( ion with the subsequent generation of l...)Near Infrared Technology and Optical Agents For Molecular Imaging

ion with the subsequent generation of light upon contact with luciferin. This approach requires the creation of modified cells, and analysis is limited to cells expressing the luciferase gene. Thus it is not translatable to clinical practice.

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
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