Strategies in In Vivo Molecular Imaging

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Abstract
After completing this article, readers should be able to: 1. Describe the emerging field of in vivo molecular imaging. 2. List the various imaging modalities that have been used to detect molecules and track cells in vivo and discuss the basic molecular strategies and principles. 3. Describe several of the selective events, such as binding, enzymatic activation, or molecular switches, that make in vivo imaging of cells and molecules possible. 4. Delineate the strengths and weakness of current imaging strategies. 5. List combined imaging modalities and their use in molecular imaging. Following cells and molecules in vivo has been a fundamental difficulty in biomedical research. The technologies in the field of in vivo molecular imaging are aimed at obtaining spatiotemporal information about in vivo processes as they occur; that is, monitoring cellular and molecular changes in response to a variety of stimuli in intact animals and eventually in humans. Understanding basic biologic and biochemical mechanisms is required to develop the tools that will enable access to this information. Traditional imaging modalities largely depend on physical parameters, such as absorption in ultrasonography, x-ray attenuation, or optical scattering, which reveal primarily structural information. For molecular imaging, unique signatures are generated using molecular biology methods that distinctly label host cells, microbial pathogens, tumor cells, or transplanted tissues. These can be detected in vivo by conventional and novel imaging modalities and can be used to examine molecular mechanisms of pathogenesis, facilitate earlier disease detection, and permit evaluation of therapeutic outcomes. The molecular targets for imaging may include DNA, RNA, protein, or protein function. Imaging of inherent levels of DNA, RNA, and protein is difficult, but protein function can be exploited readily for imaging and can be used in reporter gene approaches to create surrogate markers for processes such as messenger RNA (mRNA) and protein synthesis.1,2, …