Peripheral nerve stimulation limits of a high amplitude and slew rate magnetic field gradient coil for neuroimaging

Abstract

To establish peripheral nerve stimulation (PNS) thresholds for an ultra-high performance magnetic field gradient sub-system (simultaneous 200 mT/m gradient amplitude and 500 T/m/s gradient slew rate; 1-MVA per-axis: referred to as MAGNUS) designed for neuroimaging with asymmetric transverse gradients and 42-cm inner diameter, and to determine PNS threshold dependencies on gender, age, patient positioning within the gradient sub-system, and anatomical landmarks. The MAGNUS head-gradient was installed in a whole-body 3.0T scanner with a custom 16-rung bird-cage transmit/receive radiofrequency coil compatible with phased-array receiver brain coils. Twenty adult subjects (10 male, mean±s.d. age=40.4±11.1 years) underwent the imaging and PNS study. The tests were repeated by displacing subject positions by 2–4 cm in the superior-inferior and anterior-posterior directions. The X-axis (left-right) yielded mostly facial stimulation, with mean Δ G min =111±6 mT/m, chronaxie=766±76 μsec. The Z-axis (superior-inferior) yielded mostly chest/shoulder stimulation (123±7 mT/m, 620±62 μsec). Y-axis (anterior-posterior) stimulation was negligible. X- and Z-axes thresholds tended to increase with age, and there was negligible dependency with gender. Translation in the inferior and posterior directions tended to increase X- and Z-axes thresholds, respectively. Electric field simulations showed good agreement with the PNS results. Imaging at MAGNUS gradient performance with increased PNS threshold provided a 35% reduction in noise-to-diffusion contrast as compared to whole-body performance (80 mT/m gradient amplitude, 200 T/m/sec gradient slew rate). The PNS threshold of MAGNUS is significantly higher than that for whole-body gradients, which allows for diffusion gradients with short rise-times (under 1 msec), important for interrogating brain microstructure length-scales.