The rapidly switched magnetic gradient fields used in MRI for spatial encoding induce electric fields in the human body in accordance with Faraday's law which, if of sufficient magnitude, can produce nerve and muscle stimulation (see Section 1.4.4). The induced electric field is proportional to dB/dt, the time rate of change of the magnetic field. From a safety standpoint, the primary concern with regard to time-varying magnetic fields is cardiac fibrillation, because it is a life-threatening condition. In contrast, peripheral nerve stimulation is of practical concern because uncomfortable or intolerable stimulations would interfere with the examination (e.g., patient movements) or would even result in a termination of the examination.
Recommendations on limiting patient and volunteer exposure to time-varying magnetic fields are based primarily on the extensive investigations on peripheral nerve stimulation in humans performed at Purdue University (Schaefer et al., 2000; Nyenhuis et al., 2001). In the reported studies, data were obtained for the perception threshold, the threshold for uncomfortable stimulation, and the threshold for intolerable stimulation during exposure to gradient fields. The results indicate that the lowest percentile for intolerable stimulation is approximately 20% above the median perception threshold for peripheral nerve stimulation, which can be parameterized by the following empirical relationship:
In Eq. (1.73), t is the effective stimulus duration (in ms) defined as the duration of the period of monotonic increasing or decreasing gradient.
The maximum recommended exposure level for time-varying magnetic fields is set equal to a dB/ dt of 80% of the median perception threshold given in the relationship above for normal operation, and 100% of the median for controlled operation. As shown in Figure 1.32, the threshold for cardiac stimulation (Reilly, 1998) is well above the median perception threshold for peripheral nerve stimulation, except at very long pulse durations which are, however, not relevant for MR procedures.
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