In Vivo Molecular Fluorescence Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 262

response, the separation of the phosphorescence signal from the excitation and auto-
fluorescence signals (background noise) can be obtained by simple time gating. In
addition, implementation of a time-resolved phosphorescence imaging system is
simpler and less expensive than a time-resolved fluorescence system.

9.3 general BehavIor oF lIght In BIologIcal tIssue

This section describes the mechanisms that govern light transport through biological
tissues. The main optical characteristics of biological tissue defined by absorption coef-
ficient, scattering coefficient, and refractive index are discussed in the following text.

9.3.1 absorption

Absorption is a process of absorbing the energy of a photon by a molecule to transfer
its electron from a lower energy level to a higher energy level. Mean absorption path
length is the mean free path that a photon travels in a medium before absorption.
Absorption coefficient μ a is defined as reciprocal of mean absorption path length and
is the probability of a photon being absorbed by a medium per unit path length.
If the media contains several chromophores with different extinction coefficient,
the mean absorption coefficient can be defined as [34]

a = ()

∑ n

µ ε λ

C

(9.1)

n

where n is the number of chromophores, ε n ( λ ) and C n are the extinction coefficient
and concentration of each chromophore, respectively. According to the Bear-lambert
law, light attenuates as it propagates in an absorbing media (nonscattering) based on
the following equations:

dI
I

=− µ dx , Ix I ( ) = exp( − µ x ) (9.2)

a 0 a

where I ( x ) is the light intensity at the distance x from the source, along the light
propagation direction. I 0 is the light intensity at the source location ( x = 0).
optical absorption in biological tissue (in NIr region) originates primarily from
oxy- and deoxyhemoglobin, melanin, lipid, and water [35, 36]. Figure 9.4 shows the
absorption spectra of some of these components in the NIr window.
Since the absorption coefficient of tissue chromophores is low in the NIr region
(650-900 nm) compared to other wavelengths, light can penetrate deeply (a few cen-
timeters) into tissue. At wavelengths lower than 650 nm, absorption by hemoglobin
limits the penetration of photons into tissue, and at wavelengths higher than 900 nm,
absorption by water dominates.