Making MRI Quieter William A. Edelstein a , Robert A. Hedeen a , Richard P. Mallozzi a , Sayed-Amr El-Hamamsy a, *, Robert A. Ackermann a , Timothy J. Havens b a GE Corporate R&D, Schenectady, NY, USA 12309 b GE Medical Systems, Florence, SC, USA 29501 Received 20 August 2001; accepted 17 December 2001 Abstract We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems. © 2002 Elsevier Science Inc. All rights reserved. Keywords: acoustics; noise; acoustic noise; MRI acoustic noise; sound 1. Introduction Acoustic noise in MRI scanners has long been a concern for patient comfort and, on occasion, for patient safety [1,2]. Acoustic noise interferes with communication between MRI physicians and operators. Acoustic noise also provides an unwanted stimulus and interference during fMRI studies. When we began our work, the standard GE product scanner had a heavy ( 1000 kg) epoxy-potted shielded gradient supported on the cryostat via stiff aluminum brack- ets bolted to the bottom of the gradient assembly and the cryostat, one at each end of the magnet. In this configura- tion, fiberglass endcaps make a seal to the cryostat end flanges and to a cylindrical patient bore tube that surrounds the imaging subject. A whole-body birdcage rf coil, made of a large printed circuit board, is securely glued onto the outside of the patient bore tube. The endcaps, the cryostat flanges and inner bore, and the patient bore enclose the gradient assembly. However, the endcaps have holes for airflow. Thus sound generated by the gradient coils could reach the patient bore or scan room via the endcap holes. Also, vibrations might be mechanically conveyed from the gradient assembly which could cause the cryostat or patient bore tube to vibrate and radiate sound. We have studied acoustic noise in many modified con- figurations of this scanner. It has been widely assumed that acoustic noise comes directly from vibrations of the gradi- ent assembly. However, when we vibrationally isolated the gradients in an airtight enclosure, we found that the primary sources of acoustic noise are eddy-current-induced vibra- tions of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a “Quiet” reduced-acoustic-noise demonstration system. This scanner incorporates a variety of noise-reduction measures includ- ing a vacuum-enclosed, vibrationally isolated gradient as- sembly, a low-eddy-current rf coil and a non-conducting cryostat inner bore (NCCB). This Quiet scanner has re- duced, by about 20 dBA from the standard scanner, the * Corresponding author. Tel.: +1-518-461-6122. E-mail address: wedelst1@nycap.rr.com (W.A. Edelstein) Magnetic Resonance Imaging 20 (2002) 155–163 0730-725X/02/$ – see front matter © 2002 Elsevier Science Inc. All rights reserved. PII: S0730-725X(02)00475-7