efficacies of endogenous cells versus those of the exogenous transplanted stem cells.

PET scan technology is being developed to inform clinicians about whether drugs can bind to the appropriate targets. For example, clinicians are using PET scans to determine if treatments are effective by looking at the uptake of glucose, which tumors need to nourish their growth (Van den Abbeele and Badawi, 2002; Pollack, 2004). These effects can be observed before structural changes in the tumor can be detected.

Two caveats about the use of PET scans must be kept in mind. First, current technology does not have enough resolution to allow complete visualization through the entire diameter of the spinal cord. Furthermore, the current spatial resolution of commercial PET scanners is 4 mm but 2.5 mm resolution has been achieved in research instruments that use motion compensation. Second, information obtained from PET scans is based on metabolic events that correlate to neural activity and may not directly correspond to the location where the changes in activity are occurring. Therefore, the images generated by PET scans could be misleading because they may not accurately represent the spatial specificities of the changes (Ugurbil et al., 2003). However, refinements to PET scans could provide important information about the cellular states of the injury, such as gene activation or suppression in response to the injury; this would provide physicians with the ability to quantify responses to different spinal cord injury treatments (Brooks et al., 2003) and to identify functional changes before the onset of structural changes identifiable by MRI (National PET Scan Management, LLC, 2004). PET ligands have been developed that can detect glucose metabolism, inflammation, and receptor abundance, including agents that track the N-methyl-D-aspartate (NMDA) receptor activity and proteases. PET measures very different process than does MRI whose spatial resolution is superior. However, PET contrast resolution for identification of proteins can be hundreds of times greater than MRI depending on the target. The potentials of PET for assessing the severity of injury and the responses to therapy await application of high resolution systems with recently developed radiopharmaceuticals.

Improvements to PET and MR technologies enable investigators to visualize the molecular signatures of damage and repair to the CNS. In an attempt to examine the activities of specific neuronal circuits, imaging markers that mimic neurotransmitters and receptors that are nonradioactive are