title: MRI contrast from relaxation along a fictitious field (RAFF).
abstract: 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 subadiabatic 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 (T(RAFF)) was found to differ from laboratory frame relaxation times (T(1) and T(2)) and rotating frame relaxation times (T(1ρ) and T(2ρ)). 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 T(RAFF) was found to be insensitive to the initial orientation of the magnetization vector. In the RAFF method, the useful bandwidth did not decrease as the train length increased. Finally, as compared with an adiabatic pulse train of equal duration, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans.
authors.affiliation: Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Timo
authors.initials: T
authors.lastname: Liimatainen
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Sorce
authors.forename: Robert
authors.initials: R
authors.lastname: O'Connell
authors.forename: Michael
authors.initials: M
authors.lastname: Garwood
authors.forename: Shalom
authors.initials: S
authors.lastname: Michaeli
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity