Designing a Hyperbolic Secant Excitation Pulse to Reduce Signal Dropout in GE-EPI

Stephen James Wastling¹, Gareth John Barker¹

GE-EPI images suffer from signal dropout caused by susceptibility gradients. This can be reduced using Hyperbolic Secant (HS) RF excitation pulses with quadratic phase profiles. We determine by Bloch simulation the HS pulse parameters to give the most uniform signal response across the range of susceptibility gradients observed in the human head and show that the previous theoretical model for this is inaccurate. We also derive an expression for the bandwidth of a HS pulse used for excitation that is flip angle dependent. Finally using our optimised pulse we demonstrate recovery of signal in regions of dropout in six subjects.