Experimental procedure to determine Intralipid optical properties

  • Measure diffusely reflected light, M, from surface of a 500-ml beaker of 10% Intralipid irradiated with normal incident light. Measure diffusely reflected light, Mstd, from a diffuse reflectance standard, Rstd, (Labsphere Inc. , NH, USA) using the same measurement geometry. Calculate the diffuse reflectance, R = (M/Mstd)Rstd.

  • Calculate the ratio

    N' = mus'/mua

    The Adding-Doubling method can be used to accurately characterize N' = function(R). Flock et al. used the function(R) based on the Adding-Doubling method to relate R to the reduced albedo, a' = mus'/(mua + mus'). Then N' = a'/(1 - a'). An approximate expression for N' is:

    N' = A2/(3(-ln(R))2) - 1.

    The factor A is approximately 7 or 8 for most tissues, but increases at high R values. The factor A also depends on the refractive index of the medium. For more on this approximate expression, see the NewsEtc. article.

  • Add a small volume of absorber (India ink) to the Intralipid such that the reflectance, R+ink, is significantly reduced, perhaps to about 50%. Measurement of R+ink is made as above for the original Intralipid without ink. The amount of added absorption due to added ink to the Intralipid is known:
    dmuaink = (muaink_stock)(ink volume added)/(volume of Intralipid + ink volume added)

    where muaink_stock is the absorption of the ink stock.

  • Determine N'+ink based on R+ink, as was done for N' above.

  • Because N'+ink/N' = mua/(mua + dmuaink), the absorption of the Intralipid WITHOUT ink is calculated:
    mua = dmuainkN'+ink/(N' - N'+ink)

  • Calculate mus' = mua N'.

  • Measure the on-axis, narrow solid angle transmission, T = exp(-(mus + mua)L), through a cuvette with various dilution fractions (f) of the 10% Intralipid stock. The pathlength of the cuvette is L [cm]. Determine the scattering coefficient of the original 10% Intralipid:
    mus = (-ln(T)/L)/f - mua

    for each dilution f. The value of mua is about 0.01 cm-1 in this wavelength range has little effect on the value of mus. All the values of mus should be in agreement until too high concentrations of Intralipid are tested and multiple scattering causes the apparent mus to decrease.

  • Calculate the anisotropy, g = 1 - mus'/(mus - mua).

  • In summary, measurements of R, R+ink, and T yield mua, mus, and g, and the lumped term mus' = mus(1-g).