In Vivo Molecular Fluorescence Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 258

lesions due to the leakiness of their vascularization or as targeted probes, which
can attach to specific disease biomarkers [18, 19]. The targeted probes can bind to
the disease-related biomarkers specifically and stay longer, thus improving the
signal-to-background ratio significantly. Fluorescent probes, based on their struc-
ture, can also provide new information about tissue microenvironment, such as
pH, temperature, or hypoxia [20-24].
The characteristics of a suitable imaging agent in general and of a fluorescent
imaging agent in particular can be summarized as follows [25]:

• High accumulation in the target region with good specificity: Have a stable
binding to a specific biomarker of the disease
• Fast washout time from the blood and normal tissues and other organs (however, the
clearance of the probes from the body has to be slow enough to allow accumulation
in the target site for sufficient binding of the probes to the receptors)
• High sensitivity: Generates sufficient contrast and signal-to-background ratio
at low concentration levels (in nanomolar to micromolar range) in the region of
interest (roI) in the minimum amount of time
• Has minimum effect on the normal tissue and organ, low level of toxicity, and
side effects
• Does not interfere with drugs and therapies [19]

Fluorescence imaging has the potential to monitor multiple biomarkers simulta-
neously by using different fluorescent probes with different emission wavelengths [26,
27]. Multicolor imaging can be used to characterize simultaneously several disease
biomarkers and/or to evaluate the effect of multiple drugs in vivo . It can also be used
to study the pharmacokinetics of several drugs or probes with different clearance rates
at the same time, which is unique among imaging modalities.
This chapter is organized as follows: Section “Basics of fluorescence” intro-
duces the physics behind fluorescence imaging. In Section “General behavior of
light in biological tissue”, the optical tissue properties and tissue photon inter-
action are discussed. Techniques being employed in diffuse optical fluorescence
imaging instrumentations are reviewed in Section “Diffuse fluorescence optical
imaging instruments.” Forward models, in Section “Modeling of light propaga-
tion in tissue,” and planar and tomography reconstruction algorithms, in Section
“Imaging algorithms” are briefly introduced. They are followed by a summary
in Section “Summary.”

9.2 BasIcs oF Fluorescence

Some materials can emit photons at a different wavelength when they absorb light
or other electromagnetic radiation. Fluorescence can be described as absorption of a
photon at one wavelength and re-emission at a different, usually longer wavelength.
Molecules that are able to absorb and emit photons are known as fluorophores. Figure 9.1