intermingled biological tissue components have different optical properties, then
along some path through a tissue volume, various physical parameters (such as the
refractive index, absorption coefficients, and scattering coefficients) typically
change at material boundaries.
Absorbed light can be converted into heat, be radiated in afluorescent process,
or be consumed in photochemical reactions. As shown in Figs.6.8and6.18, the
strength of the absorption coefficients for different tissue components determines
how far light can penetrate into a specific tissue at a particular wavelength and also
determines how much energy a specific tissue absorbs from a particular optical
source.
Scattering of photons in tissue is another significant factor in the behavior of
light-tissue interactions. Together, absorption and multiple scattering of photons
cause light beams to broaden and decay as photons travel through tissue. Although
light can penetrate several centimeters into a tissue, strong scattering of light can
prevent observers from getting a clear image of tissue abnormalities beyond a few
millimeters in depth. Scattering of photons can be either an elastic process or an
inelastic process. Elastic scattering effects are used in many biophotonics applica-
tions such as optical coherence tomography, confocal microscopy, and elastic
scattering spectroscopy. Raman scattering is a major inelastic scattering process
used in biophotonics and is the basis for Raman vibrational spectroscopy. This
technique is of importance for studying biological molecules and for diagnosing
and monitoring the progress of diseases such as cataract formations, precancerous
and cancerous lesions in human soft tissue, artherosclerotic lesions in coronary
arteries, and bone and dental pathologies.
Light-tissue interactions can be classified into six generic categories that are
commonly used to describe therapeutic and surgical applications. These interactions
can be categorized as photobiomodulation, photochemical interactions, thermal
interactions (e.g., coagulation and vaporization), photoablation, plasma-induced
ablation, and photodisruption. The degree of light-tissue interaction depends on
tissue characteristics (such as the coefficients of reflection, absorption, and scat-
tering) and the parameters of the irradiating light.
The phenomenon of random interference patterns, or specklefields, in relation to
the scattering of laser light from weakly ordered media such as tissue can be both
beneficial and a limitation in biophotonics imaging. The appearance of speckles
arises from coherence effects in light-tissue interactions. Among the applications
areas of this effect are the study of tissue structures and cellflow monitoring.
An important tool in biophotonics is the concept offluorescence, which is the
property of certain atoms and molecules to absorb light at a particular wavelength
and, subsequently, to emit light of a longer wavelength after a short interaction
time. This physical phenomenon is widely used in a variety of biophotonics
sensing, spectroscopic, and imaging modalities.
6.8 Summary 191