Why an MRI room needs a Faraday cage
MRI reconstructs images from extremely weak radio-frequency signals emitted by tissue. The ambient environment — broadcast radio, mobile networks, Wi-Fi, lift motors, switching power supplies, LED drivers — produces RF noise many orders of magnitude stronger than that signal. Without an enclosure, the noise appears in the image as zipper artefacts, streaks and bands.
The enclosure is a continuous conductive shell around the examination room: a Faraday cage. It also works in the other direction, containing the scanner's own RF transmission so it does not interfere with equipment outside the room.
What the shield consists of
- Shielding panels — copper (most common) or galvanised steel sheet, applied to walls, floor and ceiling, jointed so the shell is electrically continuous.
- RF door — the most mechanically demanding element, sealing with finger stock or a pneumatic seal against a conductive frame. Doors are available as single-leaf, double-leaf or sliding, with or without an integrated window.
- RF window — a viewing panel between the examination room and the control room, shielded with fine conductive mesh laminated between glass.
- Filter panel and filter cabinet — where every electrical conductor crosses the shield through a dedicated filter that passes the working signal and blocks RF.
- Waveguides — tubes that let non-conductive services (medical gases, water, fibre) cross the shield without breaking it, sized so RF cannot propagate through.
- Interior finishes — wall cladding, suspended ceiling, lighting and flooring installed inside the shell, all specified so they neither compromise the shield nor introduce ferromagnetic material into the magnetic field.
Penetrations are where shields fail
A copper box is easy. A copper box with a door, a window, forty electrical circuits, medical gases, chilled water, ventilation ducts, a quench pipe and a fibre run is engineering. Every one of those crossings must be treated:
- Mains power, lighting circuits, nurse call, intercom, patient monitoring and camera cabling pass through filters sized for their current and signal type
- Ventilation ducts pass through honeycomb waveguide vents
- Non-conductive pipes pass through waveguide tubes of a defined length-to-diameter ratio
- Fibre optic runs pass through waveguides — never a filtered conductor
- The quench pipe passes through a dedicated, correctly sealed penetration
The single most common cause of a failed shielding test is an untreated or badly terminated penetration added late in the build by a trade who did not know the wall was a shield.
Shielding effectiveness and testing
Performance is expressed in decibels of attenuation across the frequency range of interest, which is centred on the scanner's operating frequency — approximately 64 MHz for a 1.5 T system and 128 MHz for 3 T. Each MRI manufacturer publishes a required attenuation figure for its systems in the site planning documentation, and that figure, not a generic number, is the contractual requirement.
The enclosure is tested after installation and before equipment delivery, using a transmitter and receiver moved systematically around the shell with particular attention to the door seal, window perimeter, panel joints and filter panel. Results are recorded in a test report. Testing is normally repeated if the shell is later modified.
The recognised reference method for measuring the shielding effectiveness of enclosures is IEEE Std 299; manufacturers may specify their own procedure in addition.
RF shielding is not magnetic shielding
Two different problems are often confused:
| RF shielding | Magnetic shielding | |
|---|---|---|
| Purpose | Blocks radio-frequency noise in both directions | Contains the static magnetic field (the fringe field) |
| Material | Copper or galvanised steel sheet | Steel plate, or the magnet's own active shielding |
| Always required? | Yes, for every MRI installation | Only where the 5 gauss line would otherwise extend into occupied or sensitive areas |
Modern actively shielded magnets keep the fringe field compact, so passive magnetic shielding is needed less often than it once was — but it must still be assessed against the room layout, not assumed away.
Where shielding sits in the project sequence
- Room layout and equipment selection fixed; manufacturer site planning guide obtained
- Shielding design produced: shell dimensions, door and window positions, filter panel location, penetration schedule
- Civil works completed to the tolerances the shield requires — level floor, square openings, structural support for the door
- Shield installed: floor, walls, ceiling, door, window, filter panel
- Services routed through filters and waveguides, coordinated with electrical and mechanical trades
- Shielding effectiveness test performed and report issued
- Interior finishes installed — cladding, ceiling, lighting, flooring, all non-ferromagnetic inside the field
- Equipment delivered and installed
The shield must be complete and tested before the magnet arrives. Retrofitting a missed penetration after the equipment is in place is disproportionately expensive.
Common and expensive mistakes
- Drilling into the shell after testing to mount a screen, rail or dispenser
- Ferromagnetic fixings, ceiling grids or furniture inside the magnetic field
- A door seal damaged during the fit-out and never re-tested
- Late additions — an extra camera, an extra socket — wired straight through the wall
- Lighting drivers or LED fittings inside the room that inject noise; fittings must be selected as MRI-suitable
- Assuming a generic attenuation figure instead of the equipment manufacturer's requirement
Quick summary
- The shield is a continuous conductive shell — floor, walls and ceiling, not just walls
- Every conductor crossing it needs a filter; every non-conductive service needs a waveguide
- The attenuation requirement comes from the MRI manufacturer's site planning guide
- Test before the magnet arrives, and re-test after any modification
- RF shielding and magnetic shielding are different problems with different solutions
Frequently asked questions
What is an MRI Faraday cage made of?
Most MRI RF shields are built from copper sheet, though galvanised steel is also used. The material is applied to the walls, floor and ceiling of the examination room and jointed so that the shell is electrically continuous, then completed with an RF door, an RF window, a filter panel for electrical penetrations and waveguides for non-conductive services.
How much RF attenuation does an MRI room need?
The requirement comes from the MRI manufacturer's site planning documentation for the specific system, expressed in decibels across the frequency range around the operating frequency — roughly 64 MHz at 1.5 T and 128 MHz at 3 T. Use the manufacturer's figure as the contractual requirement rather than a generic number, and verify it by measurement after installation.
Can an existing room be converted into an MRI room?
Often yes, provided the structure can carry the equipment load, the access route allows the magnet to reach the room, there is a viable quench pipe route to outside air, and there is space for the shield, the filter cabinet and the technical room. The shield is built as a shell inside the existing structure, so the finished internal dimensions will be smaller than the raw room.
Does the RF shield need to be re-tested after building work?
Yes. Any penetration, fixing or modification to the shell after the original test can compromise attenuation, and the only way to know is to measure. Treat the shielded shell as a sealed boundary that no other trade may drill, cut or route through without the shielding contractor being involved.
Related guides
Preparing a Room for MRI Installation
Room dimensions, floor loading, access routes, quench pipe, HVAC, electrical supply, the 5 gauss line and zoning — what .
ProjectsWhat a Turnkey Radiology Project Covers
The full scope of a turnkey radiology project.
LogisticsMRI Deinstallation and Transport Requirements
The full sequence for removing and shipping an MRI scanner.
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Start an enquiry Call +40 771 028 545Published 2026-08-12 by Radiology Invest Group, Bucharest, Romania. General information for planning purposes; always confirm requirements against the equipment manufacturer's documentation and the applicable authority at your destination.