The effect of and correction for through-slice dephasing on 2D gradient-echo double angle B1+ mapping
Gabriela Belsley, Damian J. Tyler, Matthew D. Robson, Elizabeth M. Tunnicliffe
Abstract
Purpose
To show that B0 variations through slice and slice profile effects are two major confounders affecting 2D dual angle B1+ maps using gradient-echo signals and thus need to be corrected to obtain accurate B1+ maps.
Methods
The 2D gradient-echo transverse complex signal was Bloch-simulated and integrated across the slice dimension including nonlinear variations in B0 inhomogeneities through slice. A nonlinear least squares fit was used to find the B1+ 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 B1+ factor scales by approximately 3.8% for every 10 Hz/cm variation in B0 along the slice direction. Higher accuracy phantom B1+ maps were obtained after applying the proposed correction; the root mean square B1+ error relative to the gold standard B1+ decreased from 6.4% to 2.6%. In vivo whole-liver T1 maps using the corrected B1+ map registered a significant decrease in T1 gradient through slice.
Conclusion
B0 inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle B1+ 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 T1 mapping using a spoiled gradient recalled echo sequence.