A Faraday cage is a conductive enclosure used to shield the inner space from electromagnetic interference. In MR imaging, this type of structure provides radiofrequency shielding to the scanning room to minimize occurrence of interference-associated imaging artifact.
The cage is usually formed by a conducting material which effectively prevents an external static electrical field from affecting the space within the cage. In essence, the cage acts as a hollow conductor whereby the electrical charge flows around the protected space instead of through it. The charge of the cage material itself will align directly opposite to the potential difference of the external static electric field. This produces a counteracting electrical field within the cage to cancel out the external field.
Application in MRI
Radiofrequency shielding in MRI is crucial to prevent both external radiofrequency pulses from interfering with the detector and the radiofrequency pulses generated by the scanner from interfering with the external environment.
All radiofrequency shielding uses the concept of a Faraday cage to some extent and usually includes shielding the entire MRI room and control unit. The metal cage can be seen running through the special glass of the window separating the control room from the scanner itself.