Accurate B1 Mapping with Actual Flip Angle Imaging (AFI) in the Presence of Fat

Actual Flip angle Imaging (AFI) is an efficient B1 mapping method requiring the proper spoiling of transverse magnetization. Optimal spoiling can be achieved using large spoiling gradients enabling water diffusion-based signal decay. However, spoiling the non-aqueous signal like from fat is typically ignored in AFI optimizations. We demonstrate that infinitesimal diffusion in the fat signal makes fat spoiling in AFI unachievable in the reasonable scan time and that incomplete fat spoiling is a major source of previously unexplained AFI errors. We propose method to minimize them using a superposition model of the spoiling artifacts and chemical shift encoded fat/water separation.