Influence of Peripheral Nerve Geometry and Local Anatomy on Magnetic Stimulation Time Constant

Background Introduction

Rapidly switching magnetic resonance imaging (MRI) gradient fields produce sufficiently strong electric fields within the human body, leading to peripheral nerve stimulation (PNS), which limits improvements in imaging speed and resolution. PNS threshold strength-duration curves are widely used to characterize stimulation thresholds for periodic waveforms, parameterized by chronaxie and rheobase values. Current MRI safety standards rely on a single chronaxie value to characterize the response of all nerves. However, experimental results show that the chronaxie values of peripheral nerves vary by an order of magnitude. Given the observed variability in chronaxie values and the importance of these values in MRI safety models, it is crucial to understand the mechanisms leading to chronaxie variability.

Source of the Paper

This study was conducted by Natalie G. Ferris, Valerie Klein, Bastien Guerin, Lawrence L. Wald, and Mathias Davids, researchers affiliated with the Biophysics Graduate Program at Harvard University, the Harvard-MIT Division of Health Sciences and Technology, and Harvard Medical School. The paper was published in the Journal of Neural Engineering.

Research Process

Research Methods

The researchers used a coupled electromagnetic-neuronal dynamic peripheral nerve stimulation model to evaluate the geometric sources of chronaxie variability. They investigated the impact of the stimulation magnetic field coil position relative to the body, local anatomical features, and nerve trajectories on the driving function and chronaxie.

The study included the following steps: 1. Electromagnetic-neuronal dynamic PNS model: First, the electric field induced by a given coil geometry in the human model was predicted, then the electric field was projected onto neural maps and integrated to obtain the potential along the nerve. Subsequently, the neuron response within the neural environment was calculated. 2. Study subjects: Two different leg models were used: a realistic leg model and a simplified cylindrical leg model. The effect of coil position and axial angle on chronaxie and rheobase was studied.

Results Analysis

1. Realistic leg model: By changing the coil’s z-position relative to the leg and its rotational angle, the effect of these changes on the PNS threshold, chronaxie, and rheobase of different nerves was investigated.
2. Simplified leg model: The influence of different geometric parameters (such as bend angle, radius, bone gap length, and height) on electric field morphology, chronaxie, and rheobase was analyzed.

Main Results

Realistic Leg Model

Changing the coil position and rotational angle significantly affects the PNS threshold. In some positions, the most sensitive nerve’s location changes, resulting in changes in chronaxie and rheobase. For example, when the coil moves from z=0m to z=0.1m, the most sensitive location changes from the common peroneal nerve to the medial peroneal nerve, with the threshold increasing from 72mT to 157mT, and the chronaxie decreasing from 459μs to 317μs.

Simplified Leg Model

This simplified model allowed researchers to better understand the impact of nerve trajectory and local anatomy on electric field formation. For example, increasing the bend angle (at a constant bend radius) reduces chronaxie, while increasing the bend radius (at a constant angle) increases chronaxie. In the bone gap model, gap length and height significantly affect the “hotspot” morphology of the electric field as well as its chronaxie and rheobase.

Driving Function Analysis

The driving function (DF) was used to determine the response to electric field changes caused by different geometric features. For example, the DF for bent nerves showed that a smaller bend radius (e.g., 1mm) has a steeper electric field slope, resulting in a larger DF amplitude and accordingly, a shorter chronaxie.

Electric Field Induced Membrane Potential Changes

By calculating the changes in transmembrane potential induced by the electric field, the researchers analyzed how different geometric features affect the temporal changes in membrane potential. In some geometric scenarios, the rate of charge accumulation and dissipation affects the stimulation threshold and chronaxie. For example, bent nerves with a small 1mm bend radius differ significantly in chronaxie from those with a large 9mm bend radius, despite having little initial charge difference.

Conclusion and Significance

The study indicates that one mechanism behind MRI magnetic stimulation chronaxie variability is the differential deposition and subsequent time distribution of charge on the axon generated by the stimulation pattern. For nerves with fixed physiological and intrinsic parameters, the dissipation rate of transmembrane potential affects the stimulation threshold and chronaxie. This study reveals that by enhancing the understanding of the mechanisms behind chronaxie variability, it is possible to generalize the actual PNS threshold descriptions and improve MRI and MPI safety standards.

Research Highlights

1. Clarified the impact of nerve trajectory and local anatomy on chronaxie.
2. Established a PNS prediction framework incorporating a coupled electromagnetic-neuronal dynamic model.
3. Identified charge dissipation rate as a fundamental factor in chronaxie variability.

Research Value

These findings hold significant implications for current gradient safety monitoring approaches, highlighting the limitations of setting PNS safety thresholds based on a single chronaxie value and emphasizing the importance of using comprehensive experimental data or realistic human models to capture chronaxie variability. The long-term goal of this research is to understand the sources of chronaxie variability to more accurately reflect patient-specific anatomical features, positions, and coil types, thereby optimizing MRI and MPI safety standards.