Benefits and Pitfalls

B₁ mapping is of interest for diverse MRI applications, and several mapping techniques have been developed. The DAM method consists of acquiring two scans at two different flip angles. To avoid the dependence of the signal on T₁, long repetition times are required to allow the recovery of the longitudinal magnetization between pulses Yarnykh, 2007Insko & Bolinger, 1993. The AFI method overcomes this practical limitation by repeating the pulse sequence at a fast rate to achieve a pulsed state of magnetization and shorter time delays between pulses. In addition, due to scan-time constraints, B₁ mapping methods are often implemented in 2D Chavez & Stanisz, 2012. However, the accuracy of the measurements of 2D B₁ mapping techniques is compromised by the slice profile effects due to the problem of nonuniform excitation across slices Yarnykh, 2007Chavez & Stanisz, 2012. The AFI method on the other hand, adresses this issue using a fast 3D implementation leading to scans with an excellent anatomical coverage in clinically feasible times, with an increase in signal-to-noise ratio compared to 2D multislice acquisitions.

The performance of the AFI method is based on the following assumptions. First, the two images acquired at different times should be registered to avoid motion effects. It is also assumed that the signal is insensitive to the main magnetic field non-uniformities and chemical shift effects that are canceled out when taking the signal ratio r Yarnykh, 2007. Despite some clear advantages over other B₁ mapping techniques, the application of spoiler gradients to mitigate the T₁ dependence can be a limitation due to significant levels of RF power depositions Sacolick et al., 2010. Furthermore, it is necessary to adapt the AFI pulse sequence to different scanner manufacturers, and programming experience is required to bring this technique into the clinic.