Electron spin relaxation time measurements using radiofrequency longitudinally detected ESR and application in oximetry

Longitudinally detected ESR (LODESR) involves transverse ESR irradiation wi th a modulated source and observing oscillations in the spin magnetization parallel to the main magnetic field. In this study, radiofrequency-LODESR w as used for oximetry by measuring the relaxation times of the electron. T-1 e, and T-2e were measured by investigating LODESR signal magnitude as a fun ction of detection frequency. We have also predicted theoretically and veri fied experimentally the LODESR signal phase dependence on detection frequen cy and relaxation times. These methods are valid even for inhomogeneous lin es provided that T-1e much greater than T-2e. We have also developed a new method for measuring T-1e valid for inhomogeneous spectra, for all values o f T-1e and T-2e, based on measuring the spectral area as a function of dete ction frequency. We have measured T-1e and T-2e for lithium phthalocyanine crystals, for the nitroxide TEMPOL, and for the single line agent Triarylme thyl (TAM). Furthermore, we have collected spectra from aqueous solutions o f TEMPOL and TAM at different oxygen concentrations and confirmed that T-1e values are reduced with increased oxygen concentration. We have also measu red the spin-lattice electronic relaxation time for degassed aqueous soluti ons of the same agents at different agent concentrations. T-1e decreases as a function of concentration for TAM while it remains independent of free r adical concentration for TEMPOL, a major advantage for oxygen mapping. This method, combined with the ability of LODESR to provide images of exogenous free radicals in vivo, presents an attractive alternative to the conventio nal transverse ESR linewidth based oximetry methods. (C) 2001 Academic Pres s.