In the presence of radiofrequency irradiation, relaxation of magnetization aligned with the effective magnetic field is characterized by the time constant T1ρ. On the other hand, the time constant T2ρ characterizes the relaxation of magnetization that is perpendicular to the effective field. Here, it is shown that T2ρ can be measured directly with Carr-Purcell sequences composed of a train of adiabatic full-passage (AFP) pulses. During adiabatic rotation, T2ρ 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 T2ρ relaxation in the fast-exchange regime, with time constant defined as T2ρ,ex. The derived theory predicts the rate constant R2ρ,ex( = 1/T2ρ,ex) to be dependent on the choice of amplitude- and frequency-modulation functions used in the AFP pulses. Measurements of R2ρ,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.
Bibliographical noteFunding Information:
This research was supported by NIH Grants P41 RR08079 and RO1 CA92004, Keck Foundation, National Foundation for Functional Brain Imaging and the US Department of Energy. Authors thank Dr. Slobodan I. Macura for helpful discussions.
- Adiabatic pulse
- Chemical exchange
- Rotating frame
- Transverse relaxation