Purpose: Fluctuations of the background magnetic field (B0)
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 B0-fluctuations in the brain due to
body and breathing motion are reflected in external field probe
Methods: Field probe measurements were compared with
scanner acquired B0-maps from experiments with breathing
and shoulder movements. A realistic simulation of B0-fluctuations
caused by breathing was performed, and used for testing
different sets of field probe positions.
Results: The B0-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 3rd 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.
- Field fluctuations
- NMR field probes
- 7 Tesla
- Dynamic shimming
- Field mapping