flip angle of less than 90 used in an MR imaging experiment. A flip angle of 90 converts all the longitudinal magnetization Mz into transverse magnetization Mxy . A flip angle a of less than 90 only "partially" converts the longitudinal magnetization, leaving a fraction cosa along the longitudinal direction (see gradient-echo pulse sequence (I), Fig. 2). When the repetition time TR is shorter than T1, use of a partial flip angle can lead to higher equilibrium signal intensity. The maximum signal intensity assuming no transverse coherence (e.g. TR longer than T2) is given by the Ernst angle for which cosa = exp(-TR/T1). For spin echo pulse sequences employing an odd number of 180 pulses (e.g. a single-echo sequence with one 180 pulse), an effect equivalent to the use of a partial flip angle is obtained by using a flip angle greater than 90 to offset the inversion of the remaining longitudinal magnetization by the 180 pulse.
NP
