Abstract:
:The increased magnetic susceptibility effects at higher magnetic fields increase the demands for shimming of the B0 field for in vivo MRI and MRS. Both static and dynamic techniques have been developed to compensate for susceptibility-induced field inhomogeneities. In this study, we investigate the impact of and need for both static and dynamic higher order B0 shimming of magnetic field homogeneities in clinical breast MRI at 7 T. Both global and local field variations at lipid-tissue interfaces were observed in the magnetic field using TE-optimized B0 mapping at 7 T. With static B0 shimming, a field homogeneity of 39 ± 11 Hz (n = 48) was reached in a single breast using second-order shimming. Further compensation of the residual local field inhomogeneities caused by lipid-tissue interfaces does not seem to be feasible with shallow spherical harmonic fields. For bilateral shimming, the shimming quality was significantly less at 62 ± 15 Hz (n = 22) over both breasts, even after (simulated) fourth-order shimming. In addition, a substantial time-dependent field instability of 30 Hz peak to peak, with significant higher order field contributions, was observed during regular breathing. In conclusion, TE-optimized B0 field mapping reveals substantial field variations in the lipid-rich environment of the human breast, in both space and time. The static field variations could be partially minimized by third-order B0 shimming, providing sufficient lipid suppression. However, in order to fully benefit from the increased spectral dispersion at high fields, the significant magnetic field variations during breathing need to be considered.
journal_name
NMR Biomedjournal_title
NMR in biomedicineauthors
Boer VO,Luttje MP,Luijten PR,Klomp DWdoi
10.1002/nbm.3096subject
Has Abstractpub_date
2014-06-01 00:00:00pages
625-31issue
6eissn
0952-3480issn
1099-1492journal_volume
27pub_type
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