CHARACTERIZATION OF ACOUSTIC NOISE AND MAGNETIC FIELD FLUCTUATIONS IN A 4 T WHOLE-BODY MRI SCANNER

Abstract

High-field, high-speed magnetic resonance imaging (MRI) can generate high levels of sound within and around the scanner. The process that produces the gradient magnetic field is the primary cause of this noise. With the push to greater background magnetic field strength and gradient field switching speed, in order to improve image quality and resolution, the noise situation is becoming worse. The noise levels generated increase the stress levels experienced by MRI patients. It is also suspected that the acoustic noise generated by the scanner during operation causes background magnetic field oscillations, which in turn degrade the image quality and resolution. Before any attempt to suppress the sound generated by the gradient coil cylinder can be made, the sound field and generating process both need to be characterised. This paper presents both the simulation and measurement results of a study of the sound radiation from a gradient coil cylinder within a 4 T MRI scanner. The acoustic frequency response function and the magnetic fluctuation frequency response function were also measured using triangular and trapezoidal gradient impulses of varying amplitudes and widths to excite the gradient coil. The measurement results and the FEA results for the sound pressure level distribution are presented and show close agreement. The results of the acoustic frequency response and the magnetic fluctuation frequency response measurements show the existence of strong responses in both acoustic and uniform magnetic field fluctuation spectra, while the gradient magnetic field fluctuation spectra show no such responses. In addition, the dominant amplitude peaks in both spectra fluctuate similarly with respect to varying trapezoidal gradient impulse flat-top widths.