[Limits of exposure to magnetic fields and national standards for the safety of the patient during magnetic resonance examination].

This review summarizes experimental bases, limitations and rationale of guide-lines and recommendations adopted by national health authorities for the safety of patients exposed to the magnetic fields utilized during magnetic resonance (MR) examinations: static magnetic field (B0); time-varying magnetic field gradients (dB/dt) and radiofrequency magnetic fields. Although the mechanisms of interaction of these fields with the living matter are today sufficiently understood to identify the main categories of potentially associated health hazards, epidemiological studies and dosimetric concepts are still far from the levels of definition of protection standards adopted in the field of ionizing radiations. The definition of safety thresholds and levels of exposure to magnetic fields during MR examinations has evolved in the last decade and the matter is still under study today. This article summarizes the main steps in this continuous updating process and discusses limitations and criteria, which recently suggested the adoption in some Countries (including Italy) of a two-tier system for the levels of exposure of patients to the magnetic fields generated by clinical MR equipment.