| The threshold for magnetic stimulation of cardiac tissue is a continuing subject of speculation as a design constraint for future magnetic resonance imaging (MRI) systems. Experiments were conducted to determine this threshold for in vitro cardiac tissue, specifically rabbit papillary muscle, using sinusoidal waveforms to mimic MRI gradient fields. While a reliable threshold could not be established with the available equipment, analysis of the experiments raised questions regarding sinusoidal stimulus waveforms and the comparative excitability of neural and cardiac tissues.; The threshold for sinusoidal electric field stimulation at a frequency f of 1 kHz is found to be 2.6 V/cm, several times higher than predicted by Reilly. In this frequency range, the tissue threshold is proportional to {dollar}fsp{lcub}0.9{rcub}{dollar} while the magnitude of the electric field induced by a stimulating magnet is proportional to f. Consequently, while magnets with a higher-frequency driving current induce a stronger electric field, the tissue threshold increases commensurately.; While unable to stimulate cardiac tissue, the Berkeley magnet could readily produce sensation (neuromuscular stimulation) in a fingertip. The disparity in magnetic field threshold between cardiac and neural tissue simple mirrors the well-known disparity in electric field threshold. From numerical simulations, the neural threshold is found to be an order of magnitude less than the cardiac threshold. Myelinated structure, rather than membrane kinetics, is found to be responsible for the greater excitability of neural tissue.; From the standpoint of MRI safety, the lower threshold of neural tissue and the nearly linear increase in cardiac tissue threshold with frequency suggest that, in the absence of underlying pathology, neuromuscular stimulation will occur well in advance of inadvertent cardiac stimulation. |
