Direct measurement of fast electron spin-lattice relaxation:: Method and application to nitroxide radical solutions and Gd3+ contrast agents

A modulation technique for direct measurement of extremely fast electron spin-lattice relaxation is described. The method is based on longitudinal magnetization detection. It is applied here to study the spin-lattice relaxation times T-1e in aqueous solutions of perdeuterated nitroxide radical (T-1e approximate to 200 ns) with various concentrations as well as to two Gd DOTA complexes (T-1e approximate to 1.8-3.2 ns), which can be employed as paramagnetic contrast agents in magnetic resonance imaging.