Biophotonics_Concepts_to_Applications

For attenuation that is exclusively due to absorptiona= 0, whereasa= 1 when
only scattering occurs. For the casea= 0.5, the coefficients of absorption and
scattering are of equal magnitude, that is,μa=μs.
Similar to the scattering efficiency given by Eq. (6.17), theabsorption efficiency
Qais defined as the fraction of the incident light beam absorbed per unit geometrical
cross-sectional area of the absorbing particle. Combining the absorption and scat-
tering efficiencies then gives thetotal extinction efficiencyQext, which is the sum of
the absorbed and scattered energy of the light beam

Qext¼QaþQs ð 6 : 22 Þ

Example 6.9Suppose that a stomach muscle tissue sample has an absorption

coefficient of 3.3 cm−^1 and a scattering coefficient of 30 cm−^1 for a Nd:YAG

laser emitting at a wavelength of 1064 nm. What is the optical albedo of the

tissue?

Solution: From Eq. (6.21) the optical albedo is

a¼

ls

lt

¼

ls

laþls

¼

30

3 : 3 þ 30

¼ 0 : 90 :

6.5 Light-Tissue Interaction Mechanisms

Interactions between natural light and biological tissue are important for life, but
also can cause harmful effects. On the beneficial side, plants require sunlight for the
photosynthesis process needed for their survival, and humans depend on UV light
to produce essential vitamin D in their skin. However, too much exposure to natural
light can produce skin inflammations, can lead to skin cancer, and can cause
wrinkles and age spots.
On the other hand, interactions between artificial light and biological tissue is a
major aspect of biophotonics that is used extensively in medical and cosmetic
therapy for procedures such as treatments of vascular lesions, removal of pigmented
lesions and tattoos, speeding of wound healing, resurfacing of skin to remove scars
and cosmetic defects, and for cancer detection and treatment. In addition, numerous
laser-based imaging techniques using a variety of light-tissue interaction processes
are deployed worldwide.
Depending on the characteristics of a specific light source and the properties of
the irradiated tissue, a wide range of light interactions with biological tissue can
occur. The main tissue properties involved in these interactions are the coefficients
of reflection, absorption, and scattering together with heat conduction and heat
capacity of the tissue. Because the absorption and scattering coefficients of tissue

170 6 Light-Tissue Interactions