Gradient-induced acoustic and magnetic field fluctuations in a 4T whole-body MR imager

Both the acoustic and magnetic fluctuation frequency response functions for a Siemens AS25 body gradient coil inside a 4 Tesla whole-body MR system were measured and analyzed in this study. In an attempt to correlate the acoustic noise inside the gradient coil with magnetic field oscillations, triangular and trapezoidal gradient impulses of varying amplitudes and widths were used to excite the gradient coil. The acoustic and magnetic responses to these inputs were measured. The results show the existence of discrete resonances in both acoustic and uniform magnetic field fluctuation spectra, while gradient magnetic field fluctuation spectra show no such resonances. In addition, the dominant amplitude peaks in spectra fluctuate similarly with respect to trapezoidal gradient impulse flat-top widths. This implies that these phenomena are correlated, and that the trapezoidal impulse flat-top width may be used as a way to suppress both acoustic noise and uniform magnetic field oscillations. Magn Reson Med 44:532–536, 2000. © 2000 Wiley-Liss, Inc.