Safety, Bioeffects, and RF Heating
Core concepts
MR safety has three main physical hazards: the static B₀ field (projectile effects, vertigo), time-varying gradients (peripheral nerve stimulation, acoustic noise), and RF fields (heating, SAR). SAR is proportional to B₀², flip angle², duty cycle, and inversely to TR. The IEC 60601-2-33 standard defines operating modes: normal, first-level controlled, and second-level controlled.
Key safety rules
- Implants must be MR-conditional or MR-safe; never assume compatibility.
- The missile effect is the dominant static-field hazard; screen all patients.
- RF heating is worst for conductive loops, tattoos, and long wires.
- Acoustic noise from gradient switching requires hearing protection.
- Quench: cryogen release; follow evacuation procedures.
Traps
Questions test whether SAR doubles with field strength squared, whether flip angle affects SAR (it does, quadratically), and whether gradient slew rate affects PNS (it does).
Rule of thumb
Moving from 1.5 T to 3 T roughly quadruples SAR for identical sequence parameters; reduce flip angle, increase TR, or lower duty cycle to stay within limits.