New phase-based B₁ mapping method using two-dimensional spin-echo imaging with hyperbolic secant pulses

Purpose: To propose a new phase-based B₁-mapping method that exploits phase information created by hyperbolic secant (HS) pulses in conventional 2D spin-echo imaging.

Methods: In this B₁-mapping method, HS pulses are used to accomplish π/2 excitation and π refocusing in standard multislice spin-echo imaging. When setting the ratio of pulse lengths of the π/2 and π HS pulses to 2:1, the spin-echo phase is independent of offset frequency and varies as a function of B₁ strength. To eliminate undesired phase accumulations induced by unknown factors other than the B₁ 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.

Results: The B₁ maps obtained by using the proposed method were in accordance with the B₁ maps obtained using previous methods in both phantom and in vivo experiments.

Conclusion: The proposed method is easy to implement without any sequence modification, is insensitive to B₀ inhomogeneity and chemical shift, and is robust in a reasonably wide range of B₁ field strength.