GOALS: Gain familiarity with tissue optical property spectra as associated length and time scales for radiative transport

Select "Spectral Panel". Note that "Tissue Types" lists various tissues. Each tissue is modeled consisting of individual chromophores (e.g. water, blood, fat, etc.), each of which has a certain concentration pertinent to a particular tissue. In this exercise we will study how absorption and scattering properties of tissues (as well as their constituting chromophores) vary with the wavelength of illuminating radiation.

I. Absorption Spectra of Tissue Constituents

Goal: This portion of the GUI Interaction is to provide an introduction to the functionality of the Spectral Panel.

1. Select Custom in Tissue Types.
2. In Absorber Concentrations set concentrations to 1 μM for Hb and 0 μM for the other optical absorbers.
3. Enter "Hb" in "Plot Label" box.
4. Click the Plot μ_a Spectrum button at the bottom of the panel.
5. Plot provides μ_a as a function of wavelength, λ, for Hb ("pure" Hb spectrum).
6. Confirm that the output is consistent with results shown in lecture 1 for 600 nm<λ<1000 nm.
7. Confirm the Hold On checkbox is checked (under the graphing area on the left).
8. Repeat the steps I.1-I.6 for HbO₂.
9. Click the Clear All button under the graphing area.

II. Properties of Rayleigh and Mie Scattering

Goal: This portion of the GUI Interaction is provide insight into the characteristics of Rayleigh and Mie Scattering.

1. Launch the Particle Scattering tool in MATLAB (Type ParticleScatteringTool in MATLAB command window).
2. Enter parameters representative of a small cellular organelle: Scatterer diameter of 0.1 μm, Scatterer refractive index of 1.4 and Medium refractive index of 1.37.
3. Confine your attention to the visible and NIR spectral region: In the Wavelength Box entering Start, End, and Step Values of 400, 1000, and 5, respectively.
4. Click the Run Simulation button and wait for about 10 seconds for the computation to run.
5. In the directions that follow make sure to take note of characteristic values and wavelength dependence of both the scattering cross-section and scattering amplitudes.
6. Examine carefully the plot of the Scattering Cross Section vs Wavelength shown in the lower left corner.
7. Examine Spectral and Scattering Angle dependence of the Scattering Amplitude.
8. Examine the impact of polarization state of the incident light using the Radio buttons.
9. Repeat the steps II.2-II.8 for parameters representative of a mitochondrion: Scatterer diameter of 0.7 μm, Scatterer refractive index of 1.42 and Medium refractive index of 1.37. Note key differences relative to scattering characteristics of a small organelle.
10. Repeat the steps II.2-II.8 for parameters representative of a cell nucleus: Scatterer diameter of 4 μm, Scatterer refractive index of 1.39 and Medium refractive index of 1.37. Note key differences relative to scattering characteristics of the small organelle and mitochondrion.

III. Tissue spectra

Goal: It is known that the liver is a highly cellular and blood filled tissue, while skin, by contrast, has less cellular content and more extracellular matrix proteins. As you do this excercise examine whether the μ_a and μ_s' spectra are consistent with the known composition and morphological properties of these tissues. Comment on the similarities / differences in their spectra.

1. Select Skin in Tissue Types.
2. Notice the defaults in Absorber Concentrations.
3. Enter Skin in Plot Label box.
4. Plot μ_a of skin versus wavelength.
5. Confirm that the Hold On checkbox is checked.
6. Repeat the steps II.1-II.4 for tissue type Liver.
7. Click the Clear All button.
8. Plot μ'_s spectra on the same axis for these tissue types.
9. Record the minimum and the maximum values of μ_a and μ_s' for these two tissues. You can either hover the mouse over the nodes on the plots to see the corresponding numeric value or in the Plot View window, click the "Curve" Radio Button in the Normalization Controls. This operation divides the second plot μ'_s results (and any other plots plotted after the first) in the plot view window by the results for the first plot for μ_a. Thus the first result gets transformed to a series of '1' values while the second result is represented as μ'_s / μ_a.
10. Estimate minimum and maximum of the ratio μ_s' / μ_a within the spectral range λ = 600-1000nm.
11. Using the data collected in III.9-III.10 and the definitions given in lecture 1, estimate minimum/maximum values of l_s,l_abs,l ^* within the spectral range of λ = 650-1000nm. Assume g = 0.8.

Additional Question:

1. You have designed a device that can detect changes in μ_a as small as 0.01 mm^-1 at λ = 600nm. Assuming that changes in your system are limited to changes in Hb concentration, what is the smallest Hb concentration change that you can detect? Similarly for HbO₂ what is the smallest concentration change that you can detect?