Sound generation in gradient coil structures for MRI

Abstract

When supporting plates of plastic material are subjected to alternating transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a modulation of the plate surface that launches an acoustic wave in air along the magnetic field axis. We have extended our previous theory describing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer diffraction at a distance from the plate surface. The extended theory predicts that the observed acoustic signal midpoint and normal to the plate surface gives a variation with frequency in approximate agreement with our previous measurements. The acoustic output off axis shows acoustic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material. The new results could have important ramifications for the minimization of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centrally and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are estimates of the compressional wave velocities deduced from frequency response data recorded at the center-point of a number of different plates.

Active acoustic control in gradient coils for MRI

“…Torsional and bending or Chladni resonances of the plate, although ever-present, do not seem to dominate the acoustic output, as in our earlier work (10), and have been ignored in our analysis. However, we suspect that the sometimes erratic behavior of current phases and amplitudes is attributable to these effects.…”

“…A preliminary report is published elsewhere (9). This work is based on an earlier study of noise generation in a rigid rectangular plate (10) forming one sector of a rectangular coil gradient assembly. This study has led us to a better understanding of the fundamental process of acoustic noise generation.…”

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

“…Although the data gathered to date do not allow us to conclusively identify the mechanism underlying this correlation, we believe that the existence of resonant peaks in the acoustic spectrum strongly suggests that mechanical resonances of the gradient coil structure are the source of this acoustic energy (11,12). We speculate that alterations in mechanical properties of the gradient coil (material stiffness, composite inclusions, etc.)…”

Acoustic noise and functional magnetic resonance imaging: Current strategies and future prospects

“…Scanner acoustic noise is derived from the mechanical oscillation of the gradient coils placed in a magnetic field (24,25). The Lorentzian forces induced when an electrical current is applied to these coils make them physically move.…”