Shimming

Spectral overlap and hence quantification precision and discriminability of metabolites strongly depend on shim quality.  In addition, frequency selective solvent and artifact suppression sequences fail in presence of major B₀ inhomogeneities. Also phase encoding or even more pronounced echo-planar encoding for magnetic resonance spectroscopic imaging is hampered by B₀ inhomogeneities. Hence successful B₀ inhomogeneity correction is absolutely crucial for any successful application of in vivo magnetic resonance spectroscopy.

At high and ultra-high field strength the impact of macroscopic and mesoscopic susceptibility differences on B₀ inhomogeneities is more pronounced in comparison to lower field strength. In addition, respiratory motion causes a fluctuation in the locally experienced B₀ field, which is especially problematic for MRS applications in the spinal cord, the myocardium or the liver.

To overcome high-field related problems additional shim coils that allow for B₀ inhomogeneity correction of higher orders in space and stronger shim currents are necessary. This project investigates B₀-mapping based shim algorithms to accurately determine higher order shim corrections, additional passive and active insert shim elements as well as dynamic shim update in order to compensate for respiratory motion and to optimize shimming in multi-slice MRSI examinations.