The effect of and correction for through-slice dephasing on 2D gradient-echo double angle B 1 + mapping

To show that $B_0$ 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.

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.

For our RF excitation pulse, the error in the $B_1^{+}$ factor scales by approximately 3.8% for every 10 Hz/cm variation in $B_0$ along the slice direction. Higher accuracy phantom $B_1^{+}$ maps were obtained after applying the proposed correction; the root mean square $B_1^{+}$ 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.

$B_0$ 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.