Small Animal Optical Imaging
There are two types of optical imaging that can be used to report gene or disease marker expression in vivo: Bioluminescence and fluorescence imaging.
Bioluminescence requires the expression of luciferase (firefly or renilla) as a gene reporter and the administration of luciferin as an optical probe of the reporter. When the enzyme encounters its substrate, a photon is liberated providing a single photon imaging event, termed bioluminescence. Since the rate of generation of bioluminescence is determined by the diffusional encounter of the enzyme and substrate in the presence of oxygen and ATP, the photon count rate in bioluminescence imaging can be considered to be at least comparable to the photon count rates associated with the decay of radiotracers in nuclear imaging. In order to remove background “noise” bioluminescence must be performed under “light-tight” conditions within an imaging box. At the Frensley Imaging Center, we employ bioluminescence measurements using a sensitive CCD camera coupled to a light tight box outfitted for maintaining an anesthetized animal. Investigators interested in bioluminescence imaging must provide their own animal model. Currently, tomography is not available for bioluminescence imaging.
Fluorescence imaging requires either the expression of a fluorescent gene reporter, such as green fluorescent protein or red fluorescent protein, or the administration of an exogenous fluorescent imaging probe. In contrast to bioluminescence, fluorescence imaging requires illumination of the animal with excitation light which can activate the fluorophore. Since fluorophores can be repeatedly activated by tissue propagated excitation light, the photon count rate can be significantly higher than in nuclear imaging or bioluminescence. However, visible light can be dramatically attenuated as well as excite endogeneous fluorophores, resulting in a high background and low imaging sensitivity. A specialty of the laboratory resides in the development of near-infrared fluorescence imaging agents and techniques which feature a low autofluorescence background, high sensitivity, and high photon count rates. Planar imaging is used in the center to dynamically track viruses, cells, and particles, as well as to perform molecularly targeting studies. Tomography is currently under validation.