IN-VIVO IMAGING OF A STABLE PARAMAGNETIC PROBE BY PULSED-RADIOFREQUENCY ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

Imaging of free radicals by electron paramagnetic resonance (EPR) spec troscopy using time domain acquisition as in nuclear magnetic resonanc e (NMR) has not been attempted because of the short spin-spin relaxati on times, typically under 1 mu s, of most biologically relevant parama gnetic species. Recent advances in radiofrequency (RF) electronics hav e enabled the generation of pulses of the order of 10-50 ns. Such shor t pulses provide adequate spectral coverage for EPR studies at 300 MHz resonant frequency, Acquisition of free induction decays (FID) of par amagnetic species possessing inhomogenously broadened narrow lines aft er pulsed excitation is feasible with an appropriate digitizer/average r. This report describes the use of time-domain RF EPR spectrometry an d imaging for in vivo applications, FID responses were collected from a water-soluble, narrow line width spin probe within phantom samples i n solution and also when infused intravenously in an anesthetized mous e. Using static magnetic field gradients and back-projection methods o f image reconstruction, two-dimensional images of the spin-probe distr ibution were obtained in phantom samples as well as in a mouse. The re solution in the images was better than 0.7 mm and devoid of motional a rtifacts in the in vivo study. Results from this study suggest a poten tial use for pulsed RF EPR imaging (EPRI) for three-dimensional spatia l and spectral-spatial imaging applications. In particular, pulsed EPR I may find use in in vivo studies to minimize motional artifacts from cardiac and lung motion that cause significant problems in frequency-d omain spectral acquisition, such as in continuous wave (cw) EPR techni ques.