Currents induced in the body by dB/dt can cause peripheral nerve or muscle stimulation. This stimulation may result in a slight tingling sensation or a brief muscle twitch that may startle the subject, but is not recognized as a significant health risk. Threshold sensations such as these should not be ignored, however, because this sensation may escalate to unpleasant or painful at higher levels of dB/dt. Since the margin between barely perceptible and unpleasant has been reported to be on the order of 1.5 (Schaefer et al., 2000), it is important that subjects report any sensation during scanning so that corrective action can be taken.
The FDA has altered its guidelines for dB/dt based upon recent research. In their 1998 document (U. S. Food and Drug Administration, 1998), the FDA stated "The original MRDD Guidance had established a level of concern for dB/dt at 20 T/sec for pulse duration over 120 microseconds. As an alternative, a manufacturer could demonstrate that the rate of change of the gradient field was not sufficient to cause peripheral nerve stimulation by an adequate margin of safety. The development of echoplanar and similar fast imaging techniques, and the clinical benefits which they provide, caused a re-evaluation of this policy. Evidence was presented that although peripheral nerve stimulation could potentially startle a patient and cause motion which could interfere with image acquisition, the sensation is not harmful. However, painful stimulation should be avoided." Thus, the current FDA standard is based upon the threshold for sensation, rather than a specific numerical value. With regard to dB/dt, values below that resulting in painful stimulation are considered non-significant risk by the FDA (1997). This reflects, in part, the difficulty in calculating the distribution of current with the body, a process that relies upon elaborate modeling.
The best means to address the discomfort of peripheral nerve stimulation is to instruct subjects not to clasp their hands together during scanning, and this causes a conductive loop that may potentiate dB/dt effects. Subjects should also be instructed to report any tingling, muscle twitching, or painful sensations that might occur during scanning.
It is theoretically possible to induce currents of sufficient to influence cardiac function and, in the extreme, cause ventricular defibrillation. Reilly (Reilly, 1989, 1991, 1992, 1993) investigated the dB/dt necessary to produce cardiac stimulation. His studies indicate that there is a large margin of safety between the dB/dt levels necessary for perceptible nerve stimulation and necessary to cause cardiac stimulation for ramp durations below 1000 μsec. In examining the experimental literature obtained in studies of dogs and the simulation curves of Reilly, Schaefer and colleagues (2000) state that the cardiac stimulation threshold for the most sensitive 1% of the population should require 20 times the energy required for peripheral stimulation. Furthermore, the mean defibrillation threshold should require 500 times the energy required for peripheral stimulation. For subjects experiencing a dB/dT sufficient for peripheral nerve stimulation at a 100 μsec ramp duration, the probability of cardiac stimulation is only 2 x 10-29.
Source: http://www.biac.duke.edu/
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