In Vivo Nitroxide Spin Label Imaging by Overhauser Effect

Double resonance is a well known development of magnetic resonance in which one resonant transition of a system is excited while simultaneously a different transition is monitored. Since the first works of the early fifties, double resonance has been widely applied in NMR and in EPR [1]. Only recently, double resonance has been used for imaging [2] and for in vivo applications [3]. The possibility to monitor the tissular oxygen content is emphasized here because the oxygen content of tissues is an extremely important parameter for the energetic metabolism and therefore is involved in numerous physiological and pathological processes. Free radicals are commonly studied by EPR measurements performed at X or Q band (9.5 and 35 GHz respectively). At these frequencies, losses in the magnetic and electric field require the use of flat cells to achieve a good penetration of the microwave into samples of physiological conductivity. For EPR imaging, radiofrequency of 250 MHz was recently selected to obtain a good penetration into small animals [4] but the images display a very poor resolution. Double resonance of free radicals dissolved into water combines the advantages of continous wave EPR sensitivity and pulsed NMR resolution. In order to achieve a good penetration of the RF waves, a low magnetic field of 6.8 mT was choosen. In this case the electronic resonance (i/ ) of the free radical (Fremy's salt: 0N(S03)2) is 197.4 MHz and the nuclear resonance (i/ ) of the proton of the water is 289 kHz. At equilibrium the population of each electronic and nuclear state is given by the Boltzman distribution. If, using a strong RF wave at v , the electronic polarization is forced to a different value from that at equilibrium, then the steady state polarization of the protons becomes : 1 = f p