5-Gauss Line
Boundary around an MRI magnet where the fringe field drops below 5 Gauss (0.5 mT). Below 5 Gauss is generally considered safe for cardiac pacemakers and most active implants per historical safety thresholds; above 5 Gauss, ferromagnetic objects can be drawn into the bore as projectiles and active implants may malfunction. The basis for MRI access control — Zone IV is inside the 5-gauss line.
Siting footprint
- 1.5T conventional unshielded magnet — typical fringe-field footprint extends 8–12 ft from magnet center.
- 1.5T active-shielded magnet (modern) — ~4–6 ft from magnet center; major siting flexibility improvement on recent product cycles.
- 3T magnets — typically larger footprint than equivalent-shielded 1.5T.
- Sealed-bore / zero-boil-off magnets often combine smaller fringe field with reduced cryogen-management burden.
- Manufacturer fringe-field maps are mandatory documentation; the OEM provides 3D field plots for siting.
Access control
- Physical barrier at boundary — wall, door, controlled-access threshold.
- Signage / floor markings at the 5-gauss line and at Zone III / IV transitions.
- Ferromagnetic detection at door (Kopp Development, Metrasens) on modern installs — required by Joint Commission for many programs.
- Zone IV access protocols — only screened personnel, no loose ferromagnetic items, no patient transport without screening.
Why it matters to install planning
- Adjacent-room layout — equipment and personnel in adjacent rooms (above, below, beside) above the 5-gauss line is restricted. Pacemaker patients, certain surgical equipment, computers with magnetic media all have constraints.
- Floor-above considerations — fringe field extends vertically; floor above the magnet may be in 5-gauss footprint.
- Multi-magnet rooms — fields combine; site-planning fringe maps for both magnets simultaneously.
Lifecycle context
- Refurb relocations — moving an MRI to a new room requires re-mapping the fringe field. Pre-existing rooms must be re-surveyed.
- Decommissioning — even at zero ramp current, the bore retains permanent ferromagnetic residue; final survey before vault repurposing.