RF Screen (Faraday Cage)
The RF screen is a copper lining placed in the ceiling, walls and floor of the magnet room (the Inner Controlled Area). It keeps out spurious signals both from foreign transmitters and from the driving electronics of the MR system itself. This is required because the signal returned from the patient is very small.
The RF screen is a Faraday cage: a closed conductive structure. This includes the windows into the room: look closely at them and you will see a fine conductive mesh embedded in the glass. Waveguides (pipes) can go through the RF screen without constituting a breach in the shielding as long as their length-diameter ratio is 5:1. Pneumatic lines can be used for switches, and fibre optics for signals. Current-carrying conducting wires (e.g. power) are required to be filtered at the penetration panel. Medical gases are transported using non-conductive pipes, passed through waveguides.
The strength of signals involved in MRI are about 1000 times weaker than the spurious signals mentioned above.
Further reading on this topic:
Books: MRI From Picture to Proton p187