Abstract
In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T 1 ρ . On the other hand, the time constant T 2 ρ characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T 2 ρ can be measured directly with Carr–Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T 2 ρ 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 2 ρ relaxation in the fast-exchange regime, with time constant defined as T 2 ρ,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 2 ρ,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.
Published Version
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