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.
Thorium-induced Anatomical and Histopathological Changes in Liver of Swiss Mice
ObjectivePresent study is aimed to investigate the anatomical and histopathological changes in liver tissues of Swiss albino male mice, one month after intravenous administration of thorium (232Th; 4 and 40 mg/kg).MethodsSynchrotron X-ray micro-CT imaging, CD31 immuno-cytochemistry, hematoxylin & eosin staining and Periodic acid-Schiff staining were performed to study changes in liver anatomy, blood vessels/capillaries, histology and glycogen content of liver, respectively.ResultsSynchrotron X-ray micro-CT imaging of liver showed loss of blood vessels in mice treated with thorium (4 mg/kg), which was more prominent at higher dose of thorium (40 mg/kg). These thorium-induced changes in liver were correlated with the decrease in CD31 positive cells and loss of tissue architecture. A dose-dependent increase in glycogen content was also observed in the liver of thorium-treated mice.ConclusionOur results provide novel insight about the effects of thorium on liver, which may have significant implications in understanding the mechanism of thorium thorium-induced hepatotoxicity.