International Journal on Magnetic Particle Imaging IJMPI
Vol. 10 No. 1 Suppl 1 (2024): Int J Mag Part Imag

Short Abstracts

Measured PNS Thresholds in a Human Head MPI Solenoid from 200 Hz to 88.1 kHz

Main Article Content

Alex Barksdale (MIT Department of Electrical Engineering and Computer Science, Cambridge, MA, United States; Martinos Center for Biomedical Imaging, Charlestown, MA, United States), Natalie Ferris (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard Graduate Program in Biophysics, Harvard University, Cambridge, MA, United States; Harvard-MIT Division of Health Sciences and Technology, Boston, MA, United States), Eli Mattingly (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard-MIT Division of Health Sciences and Technology, Boston, MA, United States), Monika Śliwiak (Martinos Center for Biomedical Imaging, Charlestown, MA, United States), Bastien Guerin (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard Medical School, Boston, MA, United States), Lawrence Wald (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard Graduate Program in Biophysics, Harvard University, Cambridge, MA, United States; Harvard Medical School, Boston, MA, United States), Mathias Davids (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard Medical School, Boston, MA, United States), Valerie Klein (Martinos Center for Biomedical Imaging, Charlestown, MA, United States; Harvard Medical School, Boston, MA, United States)

Copyright (c) 2024 Alex Barksdale, Natalie Ferris, Eli Mattingly, Monika ?liwiak, Bastien Guerin, Lawrence Wald, Mathias Davids, Valerie Klein

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

Time-varying B-fields produced by MPI drive coils can cause peripheral nerve stimulation (PNS)[1] which can compromise subject comfort and safety[2]. Previous magnetostimulation experiments[3-5] observed deviations from the hyperbolic strength-duration relation predicted by the Fundamental Law of Magnetostimulation (FLM)[6]. We measure PNS thresholds in 8 healthy volunteers in a head solenoid over 16 frequencies between 200Hz to 88.1kHz to supplement existing literature. The head solenoid designed for our human-scale fMPI scanner[7] is a four-layer coil wound with hollow Cu wire (4mm OD, 2mm ID), consisting of 54 turns (111cm length, 27cm winding ID). A capacitor bank enabled rapid switching between 6 untuned frequencies (200Hz-700Hz), and 10 tuned frequencies (1.8kHz-88.1kHz). Stimulation waveforms consisted of 256 cycle sinusoids with exponential ramp-up envelopes (time constant 25 cycles for all frequencies). Stimulation thresholds were robustly determined by fitting sigmoid functions to subject responses over stimulation amplitude.


Measurements reveal departure from the hyperbolic FLM, showing a minimum threshold of ~4.97mT peak (3.51mT rms) at coil center between 17kHz-25kHz, increasing to ~6.44mT peak (4.55mT rms) at 66.8kHz contrasting the predicted asymptotic behavior. Only 4 out of 8 subjects reported stimulation at 88.1kHz (amplifier max 7mT peak). We fit error function CDFs to estimate mean thresholds across subjects (~6.86mT peak, 4.85mT rms at 88.1kHz). PNS thresholds may limit sensitivity in human head imaging[8]. This work provides measured PNS thresholds that critically inform drive field amplitudes for human head imaging, or drive frequency optimization for maximum sensitivity considering particle relaxation, Faraday detection, and drive amplitude at the PNS threshold.

Article Details

References

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[7] Mattingly, E., Mason, E., Sliwiak, M., & Wald, L. L. (2022). Drive and receive coil design for a human-scale MPI system. International Journal on Magnetic Particle Imaging IJMPI, 8(1 Suppl 1).
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