The effect of and correction for through-slice dephasing on 2D gradient-echo double angle B₁⁺ mapping

Purpose: To show that B0 variations through slice and slice profile effects are two major confounders affecting 2D dual angle B-1(+) maps using gradient-echo signals and thus need to be corrected to obtain accurate B-1(+) maps.Methods: The 2D gradient-echo transverse complex signal was Bloch-simulated and integrated across the slice dimension including nonlinear variations in B-0 inhomogeneities through slice. A nonlinear least squares fit was used to find the B-1(+) factor corresponding to the best match between the two gradient-echo signals experimental ratio and the Bloch-simulated ratio. The correction was validated in phantom and in vivo at 3T.Results: For our RF excitation pulse, the error in the B-1(+) factor scales by approximately 3.8% for every 10 Hz/cm variation in B0 along the slice direction. Higher accuracy phantom B-1(+) maps were obtained after applying the proposed correction; the root mean square B1+ error relative to the gold standard B-1(+) decreased from 6.4% to 2.6%. In vivo whole-liver T-1 maps using the corrected B-1(+) map registered a significant decrease in T-1 gradient through slice.Conclusion: B-0 inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle B-1(+) maps using gradient-echo sequences. Consideration of this confounder is crucial for research relying on accurate knowledge of the true excitation flip angles, as is the case of T-1 mapping using a spoiled gradient recalled echo sequence.