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PURPOSE: Applications of dynamic shimming require high field fidelity, and characterizing the shim field dynamics is therefore necessary. Modeling the system as linear and time-invariant, the purpose of this work was to measure the impulse response function with optimal sensitivity. THEORY AND METHODS: Frequency-swept pulses as inputs are analyzed theoretically, showing that the sweep speed is a key factor for the measurement sensitivity. By adjusting the sweep speed it is possible to achieve any prescribed noise profile in the measured system response. Impulse response functions were obtained for the third-order shim system of a 7 Tesla whole-body MR scanner. Measurements of the shim fields were done with a dynamic field camera, yielding also cross-term responses. RESULTS: The measured shim impulse response functions revealed system characteristics such as response bandwidth, eddy currents and specific resonances, possibly of mechanical origin. Field predictions based on the shim characterization were shown to agree well with directly measured fields, also in the cross-terms. CONCLUSION: Frequency sweeps provide a flexible tool for shim or gradient system characterization. This may prove useful for applications involving dynamic shimming by yielding accurate estimates of the shim fields and a basis for setting shim pre-emphasis.

Original publication

DOI

10.1002/mrm.24934

Type

Journal article

Journal

Magn Reson Med

Publication Date

08/2014

Volume

72

Pages

570 - 583

Keywords

dynamic field camera, dynamic shimming, eddy currents, gradient characterization, impulse response function, magnetic field monitoring, shim characterization, Algorithms, Equipment Design, Equipment Failure Analysis, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Phantoms, Imaging, Reproducibility of Results, Sensitivity and Specificity