Thresholds for interference with neuronal activity

Low frequency safety assessment with regard to unintended nerve stimulation is typically aiming to avoid field strength that have been found to cause spiking in generic neuronal models. The assumptions underlying current safety approaches are investigated here using coupled EM-neuronal dynamics modelling in a computational human anatomical model. The relevant example of MRI gradient coil induced nerve stimulation is studied, considering the impact of tissue/field inhomogeneity, coupling mechanisms, neuron models, and local, RF-coil induced heating affecting ion-channel dynamics. It is found that: the inhomogeneous field affects the stimulation threshold, end-node stimulation is not the only dominant mechanism and local field foci are relevant - as opposed to the assumptions underlying the standards -, and temperature impacts significantly neuronal dynamics, but less so stimulation thresholds. The acceptability of field smoothing is discussed. These results indicate that current safety assessment guidelines should be revisited and that quantities beyond local field or current strengths could be required to properly account for the impact of in vivo field inhomogeneity.