Prediction of motion induced magnetic fields for human brain MRI at 3 T

Maps of B₀ field inhomogeneities are often used to improve MRI image quality, even in a retrospective fashion. These field inhomogeneities depend on the exact head position within the static field but acquiring field maps (FM) at every position is time consuming. Here we propose a forward simulation strategy to obtain B₀ predictions at different head-positions.

FM were predicted by combining (1) a multi-class tissue model for estimation of tissue-induced fields, (2) a linear k-space model for capturing gradient imperfections, (3) a dipole estimation for quantifying lower-body perturbing fields (4) and a position-dependent tissue mask to model FM alterations caused by large motion effects. The performance of the combined simulation strategy was compared with an approach based on a rigid body transformation of the FM measured in the reference position to the new position.

The transformed FM provided inconsistent results for large head movements (> 5° rotation, approximately), while the simulation strategy had a superior prediction accuracy for all positions. The simulated FM was used to optimize B₀ shims with up to 22.2% improvement with respect to the transformed FM approach.

The proposed simulation strategy is able to predict movement-induced B₀ field inhomogeneities yielding more precise estimates of the ground truth field homogeneity than the transformed FM.