5-gauss line
The boundary around an MRI magnet where the fringe field drops below 5 Gauss (0.5 mT). Below 5 Gauss, the field is generally considered safe for cardiac pacemakers and most active implants per historical safety thresholds; above 5 Gauss, ferromagnetic objects can be attracted into the bore and active implants may malfunction. The basis for MRI access control — Zone IV is inside the 5-gauss line.
Why it matters to buyers: Siting requirement — the 5-gauss footprint establishes room-layout constraints. Typical footprint: ~8–12 feet from magnet center on 1.5T conventional magnet; ~4–6 feet on active-shielded magnets (modern). Active shielding is the major siting flexibility improvement on the 1.5T → 3T product cycle. Sealed-bore / zero-boil-off magnets often combine smaller fringe-field with reduced cryogen-management burden.
Why it matters to engineers: Survey magnet fringe field during siting; post the boundary with physical markers, signage, and Zone IV access controls. Fringe field maps are part of the magnet documentation. Quench-vent stack sizing and MRI-conditional implant safety zones reference the gauss-line map.