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

PURPOSE

Aim of this study was to develop a reliable B mapping method for brain imaging based on vendor MR sequences available on clinical scanners. Correction procedures for B 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 (B , TBP, B ) was derived from simulations for converting double angle method signal quotients into bias-free B maps. In vitro and in vivo tests compare results with reference B maps based on an established in-house sequence.

RESULTS

The simulation shows that C has a negligible B dependence, allowing for a polynomial approximation of C (TBP, B ). Signal quotients measured in a phantom experiment with known TBP reconfirm the simulation results. In vitro and in vivo B maps based on the proposed method, assuming TBP = 5.8 as derived from a phantom experiment, match closely the reference B maps. Analysis without B correction shows marked deviations in areas of distorted B , highlighting the importance of this correction.

CONCLUSION

Double angle method-based B mapping was set up for vendor gradient echo-echo-planar imaging sequences, using a correction procedure for slice profile imperfections and B 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.