Vibro-Acoustic Analysis of Acoustic Liners For MRI Scanners
Introduction Magnetic Resonance Imaging (MRI) scanners are popular medical diagnostic devices providing useful information about patients in a noninvasive manner. However, a disadvantage of MRI is the high level of acoustic noise generated during scanning, which is caused by the Lorentz forces acting on the gradient coils bound within an epoxy resin cylinder. As the gradient current switches direction the gradient windings vibrate, leading to the generation of sound waves [1]. Active and passive methods are commonly used for noise control. Due to the frequency limitation of Active Noise Control methods (normally effective only at relatively low frequencies), a passive noise control method using an acoustic liner was investigated by Mechefske et al. [2]. To extend their work, this paper shows some simulation results that predict the noise blocking effect of an acoustic liner, which could be used to attenuate the noise propagating into the bore of the MRI scanner (see Figure 1). Vibro-acoustic analysis was conducted to predict the acoustic performance of the liners with different thickness based on the coupling of a finite element (FE) structural model and a boundary element (BE) acoustical model.
Investigation of acoustic noise on 15 MRI scanners from 0.2 T to 3 T
Acoustic noise levels for fast MRI pulse sequences were surveyed on 14 systems with field strengths ranging from 0.2 T to 3 T. A microphone insensitive to the magnetic environment was placed close to the magnet isocenter and connected via an extension cable to a sound level meter outside the scan room. Measured noise levels varied from 82.5 ± 0.1 dB(A) for a 0.23 T system to 118.4 ± 1.3 dB(A) for a 3 T system. Further measurements on four of the closed‐bore systems surveyed showed that: 1) pulse sequence parameters (particularly FOV and TR) were more influential in determining noise level than field strength, 2) the noise level was found to vary along the z‐direction with a maximum near the bore entrance, and 3) in one of two systems tested there was a significant increase in noise with a volunteer present instead of a test object. The results underline the importance of hearing protection for patients and for staff spending extended periods in the scan room. J. Magn. Reson. Imaging 2001;13:288–293. © 2001 Wiley‐Liss, Inc.
Parameters for low energy operation of CESR
We present a detailed design for operation of the Cornell Electron Storage Ring, CESR, in the energy range covering the /spl psi/ resonances and the Charm and Tau thresholds, as well as reaching to the upper Y resonances near 11 GeV. The addition of 18 m of super-ferric wiggler magnets will partially restore low energy beam emittance and damping times. Installation of superconducting quadrupoles in the interaction region and the addition of high performance superconducting RF cavities will enhance performance at all energies. Studies of optics, beam dynamics, wiggler and vacuum system performance, beam stability, and beam-beam effects confirm operation with a luminosity of 3/spl times/ 10/sup 32/ CM/sup -2/ sec/sup -1/ at 1.88 GeV.