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_1ρ. On the other hand, the time constant T_2ρ characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T_2ρ can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T_2ρ 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_2ρ relaxation in the fast-exchange regime, with time constant defined as T_2ρ,ex. The derived theory predicts the rate constant R_2ρ,ex( = 1/T_2ρ,ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R_2ρ,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.