The see through bodyA BRIEF INTRODUCTIONBY: ABEL RUIZ

What do I mean?

Explain the phenomenon of shinning a flashlight through your hand And being able to see through it. This effect is known as the optical window or absorption window.

Key consepts

Refractive index Scattering absorption

Refractive index

We can find the index of refraction of tissue through Snell’s law which turns out to become 1.4 for most tissue.

sin(θ1)=v2sin(θ2)

Absorption

When light passes through a medium some of the photons can be absorbed and its energy converted into thermal energy.

Id=I0e(-μa*d)

Where Id is the transmitted light intensity, and μa is the absorption coefficient of medium for a given wavelength .

Another parameter of interest is the Absorbance A(which is the loss in light intensity). It can be expressed by the extinction coefficient of the compound(ε)(how well it absorbs light given a wavelength), the distance between where light enters and exits(d), and the concentration of the compound in the solution.[C]

Scattering

Because interaction of light with matter can change the direction of propagation and give rise to attenuation we must take it into effect. We can relate it to the intensities by:

Id=i0e(-μs*d)

Where μs is the scattering coefficient for a given wavelength(probability per unit length of a photon being scattered).

Anisotropy (g) Is the average value of the cosine of the scattering angle. It

ranges from (1,-1) for extremely forward to extremely backward scattering.

Attenuation

The attenuation of light in deep biological tissue depends on the effective attenuation:

) Where

In tissue g =0.9

Putting it all together

There are three different tissue components we need to look at Water – water is nearly transparent in the visible light range but

starts to absorb over the near –infrared region. Using the effective absorption coefficient we can plot the absorption vs. wavelength of water.

Blood(hemoglobin)- for hemoglobin absorption peaks at 420nm and 580nm and then gradually decreases at the wavelength increases.

Melanin- the absorption of melanin linearly decreases as the wavelength increases.

As seen, the optical window is bound within the wavelengths of (630-930nm) corresponding to red and infrared light. The lower limit is set by both the hemoglobin and melanin. While at the upper boundary it is bound by the absorption spectrum of the water. Bone was not plotted because the abortion coefficient is near 1 due to the highly dense calcium atoms that tend to absorb all photons.

credits

http://www.medphys.ucl.ac.uk/research/borg/homepages/gbranco/Chapter%202%20Fundamentals%20of%20Tissue%20Optics.pdf

http://en.wikipedia.org/wiki/Optical_window_in_biological_tissue#See_also

http://en.wikipedia.org/wiki/Near-infrared_spectroscopy Pictures provided by Dmitry Budker