Radio frequency continuous-wave and time-domain EPR imaging and Overhauser-enhanced magnetic resonance imaging of small animals: instrumental developments and comparison of relative merits for functional imaging

Electron paramagnetic resonance (EPR) imaging in the continuous wave (CW) and time-domain modes, as well as Overhauser-enhanced magnetic resonance imaging in vivo is described. The review is based mainly on the CW and time-domain EPR instrumentation at 300 MHz developed in our laboratory, and the relative merits of these methods for functional in vivo imaging of small animals to assess hypoxia and tissue redox status are described. Overhauser imaging of small animals at magnetic fields in the range 10-15 mT that is being carried out in our laboratory for tumor imaging and the evaluation of tumor hypoxia based on quantitative evaluation of Overhauser enhancement is also described. Alternate approaches to spectral-spatial imaging using the transverse decay constants to infer in situ line widths and hence in vivo pO2 using CW and time-domain EPR imaging are also discussed. The nature of the spin probes used, the quality of the images obtained in all the three methods, the achievable resolution, limitations and possible future directions in small animal functional imaging with these modalities are summarized.