Platforms for Gene Expression Imaging

Radionuchde-based methods, optical techniques, and magnetic resonance imaging (MRl) have each been used as platforms for gene expression imaging. Thus far, studies in the lungs have almost exclusively been limited to radionucUde-based methods. 

These platforms include planar gamma scintigraphy, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). They differ in their availability, cost, need for specialized infrastructure and technical expertise, and ability to accurately quantify tissue radioactivity noninvasively (9). 

Until recently, limitations in spatial resolution made PET studies in small animals impossible. Eor instance, most PET scanners in clinical use have an image spatial resolution of 10-15 mm (although more recent instruments can achieve resolution approaching 1-2 mm) the mouse lung, in contrast, is 10mm wide. Other issues such as scanner sensitivity (i.e., the fraction of radioactive events actually detected by the device) and the amount of radioactivity 

Overall, the fairest assessment of both PET and non-PET systems is that their use to monitor gene expression in living animals is still new and incompletely developed. Therefore, the advantages of one over another are still theoretical and systematic studies are needed to determine each method s relative benefits 

Optical imaging offers several key benefits over PET. PET radiopharmaceuticals generally need to be made on-site and require appropriate radiation safety precautions. In contrast, bioluminescence studies are simpler to conduct because the substrates are commercially available and readily prepared. Furthermore, multiple animals can be smdied at the same time, and they can be studied quickly because image acquisition times are typically short. The interval between smdies that involve repetitive imaging is also short, unlike radionuclide-based methods, which require time for radioactive decay. Finally, both the reporter gene products and substrates used in bioluminescence studies (e.g., luciferases and D-luciferin) appear to be nontoxic to mammalian cells. This is a theoretical advantage over radionuclide-based methods, which depend on the use of ionizing radiation (22). 

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