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

Magn Reson Med. 2020 Jan;83(1):352-366. doi: 10.1002/mrm.27909. Epub 2019 Aug 6.

Abstract

Purpose: To establish peripheral nerve stimulation (PNS) thresholds for an ultra-high performance magnetic field gradient subsystem (simultaneous 200-mT/m gradient amplitude and 500-T/m/s gradient slew rate; 1 MVA per axis [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 subsystem, and anatomical landmarks.

Methods: The MAGNUS head gradient was installed in a whole-body 3T scanner with a custom 16-rung bird-cage transmit/receive RF coil compatible with phased-array receiver brain coils. Twenty adult subjects (10 male, mean ± SD 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.

Results: The x-axis (left-right) yielded mostly facial stimulation, with mean ΔGmin = 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-axis and z-axis thresholds tended to increase with age, and there was negligible dependency with gender. Translation in the inferior and posterior directions tended to increase the x-axis and z-axis 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 with whole-body performance (80 mT/m gradient amplitude, 200 T/m/sec gradient slew rate).

Conclusion: 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.

Keywords: MR safety; diffusion imaging; electric field; head-only scanner; microstructure; peripheral nerve stimulation.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adult
  • Algorithms
  • Brain / diagnostic imaging*
  • Electric Stimulation*
  • Equipment Design
  • Female
  • Humans
  • Image Processing, Computer-Assisted
  • Magnetic Fields*
  • Magnetic Resonance Imaging
  • Male
  • Middle Aged
  • Neuroimaging / instrumentation*
  • Neuroimaging / methods*
  • Peripheral Nerves / diagnostic imaging*
  • Peripheral Nerves / physiology
  • Peripheral Nervous System / diagnostic imaging*
  • Phantoms, Imaging
  • Reproducibility of Results
  • Whole Body Imaging