Evolution of magnetization in a B-1 field. I. The impact of B-0/B-1 inhomogeneity and fast chemical exchange in high-resolution NMR

Since the classical treatment by Torrey in 1949 of the problem of how magne tization evolves in a continuously applied B-1 radiofrequency held, the top ic has received relatively little attention in the nuclear magnetic resonan ce literature. This article focuses on the behavior of the macroscopic magn etization as well as that of its isochromatic constituents in both "hard" a nd "soft" resonant B-1 fields by examining the effects of B-0 and B-1 inhom ogeneities and moderately fast chemical exchange. Discussion embraces the o ut-of-coil regions where spins experience large B-1 inhomogeneity and give, under suitable conditions, well-detected signals. It is shown that the eff ect of Delta B-1 inhomogeneity is formally similar to increased transverse decay and longitudinal recovery rates in a homogeneous B-1 field, and can l ead to two different types of transient phase randomization of isochromatic net magnetizations, which in turn give different macroscopic behaviors. So me practical implications relating to these considerations are also noted. (C) 1999 John Wiley & Sons, Inc.