other properties constant.
  9. Click the Run Monte Carlo Simulation button.
  10. Note the magnitude of the Mean Depth, axial and lateral penetration depths of the fluence distribution.
  11. Now repeat this exercise keeping μa constant at 0.1mm-1 and μ's values of 0 and 1mm-1, noting the mean depth of penetration.

Questions:

  1. How does increasing absorption change the magnitude, axial and lateral penetration depths?
  2. How does increasing scattering change the magnitude, axial and lateral penetration depths?


II. Visualizing Radiance versus Fluence distributions using the Monte Carlo CommandLine Application

Goal: This portion of the lab exercise is to examine the directional aspect of radiance in turbid tissues.

  1. On the Desktop, holding down the shift key, right click the MCCL folder and select Open PowerShell window here.
  2. Execute the Monte Carlo Command Line Application using the command is mc infile=infile_one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle.txt. This infile specifies a perpendicular point source impinging on the tissue at (0,0,0). The tissue is homogeneous with optical properties μa = 0.01mm-1, μ's = 1mm-1, g=0.8 and n = 1.4. N=10,000 photons are launched. Executing this command creates a folder called "one_layer_FluenceOfRhoAndZ_RadianceOfRhoAndZAndAngle" in the MCCL directory with fluence and radiance detector results.
  3. Go into the MCCL folder.
  4. 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. Enter load_results_script at the MATLAB prompt.
  5. Three figures should appear, one plot of Fluence, and two plots of radiance in the lower and upper hemispheres. Note that the lower hemisphere is notated as [0-pi/2] and the upper hemisphere as [pi/2-pi]. These angles are with reference to the positive z-axis which is into the tissue.

Questions:

  1. In what way can the angular information shown in the radiance plots be useful?
  2. 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:

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