being created, including the iron analog annexin V (Schellenberger et al., 2002), the fluorescent marker Cy5.5 (Petrovsky et al., 2003), and markers that do not become active until they reach their target. Future modification and adaptation of these technologies could be used to examine specific stages of regeneration, including those designed to detect neurite outgrowth, astrocyte scarring, oligodendrocyte myelination, and immunological response.
At present it is difficult to follow the path of cell transplants (such as stem cells, Schwann cells, and olfactory ensheathing cells) in the living spinal cord; therefore, it is difficult to draw conclusions about the efficacy of an experiment with such cells. Continued advancement of imaging techniques will provide a mechanism by which investigators and clinicians can assess the integration of grafted tissue or cells into the preexisting neuronal network or monitor the response to gene therapy by tracking the transgene location. Transgenic animal models have thus been developed. Specific populations of cells in these animals are genetically engineered to be fluorescent or to emit a fluorescent signal when they are functionally activated. Such approaches, which use two-photon confocal imaging to detect the signal, can be directly applied to spinal cord preparations in vitro and administered to intact mice and rats. With improvements in the current technology, the use and improvement of near-infrared markers might also provide researchers with a means to monitor the progression of a spinal cord injury and recovery in laboratory animals.
The promise of molecular imaging technologies can be realized only if the technologies can be successfully transferred to the clinical setting. The transfer of these technologies will require cross-disciplinary collaborations and multidisciplinary research efforts among molecular and cellular biologists, imaging scientists, nanotechnologists, and clinicians. A review article by Massoud and Gambhir (2003) identified the following goals for the transfer of molecular imaging technologies from the research laboratory to the clinic:
develop noninvasive in vivo imaging methods that detect specific cellular and molecular processes, such as gene expression and protein-protein interactions;
monitor multiple molecular events in concert;
monitor the trafficking and targeting of cells;