In a second approach, cells engineered to express fluorescent proteins can be used to mark tumors. The recombinant tumor cells can be implanted into animals and, following excitation with an appropriate light source, the fluorescence from the expressed fluorescent proteins can be detected by means of a CCD camera. The excitation and emission wavelengths of commercially available fluorescent proteins are generally in the visible region of the spectrum. As discussed in this publication, this region can be compromised by tissue autofluorescence and nonspecific background. Also, this method requires transgenic cell lines and is thus limited in its ability to detect a variety of potential targets. Similar to bioluminescent imaging, this method cannot translate to the clinic.
A more flexible and direct approach employs targeted imaging agents consisting of antibodies, receptor binding ligands, small molecules, or peptides labeled with fluorochromes. The fluorescent labels can be visualized by excitation with an appropriate light source and the emitted photons captured via a CCD camera or other optical detector.
For fluorescent imaging, there are generally three
parameters which are used to characterize the interaction
of photons with tissues. The three processes are
light absorption, light scattering, and fluorescent
emission. A fundamental consideration in optical
imaging is maximizing the depth of tissue penetration.
Absorption and scattering of light is largely a
function of the wavelength of the excitation source1.
In general, light absorption and scattering decreases
with increasing wavelength2. Be