Measuring motion-induced B<inf>0</inf>-fluctuations in the brain using field probes

Purpose: Fluctuations of the background magnetic field (B<inf>0</inf>) due to body and breathing motion can lead to significant artifacts in brain imaging at ultrahigh field. Corrections based on real-time sensing using external field probes show great potential. This study evaluates different aspects of field interpolation from these probes into the brain which is implicit in such methods. Measurements and simulations were performed to quantify how well B<inf>0</inf>-fluctuations in the brain due to body and breathing motion are reflected in external field probe measurements. Methods: Field probe measurements were compared with scanner acquired B<inf>0</inf>-maps from experiments with breathing and shoulder movements. A realistic simulation of B<inf>0</inf>-fluctuations caused by breathing was performed, and used for testing different sets of field probe positions. Results: The B<inf>0</inf>-fluctuations were well reflected in the field probe measurements in the shoulder experiments, while the breathing experiments showed only moderate correspondence. The simulations showed the importance of the probe positions, and that performing full 3^rd order corrections based on 16 field probes is not recommended. Conclusion: Methods for quantitative assessment of the field interpolation problem were developed and demonstrated. Field corrections based on external field measurements show great potential, although potential pitfalls were identified.