MR Spin Behavior During RF Pulses: T2 vs. T2' Relaxation

Introduction: We investigate the behavior of the MR magnetization vector during RF pulses in the presence of rapid transverse relaxation. The transverse relaxation is divided into two separate mechanisms: Homogeneously broadened lines which resulting in irreversible T2 amplitude decay, and inhomogeneously broadened lines resulting in dephasing and in principle refocusable T2’. While the two different mechanisms by themselves result in identical free induction decay (FID) behavior, it is shown they do not follow the same magnetization trajectory during the application of external RF pulses. Therefore, RF optimization strategies based solely on the classical Bloch equations [1-3] may benefit from additional modifications described here when imaging species with high inhomogeneously broadened linewidths (for example due to susceptibility effects) as found in bone.