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Estimation of Shielding Effectiveness for Electromagnetic Interference Using An Infinite Plate Model

X Jiang

X Jiang*, K Little , D Hintenlang , Department of Radiology, Ohio State University, Columbus, OH


SU-I-GPD-P-13 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Image artifacts caused by electromagnetic interference (EMI) have been reported for digital radiography and mammography. During a recent mammography system installation at the authors’ site, artifacts were observed on the image receptor due to an adjacent transformer and power cables, which produced magnetic fields up to 72mG in the mammography room. Magnetic shielding was determined to be necessary to reduce the field, and a target shielding effectiveness (SE) of 40dB was proposed. Discussion of EMI shielding at utility frequencies for medical imaging is scarce in the literature. In this study, we use an infinite planar sheet model to estimate the SE for various materials.

Methods: Three mechanisms contribute to the attenuation of an electromagnetic wave by a metal sheet: reflection, absorption, and multiple internal reflections. The magnitude of each contribution can be predicted by matching Maxwell’s equations to appropriate boundary conditions, and general solutions have been provided in the literature. Using these equations, we investigate the SE of various materials at 60Hz.

Results: When only moderate EMI reduction is required, a thin layer of high conductivity materials will usually provide sufficient SE through high reflection loss, e.g., ~0.8mm copper or ~1.9mm aluminum provides 20dB SE. When higher SE is required, high permeability materials are preferable due to their high absorption loss, which increases proportionally to thickness. ~5.3mm/~7.8mm M36 Si-steel provides a SE of 40dB/60dB. If even higher SE is desired, very high permeability materials such as mu-metal or permalloy may be used. Additionally, it was found that a small source-to-shield distance (≤50cm) reduces the SE of high conductivity materials while increasing that of high permeability materials.

Conclusion: EMI shielding can provide an effective approach to reduce EMI related image artifacts. An infinite plate model may be used as guidance for the selection of an appropriate material and thickness.

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