Transverse relaxation in the rotating frame induced by chemical exchange

In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T-1rho. On the other hand, the time constant T-2rho characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that Tip can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation.. Tip characterizes the relaxation of the magnetization, which under adiabatic conditions remains approximately perpendicular to the time-dependent effective field. Theory is derived to describe the influence of chemical exchange on T-2rho relaxation in the fast-exchange regime, with time constant defined as T-2rho,T-ex. The derived theory predicts the rate constant R-2rho,R-ex (1/T-2rho,T-ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R-2rho,R-ex of the water/ethanol exchanging system confirm the predicted dependence on modulation functions. The described theoretical framework and adiabatic methods represent new tools to probe exchanging systems. (C) 2004 Elsevier Inc. All rights reserved.