[Interaction mechanisms and biological effects of time-variable magnetic fields (dB/dt) in magnetic resonance for clinical use].

The first part of this paper is devoted to discuss the necessity of using time-varying magnetic fields, with frequencies up to a few kHz, during NMR tomographic and spectroscopic clinical examinations. It is then shown that these magnetic fields induce in the biological systems an electric field, with associated current densities, whose intensities span from physiological currents to currents able to depolarize the axonal membrane. Therefore particular emphasis is given to the involvement of excitable tissues, which can produce biological responses of variable intensity, from the less hazardous, such as magnetophosphenes, to muscle twitches, to more serious phenomena, such as extrasistoles and ventricular fibrillation. The thresholds above which these effects become physiologically relevant depend upon the different conditions and modalities of stimulus administration. Regarding weak electric currents, with their associated biological effects, the more important mechanisms of action at present hypothesized, along with some better studied experimental models, have been shortly outlined.