MRI Faraday cages: an alternative protocol for shielding effectiveness measurements

The periodic evaluation of the Faraday cage efficiency for diagnostic Magnetic Resonance is mandatory by Italian legislation and it is recommended by manufacturers. An inadequate shielding could affect clinical images with artifacts, reducing the quality of examinations. The International reference standard for shielding effectiveness mea-surements is represented by IEEE Standard 299-2006, but it results poorly applicable in Magnetic Resonance sites. In this study, an alternative approach is proposed to simplify the applicability of the IEEE Standard. The experimental set-up for measurements is composed by a signal generator and a transmitting antenna, coupled with the signal generator. The novelty of the proposed methodology consists in the absence of the receiver antenna; for this purpose, the Magnetic Resonance scanner is used. This approach permits to test the entire Faraday cage by means of a single measure. If the Magnetic Resonance device picks up the radiofrequency signal from outside of the cage, this would be visible on the acquired images as a "zipper" artifact. Four Faraday cages, different in building materials, were tested by using the proposed methodology, and the results were compared with those obtained by using the IEEE Standard. The presence of the "zipper" artifact on the images made us inspect the entire Faraday cage and not just the openings, finding that the radiofrequency signal entered from an area without openings, thus denoting a poor construction of the cage. The proposed methodology provides an immediate evaluation of the shielding efficiency, furnishing information on the entire Faraday cage and not only on its apertures. The method is simple, easily reproducible, and constitutes a valid alternative to the IEEE Standard; in particular, during constancy tests.