Temporal and spatial analysis of fields generated by eddy currents in superconducting magnets: optimization of corrections and quantitative characterization of magnet/gradient systems.
We propose methods for the spatial and temporal characterization of time-dependent magnetic fields generated by eddy currents after switching gradients. For an on-line determination of the temporal variations of the fields, we extract two terms from the unresolved signal of an extended sample, describing the time evolution of a frequency shift gamma delta Bz(t) and of a decay constant k(t). This procedure allows us to optimize interactively the multiexponential pre-emphasis as well as any spectral volume selection method with respect to eddy currents. Additionally, we suggest an imaging sequence which allows us to determine the spatial distribution of eddy current fields at a chosen time-point after any gradient sequence to be tested. Expansion of these eddy currents fields into spherical harmonic functions proves the existence of a higher order terms, which cannot be corrected by a standard pre-emphasis device, where time constants and amplitudes are adjusted on the X, Y, Z, and Z0 coils. The proposed numerical analysis gives a tool to characterize any magnet/gradient system quantitatively with respect to eddy current performance.
Boesch
C
C
University Children's Hospital, Zürich, Switzerland.
Gruetter
R
R
Martin
E
E
D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008280:Magnetics; D013997:Time Factors