A Simple Case of Earthing
RFI Industries was contacted about an MRI facility that had experienced a degradation of its imaging system, culminating in unreliable image quality. RFI was advised that the image deterioration had not been sudden, but had occurred over a period of time.
The MR equipment had been thoroughly checked but no specific faults could be identified. Doubts were expressed concerning the RF shielding effectiveness of the six-year-old facility. Accordingly, RFI Industries were consulted and engaged to carry out shielding effectiveness tests.
During the pre-investigation discussions it was revealed that some time previously there had been water ingress into the building. Surprising as it may seem to some, moisture and RF shielding generally are ingredients for corrosion with a resulting drop-off in RF attenuation.
Furthermore, a gradual deterioration in MR quality would be symptomatic of deterioration to the RF shielding typically resulting from crevasse corrosion between the shielding joints. RFI Industries staff therefore speculated that the solution of the problem at the facility could be of significant proportion.
Examination of the filter cable connections and door seals prior to commencing the shielding effectiveness test revealed two fundamental problems. Firstly, an earth continuity conductor had been fed through an insulated bush which had been fitted by the electrician to a spare (and sealed) position in the power filter cabinet; and secondly, the earth cables that were attached to the common earth stud were loose.
Passing unfiltered metal conductors through holes or waveguides in RF shields immediately introduces an antenna which will pass unwanted signals to and from the RF shielded area - and will certainly reduce the shielding effectiveness of the facility. It is understood that the offending wire had been installed at some time after commissioning of the facility.
The earth stud connections had vibrated loose over a period of time with resulting intermittent and inefficient operation of the filters, and the facility in general. The loose earth connection is symptomatic of variable MR quality.
Removal of the offending wire through the filter cabinet, and tightening the earth connections resulted in an immediate improvement to the MR images. A successful RF attenuation test was then carried out, and it was not necessary to remove plasterboard linings and aesthetic finishes to investigate further.
This case highlights the importance of correct installation of wiring to RF shielded enclosures, and MRI facilities in particular. Alterations to wiring associated with shielded enclosures must be carried out by tradesmen who understand the implications of not using the correct procedures and accessories.
RF shielded enclosures use filters for power and communications circuits. These filters have a leakage current which means that it is critical that the earth connections are properly and securely made, not only to ensure correct operation of the filters, but also for electrical safety.
Shielded enclosures also include purpose designed ventilation waveguides and door assemblies. Routine checks for missing or damaged gaskets (contact fingers) on doors, loose covers on filter cabinets, loose cable connections, and unauthorised conductors penetrating the shield, together with corrective action if necessary, will ensure the RF integrity of shielded enclosures is maintained. Where doubt exists, it is important to seek professional advice.
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