Safety
Zones and labeling
The MR suite is divided into four zones of increasing hazard. Zone I is freely accessible, Zone II is the reception and screening area, Zone III is the physically restricted area where screening is controlled, and Zone IV is the magnet room itself. Current terminology is MR unsafe, MR safe, and MR conditional; MR conditional items are permitted only under the conditions stated in their labeling. The 5-gauss line marks where the static field can affect devices and projectiles, and it must be visibly marked and respected.
Static field, RF, and gradients
The static field creates projectile risk and can cause vertigo and magnetophosphenes. RF energy deposits heat; SAR limits depend on field strength and patient size and are reduced by longer TR, smaller flip angle, fewer slices per TR, and less refocusing. Cables and skin-to-skin loops cause burns. Time-varying gradients can cause peripheral nerve stimulation, and acoustic noise mandates hearing protection.
Crises and contrast
A quench releases cryogen gas; evacuate, ventilate, and do not re-enter until oxygen levels are safe. Never bring a patient with an unknown or unsafe implant into Zone IV; if a code occurs in the magnet room, remove the patient to a safe area before resuscitation when feasible. Gadolinium risks include nephrogenic systemic fibrosis and tissue retention; group II macrocyclic agents are preferred in renal impairment.
Rule of thumb
SAR scales roughly with the square of field strength, so moving a protocol from 1.5 T to 3 T can roughly quadruple heating for the same sequence and requires protocol adjustment.