A comparison between human magnetostimulation thresholds in whole-body and head/neck gradient coils

Area of Science:

• Biophysics
• Medical Imaging
• Neuroscience

Background:

• Magnetic Resonance Imaging (MRI) utilizes gradient coils that can induce electric fields capable of stimulating neural tissue.
• Understanding magnetostimulation thresholds is crucial for ensuring patient safety during MRI procedures.
• Previous research has established basic principles, but coil-specific thresholds require detailed investigation.

Purpose of the Study:

• To quantify magnetostimulation thresholds in volunteers using both whole-body and head/neck gradient coils.
• To determine tissue-specific stimulation parameters (electric field rheobase E(r) and chronaxie τc) for each coil type.
• To compare the operational limits and physiological limitations of the two gradient coil systems.

Main Methods:

• Twenty volunteers underwent stimulation threshold measurements using whole-body and head/neck gradient coils in xy oblique mode.
• A 64-lobe trapezoidal waveform with specific flat-tops and rise times was applied.
• Thresholds were determined by subject perception, fitted to a linear model to extract slope (SR(min)) and intercept (ΔG(min)), and used to calculate E(r) and τc.

Main Results:

• Mean threshold parameters differed: body coil (SR(min) = 62.2 mT/m/ms, ΔG(min) = 44.4 mT/m) vs. head/neck coil (SR(min) = 87.3 mT/m/ms, ΔG(min) = 78.9 mT/m).
• Tissue stimulation parameters varied: body coil (E(r) = 1.8 V/m, τc = 770 μs) vs. head/neck coil (E(r) = 1.3 V/m, τc = 1100 μs).
• Head/neck coil operational limits were higher, but it exhibited physiological limitations across a broader operational range compared to the body coil.

Conclusions:

• Magnetostimulation thresholds and resulting tissue stimulation parameters are coil-specific.
• The head/neck coil presents a distinct safety profile requiring careful consideration of its wider physiological limitations.
• Subject thresholds were not well-correlated between the two coil types, highlighting the need for coil-specific safety assessments.