HST MEMP/Harvard Biophysics PhD Thesis Defense - Tuesday, August 20 (1:00 PM): Natalie Ferris

Waveform-based modulation of non-invasive peripheral nerve stimulation

Electromagnetic peripheral nerve stimulation (PNS) has many wide-reaching diagnostic and therapeutic applications ranging from assessment of nerve injury to rehabilitation therapy and pain management. Non-invasive electrical stimulation techniques using surface electrodes are preferred when possible. However, due to the highly local fields produced by electrodes, deeper stimulation targets such as spinal cord, nerve roots, and ganglia currently require surgically implanted electrodes.  In magnetostimulation, the electric field is induced by time-varying magnetic fields, which may enable non-invasive electrical stimulation at depth. While magnetostimulation of nerves in magnetic resonance imaging should be avoided to ensure patient comfort and safety, magnetostimulation is the primary mechanism of action in non-invasive transcranial magnetic stimulation and has been reported to have benefits over traditional electrical peripheral nerve stimulation. The induced electric field is highly shaped by the body and so generating targeted, specific magnetostimulation requires an in depth understanding of how axonal excitability depends on the spatial and temporal properties of the electric field. Regardless of how the stimulation field is generated, PNS is governed by the dynamics of the electric potential across the axon membrane through complex mechanisms including charge build up at the axon’s membrane, charge redistribution along the axon over time, and the temporal dynamics of ion channels. In this dissertation I characterize magnetostimualtion axon excitability as a function of stimulus waveform, present experimental high frequency stimulation thresholds, and propose waveform-based methods to exploit the biophysical properties of axons and reduce excitation. This work shows that a deeper understanding of axonal excitation can lead to informed waveform design and better control of magnetostimulation. This work may be useful in MRI where the detailed knowledge of anatomy and E-field induction can be used to prevent stimulation and also in clinical neurostimulation where better understanding of excitation may enable increased targeting and neuromodulation. 

Thesis Supervisor:
Larry Wald, PhD
Professor of Radiology, HMS, A. A. Martinos Center for Biomedical Imaging

Thesis Committee Chair:
Bruce Rosen, MD, PhD
Director, A. A. Martinos Center for Biomedical Imaging; Laurence Lamson Robbins Professor of Radiology, HMS

Thesis Readers:
Adam Cohen, PhD
Professor of Chemistry, Chemical Biology, and Physics, Harvard University

Bastien Guerin, PhD
Assistant Professor of Radiology, HMS, A. A. Martinos Center for Biomedical Imaging

Aapo Nummenmaa, PhD
Assistant Professor of Radiology, HMS, A. A. Martinos Center for Biomedical Imaging

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Natalie Ferris is inviting you to a scheduled Zoom meeting.

Topic: Natalie Ferris’ PhD Thesis Defense
Time: Tuesday, August 20, 2024, 1:00 PM Eastern Time (US and Canada)

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