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Abstract #0671

Digital Cross-Term Pre-Emphasis for Higher-Order Dynamic Shimming

Signe Johanna Vannesjo1, Benjamin E. Dietrich1, Matteo Pavan1, Christoph Barmet1, 2, Klaas P. Pruessmann1

1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland; 2Skope Magnetic Resonance Technologies, Zurich, Switzerland

Dynamic shimming holds promise to improve B0 homogeneity for a range of applications, however requires fast updating of the shim fields. Eddy currents limit the shim settling times and also produce strong cross-term field components. Here we show the feasibility of digital cross-term pre-emphasis applied to the input shim waveforms. The pre-emphasis is calculated based on shim impulse response functions (SIRFs) measured with a dynamic field camera. It is shown that settling times can be shortened and cross-term responses much reduced with this approach. The digital pre-emphasis allows for full flexibility in shaping the response, within hardware limitations.

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Keywords

adjust allowing always amplifiers analog application applications applied around attenuated bandwidth basing becomes biomedical body calculation calibration camera capabilities cause channel channels chosen circuits close coils compensated compensation component components comprehensive controlled cosine counteract cross decaying desired determination deviation diagonal digital distinct domain done driving dynamic eddy effectively emphasis enables equal especially example exceed except experimental exponentially fast feasibility feasible field fields filter finding flexible forward frequencies frequency full functions furthermore hardware harmonic implemented impulse indispensable individual induce induced infinite inhomogeneities input inputs instead instruments interactions inverse lies limitations linear living long make matrix maximal maximally measured measuring milliseconds must optimal optimized output performance previously principle problem produce pulse raised readouts reduced regarded remaining removes require response responses selected self sets settling several severe shape shim shimming shims since slight slope specified sphere stepped strengths strong subject suffer suppressed sweeps switch switching system targeted task term terms trapezoidal vector waveform yields