GOALS: Gain familiarity with wavelength dependence of tissue optical properties and associated length and time scales for radiative transport

Select Spectral Panel. Note that Tissue Types lists various tissues. Each tissue is modeled as a composite of individual chromophores (e.g. water, blood, fat, etc.). Each chromophore is assigned a specific concentration representative of the tissue type under consideration. In this exercise we will study how absorption and scattering properties of tissues (as well as their constituent chromophores) vary with the wavelength of light.

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. In Wavelength Range set Begin to 600, End to 1000, and Number to 36.
4. Enter "Hb" in Plot Label box.
5. Click the Plot μ_a Spectrum button at the bottom of the panel.
6. Plot provides μ_a as a function of wavelength, λ, for Hb ("pure" Hb spectrum).
7. Confirm that the output is consistent with results shown in lecture 1 for 600 nm<λ<1000 nm.
8. Confirm the Hold On checkbox is checked (under the graphing area on the left).
9. Repeat the steps I.1-I.6 for HbO₂.
10. Click the Clear All button under the graphing area.

II. 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 exercise 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. Record the minimum and maximum values of μ_a for these two tissues. If you click on the nodes of the plots, you will see the corresponding numeric values.
8. Click the Clear All button.
9. Plot μ'_s spectra on the same axis for these tissue types.
10. Record the minimum and maximum values of μ'_s for these two tissues.
11. Plot μ_a and μ'_s for Skin on the same plot, 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. Repeat for tissue type Liver.
12. Use either the minimum and maximum values from II.7 and II.10 or the values in II.11 to estimate the minimum and maximum of the ratio μ'_s / μ_a within the spectral range λ = 600-1000nm.
13. Using the data collected in II.9-II.10 and the definitions given in lecture 1, estimate minimum/maximum values of l_s,l_abs,l ^* within the spectral range of λ = 600-1000nm. 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?