New Phase-Based B-1 Mapping Method Using Two-Dimensional Spin-Echo Imaging with Hyperbolic Secant Pulses

Abstract

PurposeTo propose a new phase-based B-1-mapping method that exploits phase information created by hyperbolic secant (HS) pulses in conventional 2D spin-echo imaging. MethodsIn this B-1-mapping method, HS pulses are used to accomplish /2 excitation and refocusing in standard multislice spin-echo imaging. When setting the ratio of pulse lengths of the /2 and HS pulses to 2:1, the spin-echo phase is independent of offset frequency and varies as a function of B-1 strength. To eliminate undesired phase accumulations induced by unknown factors other than the B-1 strength, two spin-echo images are acquired using HS pulses applied with opposite frequency-sweep directions, and the resulting phase images are subtracted from each other. To demonstrate the performance of the proposed method, phantom and in vivo experiments were performed using a surface coil and a volume coil. ResultsThe B-1 maps obtained by using the proposed method were in accordance with the B-1 maps obtained using previous methods in both phantom and in vivo experiments. ConclusionThe proposed method is easy to implement without any sequence modification, is insensitive to B-0 inhomogeneity and chemical shift, and is robust in a reasonably wide range of B-1 field strength. Magn Reson Med 73:170-181, 2015.