Hardware
Sources of B0 Inhomogeneities
Although scanner manufacturers try to make magnets that are as homogeneous as possible, they are far from perfect. The manufacturing process requires many kilometers of superconducting wire to be wound to create the main magnet and can lead to inhomogeneities due to manufacturing tolerances. Moreover, large metal objects near the scanner can interact with the field created by the scanner and impact the resulting field within the scanner. This is a more important problem with higher field strength. During the installation process, the empty bore is homogenized in a process called passive shimming. During this process, small ferromagnetic pieces are introduced in the scanner at optimized locations to produce a field that counteracts the inhomogeneities. Hardware inhomogeneities are relatively small (less than 1 ppm Webb, 2016).
Specialized equipment such as field probes Dietrich et al., 2016 (e.g.: Skope Magnetic Resonance Technologies, LLC) can be used to evaluate the B0 field of the scanner while it is being installed. This equipment can also be used after installation because it is more precise than B0 field maps and offers better field temporal resolution, allowing the ability to observe eddy currents created from gradient switching.
During an imaging session, heating of the different components and of the main magnet can lead to temperature-dependent changes in the B0 field. These can be observed by a frequency drift in the field. As an example, a ~0.4Hz/min has been observed in MRS at 3T but depends on multiple factors Academic Press, 2021. Modern scanners usually have systems in place to evaluate and correct for this drift El-Sharkawy et al., 2006.
- Webb, A. G. (2016). Magnetic Resonance Technology: Hardware and System Component Design. Royal Society of Chemistry.
- Dietrich, B. E., Brunner, D. O., Wilm, B. J., Barmet, C., Gross, S., Kasper, L., Haeberlin, M., Schmid, T., Vannesjo, S. J., & Pruessmann, K. P. (2016). A field camera for MR sequence monitoring and system analysis. Magn. Reson. Med., 75(4), 1831–1840.
- Frequency drift in MR spectroscopy at 3T. (2021). Neuroimage, 241, 118430.
- El-Sharkawy, A. M., Schär, M., Bottomley, P. A., & Atalar, E. (2006). Monitoring and correcting spatio-temporal variations of the MR scanner’s static magnetic field. MAGMA, 19(5), 223–236.