A new method to measure rotating frame relaxation and to create contrast for MRI is introduced. The technique exploits relaxation along a fictitious field (RAFF) generated by amplitude- and frequency-modulated irradiation in a sub-adiabatic condition. Here, RAFF is demonstrated using a radiofrequency pulse based on sine and cosine amplitude and frequency modulations of equal amplitudes, which gives rise to a stationary fictitious magnetic field in a doubly rotating frame. According to dipolar relaxation theory, the RAFF relaxation time constant (TRAFF) was found to differ from laboratory frame relaxation times (T1 and T2) and rotating frame relaxation times (T1ρ and T2ρ). This prediction was supported by experimental results obtained from human brain in vivo and three different solutions. Results from relaxation mapping in human brain demonstrated the ability to create MRI contrast based on RAFF. The value of TRAFF was found to be insensitive to the initial orientation of the magnetization vector. Finally, as compared with adiabatic pulse trains of equal durations, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans.

No abstract

No abstract