CONSTANT-RELAXATION METHODS FOR DIFFUSION MEASUREMENTS IN THE FRINGE-FIELD OF SUPERCONDUCTING MAGNETS

A number of new NMR methods have been introduced for measuring molecular self-diffusion coefficients in the fringe field of superconducting magnets, avoiding the determination of longitudinal and transverse magnetization relaxation rates. Constant-relaxation methods based on the stimulated-echo product signals allow the determination of both constant and time-dependent diffusion coefficients. A density-operator formalism taking into account phenomenologically the effect of relaxation and diffusion encoding on the Liouville coordinates has been used to introduce and characterize an inhomogeneous mixed echo. This echo corresponds to a spatial and temporal superposition of a conventional spin echo and a stimulated echo. The mixed echo is generated by the action of a pulse sequence based on a stimulated-echo three-pulse sequence with an insertion of a Carr-Purcell-like pulse sequence in the last pulse interval. A constant relaxation method using a Carr-Purcell mixed-echo pulse sequence has been developed. It has proved to be advantageous when T-2 is not very small compared to T-1. The performance and limitations of these methods are discussed and demonstrated experimentally. (C) 1994 Academic Press, Inc.