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

Abstract

AbstractPurposeTo show that variations through slice and slice profile effects are two major confounders affecting 2D dual angle maps using gradient‐echo signals and thus need to be corrected to obtain accurate maps.MethodsThe 2D gradient‐echo transverse complex signal was Bloch‐simulated and integrated across the slice dimension including nonlinear variations in inhomogeneities through slice. A nonlinear least squares fit was used to find the 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.ResultsFor our RF excitation pulse, the error in the factor scales by approximately 3.8% for every 10 Hz/cm variation in along the slice direction. Higher accuracy phantom maps were obtained after applying the proposed correction; the root mean square error relative to the gold standard decreased from 6.4% to 2.6%. In vivo whole‐liver maps using the corrected map registered a significant decrease in gradient through slice.Conclusion inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle 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 mapping using a spoiled gradient recalled echo sequence.