B1 mapping using an EPI-based double angle approach: A practical guide for correcting slice profile and B0 distortion effects.

PURPOSE: Aim of this study was to develop a reliable B1 mapping method for brain imaging based on vendor MR sequences available on clinical scanners. Correction procedures for B0 distortions and slice profile imperfections are proposed, together with a phantom experiment for deriving the approximate time-bandwidth-product (TBP) of the excitation pulse, which is usually not known for vendor sequences. METHODS: The double angle method was used, acquiring two gradient echo echo-planar imaging data sets with different excitation angles. A correction factor C (B1 , TBP, B0 ) was derived from simulations for converting double angle method signal quotients into bias-free B1 maps. In vitro and in vivo tests compare results with reference B1 maps based on an established in-house sequence. RESULTS: The simulation shows that C has a negligible B1 dependence, allowing for a polynomial approximation of C (TBP, B0 ). Signal quotients measured in a phantom experiment with known TBP reconfirm the simulation results. In vitro and in vivo B1 maps based on the proposed method, assuming TBP = 5.8 as derived from a phantom experiment, match closely the reference B1 maps. Analysis without B0 correction shows marked deviations in areas of distorted B0 , highlighting the importance of this correction. CONCLUSION: Double angle method-based B1 mapping was set up for vendor gradient echo-echo-planar imaging sequences, using a correction procedure for slice profile imperfections and B0 distortions. This will help to set up quantitative MRI studies on clinical scanners with release sequences, as the method does not require knowledge of the exact RF-pulse profiles or the use of in-house sequences.