Accurate actual flip angle imaging (AFI) in the presence of fat

Abstract

Purpose: To investigate the effect of incomplete fat spoiling on the accuracy of B₁ mapping with actual flip angle imaging (AFI) and to propose a method to minimize the errors using the chemical shift properties of fat.

Theory and methods: Diffusion-based dephasing is the main spoiling mechanism exploited in AFI. However, a very low diffusion in fat may make the spoiling insufficient, leading to ghosts in the B₁ maps. As the errors retain the chemical-shift signature of fat, their impact can be minimized using chemical-shift-based fat signal removal from AFI acquisition modified to include multi-echo readout. The source of the errors and the proposed correction were studied in simulations and phantom and in-vivo imaging experiments.

Results: Our results support that AFI artifacts are caused by the incomplete fat spoiling present in clinically attractive short TR acquisition regimes. The correction eliminated the ghosting and significantly improved the B₁ mapping accuracy as well as the accuracy of R₁ mapping performed with AFI-derived B₁ maps.

Conclusions: The incomplete fat signal spoiling may be a source of AFI B₁ mapping errors, especially in subjects with high fat content. Achieving complete fat spoiling requires longer TR, which is undesirable in clinical applications. The proposed approach based on fat signal removal can reduce errors without significant prolongation of the AFI pulse sequence. We propose that, when attaining complete fat spoiling is not feasible, AFI mapping should be performed in a multi-echo regime with appropriate fat separation or suppression to minimize these errors.

Keywords: B1 mapping; actual flip angle imaging (AFI); fat‐water separation; spoiling.