Optical Properties of Aorta

by Steven Jacques, Oregon Medical Laser Center
The optical properties of aorta are reviewed. The aorta is the major very large artery leaving the heart which first brings blood to the rest of the body. Because of its large size, it provides large sample sizes for convenient experimental measurements. Hence, it has been widely studied. It is not the same as other vessels, but perhaps is a reasonable first approximation.

The following figures summarize the optical properties of aorta, compiled from various reported studies. The tissues are either human, pig, or dog aorta, and the inter-species differences are not as important as the inter-study differences. In other words, variation in experimental technique and tissue preparation probably dominate the differences amongst data.

The two primary optical properties that determine diffusive light transport through the aorta are the absorption coefficient µa [cm-1] and the reduced scattering coefficient µs' [cm-1]. These values can be measured at any wavelength of interest using a variety of methods. Here are two methods used to generate data in this article:

Click here for the references for the data in the following figures.

You will also find listings of the original data which you may download.

The absorption coefficient µa [cm-1].

muaClick here to expand

The absorption increases toward shorter wavelengths due to protein absorption, and toward longer wavelengths due to water absorption. In the 400-600 nm range, absorption by hemoglobin is very strong and residual hemoglobin staining of vessel walls is a strong influence. Hemoglobin contamination can be seen in some of the spectra in this region. In the central region between 600-1300, tissue absorption is lowest. Integrating sphere measurements can yield artificially high µa values due to edge losses at the sphere's ports, and the data of Keijzer and Oraevsky a are suspect in this region. The low values of van Gemert, Yoon a and Yoon b at 633 nm, and the low values of van Gemert, Essenpreis, Cheong b and Cheong c at 1064 nm are probably good. The low value at 1064 from the photoacoustic measurements of Oraevsky b are as low as water itself. Since aorta tissue has only about 80% water content (rough estimate), the tissue is greater than 80% of the pure water µa value. Although the tissue measured in Oraevsky b is diseased aorta (atherosclerotic plaque), the optical properties are not very different from that of normal aorta (see the Oraevsky b paper for comparison). The data of Keijzer shows the individual optical properties of the three layers that comprise the aorta:

It should be pointed out that the inter-specimen variation in µa for three samples was as great as the inter-layer variation for µa. So while differences in optical properties of tissue layers are real, the inter-specimen variations can be significant. In general, one needs to assess the optical properties of any particular tissue site rather than rely on some average value from a library if accuracy is important. However, for general guidance of dosimetry in medical laser applications, the data of this graph and the following graphs are helpful.

The reduced scattering coefficient µs' [cm-1].

muspClick here to expand

The scattering falls with increasing wavelength. The scattering behavior at shorter wavelengths below 600 nm is likely dominated by scattering from the periodicity and size of refractive index fluctuations of the collagen fibrils, in the size range of 70 nm to hundreds of nm. Longer wavelength behavior beyond 600 nm is increasingly dominated by scattering from the larger 2-3 µm diameter cylinders of collagen fibers composed of collagen fibrils. Other tissue components may also contribute to the overall scattering, but collagen fibers at the micro and macro scale probably dominate the scattering.

The reduced scattering coefficient, µs' = µs(1 - g), can be further subdivided into its two components, the scattering coefficient µs [cm-1] and the scattering anisotropy g [dimensionless]. Typically this is done by measuring the total attenuation of transmission (T) of a collimated beam of light by a thin tissue sample of thickness d [cm]:

µt = -ln(T)/d = (µa + µs') [cm-1].

Then one calculates:

µs = µt - µa

g = 1 - µs'/µs

The scattering coefficient µs [cm-1].

musClick here to expand

The scattering coefficient decreases with increasing wavelength for the same reasons given for the behavior of the reduced scattering coefficient. The measurement of µt is more tricky than one might think. Multiple scattering in a too thick sample can cause higher measured T and yield a lower µt. Too big a collection port such that some scattered light is still collected will also cause higher measured T and lower the calculated µt. An erroneously low µt will cause deduction of an erroneously low µs.

The anisotropy g [dimensionless].

gClick here to expand

The scattering anisotropy g tends to be constant at longer wavelengths. The anisotropy will typically drop at shorter wavelengths. Again, the measurement depends on an accurate determination of µt and the caution cited above for determination of µs is also pertinent to determination of g. An erroneously low µt will cause deduction of an erroneously low µs which will cause an erroneously low g since the value of µs' is constant at its experimentally determined value. The simultaneously low values of Cheong a for µs and g, while the µs' value is comparable to others' values, are symptomatic of an erroneously low µt measurement.

The optical penetration depth, delta [cm]

deltaClick here to expand

Finally, a parameter of practical importance is the optical penetration depth, delta [cm], which is the distance through tissue over which diffuse light decreases in fluence rate to 1/e or 37% of its initial value. In other words, the concentration of light in tissue falls exponentially with increasing distance, x (cm), from the source:

C(x) = (constant)exp(-x/delta)

where delta = 1/sqrt(3µaa + µs'))

In summary, the measurements of aorta optical properties are quite variable and offer only an approximate guide to the optical behavior of aorta or other vessels in vivo.