During magnetic resonance (MR) procedures, gradient or time-varying magnetic fields may stimulate nerves or muscles in patients by inducing electrical fields. This topic has been reviewed over the years by various experts including Bencsik, et al. (2007), Schaefer, et al. (2000), Nyenhuis, et al. (1997), and Bourland, et al. (1999). The potential for interactions between gradient magnetic fields and biological tissues is dependent on a variety of factors including the fundamental field frequency, the maximum flux density, the average flux density, the presence of harmonic frequencies, the waveform characteristics of the signal, the polarity of the signal, the current distribution in the body, the electrical properties, and the sensitivity of the cell membrane.
GRADIENT MAGNETIC FIELD-INDUCED STIMULATION
Several investigations have characterized MR system-related, gradient magnetic field-induced stimulation in human subjects. At sufficient exposure levels, peripheral nerve stimulation is perceptible as “tingling” or “tapping” sensations. At gradient magnetic field exposure levels from 50% to 100% above perception thresholds, patients may become uncomfortable or experience pain. At extremely high levels, cardiac stimulation is a concern. However, the induction of cardiac stimulation requires exceedingly large gradient fields that are more than an order of magnitude greater than those used currently by commercially available MR systems.
With regard to gradient magnetic fields, the U.S. Food and Drug Administration considers that MR procedures using rates of change (dB/dt) sufficient to produce severe discomfort or painful nerve stimulation to be a significant risk. These safety standards for gradient magnetic fields associated with present-day MR systems appear to adequately protect patients from potential hazards or injuries.
Interestingly, studies performed in human subjects have indicated that anatomical sites of peripheral nerve stimulation vary depending on the activation of a specific gradient (i.e. x-, y- or, z-gradient). For example, stimulation sites for x-gradients included the bridge of the nose, left side of the thorax, iliac crest, left thigh, buttocks, and the lower back. Stimulation sites for y-gradients included the scapula, upper arms, shoulder, right side of the thorax, iliac crest, hip, hands, and upper back. Stimulation sites for z-gradients included the scapula, thorax, xyphoid, abdomen, iliac crest, and upper and lower back. Peripheral nerve stimulation sites were typically found at bony prominences.
According to Schaefer, et al. (2000), since bone is less conductive than the surrounding tissue, it may increase current densities in narrow regions of tissue between the bone and the skin, resulting in lower nerve stimulation thresholds than expected. Modifying gradient hardware and pulse sequences may be useful strategies to avoid unpleasant peripheral nerve stimulation that occurs with certain MRI techniques, according to Weinberg, et al. (2007).
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