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

• Published in: Magnetic resonance in medicine 2020-01, Vol.83 (1), p.352-366
• Main Authors: Tan, Ek T., Hua, Yihe, Fiveland, Eric W., Vermilyea, Mark E., Piel, Joseph E., Park, Keith J., Ho, Vincent B., Foo, Thomas K. F.
• Format: Article
• Language: English
• Subjects: Adult; Age; Algorithms; Amplitudes; Brain; Brain - diagnostic imaging; Dependence; diffusion imaging; electric field; Electric fields; Electric Stimulation; Equipment Design; Female; head‐only scanner; Humans; Image Processing, Computer-Assisted; Magnetic Fields; Magnetic Resonance Imaging; Male; Medical imaging; microstructure; Middle Aged; MR safety; Neuroimaging; Neuroimaging - instrumentation; Neuroimaging - methods; Neurology; Noise reduction; peripheral nerve stimulation; Peripheral nerves; Peripheral Nerves - diagnostic imaging; Peripheral Nerves - physiology; Peripheral Nervous System - diagnostic imaging; Phantoms, Imaging; Reproducibility of Results; Slew rate; Stimulation; Subsystems; Thresholds; Whole Body Imaging
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.27909

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.