Passive shielding involves using materials with magnetic shielding properties. Common materials that are used for passive shielding are aluminum, silicon steel, and low carbon steel. Passive shielding can be very effective in resolving 60 Hz and higher frequency EMI issues, but has limited effectiveness in resolving lower frequency disturbances that can be created by moving metal objects such as subways and trains that operate on DC electric power. Passive shielding also requires no maintenance after installation and can often be the most cost effective solution for higher frequency (frequencies of 50 Hz and greater) EMI issues. However, this solution is dependent upon the level of the EMI and the amount of material required to resolve the issue.

Active shielding, on the other hand, utilizes electronics and coil systems to create a cancelling magnetic field over a predetermined volume. Active shielding, unlike passive shielding, can be very effective at lower frequencies (frequencies below 100 Hz). There are several benefits associated with active shielding solutions. First, an active system has the ability to respond to a changing environment. Therefore, if the EMI environment becomes worse, a well designed active system will be able to respond to those changes and maintain an EMI environment that meets manufacturer specifications for EMI sensitive equipment. Second, an active system requires considerably less construction. An active system typically requires the installation of coils unlike passive shielding, which often requires the installation of a six sided structure that requires interior finishes. Additionally, since passive shielding is a much less effective solution for lower frequency applications, active shielding is often a more cost effective solution to EMI issues generated by subways and moving metal. In the event that a particular site does or will experience EMI issues, solutions should be evaluated based upon the effectiveness of the solution and the cost to implement the solution.


Vibration

ETS-Lindgren uses isolators under the magnet to perform vibration surveys
Imaging equipment, particularly CT and MRI scanners, can be impacted by building vibrations. Fortunately, most vibration concerns can be addressed through careful design and construction of the area to house sensitive pieces of equipment. In existing buildings, it is critical to perform a vibration survey. A survey will quickly determine whether the existing structure meets the vibration requirements of the equipment being sited. If the equipment does not meet the vibration criteria, a good survey will analyze the cause of the vibration and provide general recommendations to resolve vibration issues. Many vibration issues can be resolved through inexpensive solutions. For example, a nearby mechanical room may not include isolation pads or isolators for mechanical equipment that induce vibrations into the structure. This may simply require placing vibration isolators under such equipment. It may also be possible to use vibration isolators on some imaging equipment such as MRIs, which would allow for the attenuation of vibrations and decoupling of the equipment from the structure due to the benefits of the isolators. However, in some situations, it may be necessary to stiffen the structure under the imaging equipment or build an isolated slab to meet the equipment's vibration criteria. The unfortunate aspect is that isolated slabs and modifications to the structure can be quite expensive. However, such a building retrofit is much less costly to perform during the process of outfitting a space and installing imaging equipment. Performing a retrofit after imaging equipment has been installed and found to be adversely affected by vibrations can be extremely expensive.

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