Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient system

In MRI, strong, rapidly switched gradient fields are desirable because they can be used to reduce imaging time, obtain images with better resolution, or improve image signal-to-noise ratios. Improvements in gradient strength can be made by either increasing the gradient amplifier strength or by enhancing gradient efficiency. Unfortunately, many MRI pulse sequences, in combination with high-performance amplifiers and existing gradient hardware, can cause peripheral nerve stimulation (PNS). This makes improvements in gradient amplifiers ineffective at increasing safely usable gradient strength. Customized gradient coils are one way to achieve significant improvements in gradient performance. One specific gradient configuration, a planar gradient system, promises improved gradient strength and switching time for cardiac imaging. The PNS thresholds for planar gradients were characterized through human stimulation experiments on all three gradient axes. The specialized gradient was shown to have significantly higher stimulation thresholds than traditional cylindrical designs (y-axis SR(min) = 210 +/- 18 mT/m/ms and DeltaG(min) = 133 +/- 13 mT/m; x-axis SR(min) = 222 +/- 24 mT/m/ms and DeltaG(min) = 147 +/- 17 mT/m; z-axis SR(min) = 252 +/- 26 mT/m/ms and DeltaG(min) = 218 +/- 26 mT/m). This system could be operated at gradient strengths 2 to 3 times higher than cylindrical configurations without causing stimulation.