Optimizing Concurrent TMS/fMRI Using Automated Intensity Adjustments

Abstract

Introduction

Transcranial magnetic stimulation (TMS) is a brain stimulation method that utilizes the interaction between magnetic and electric fields to incite activation in the brain non-invasively. It is used for scientific research and as a promising clinical diagnostic tool for various neurological and neuropsychiatric disorders. Combining TMS with functional magnetic resonance imaging (fMRI) provides an opportunity to directly assess whole-brain reaction to stimulation. However, subject motion poses a significant challenge in concurrent TMS/fMRI, leading to artefacts in fMRI scans and potential shifts in coil positioning away from the region of interest (ROI) as even minor movements can have pronounced effects: research indicates that for every millimetre increase in coil-to-cortex distance (CCD), the relative stimulating amplitude required to induce neural activation increases by ~3% [1]. Presently, this issue is, in the case of concurrent TMS/fMRI, solely addressed by head fixation methods, while automated adjustment of TMS remains poorly explored. To bridge this gap, we aim to validate the benefits of continuous CCD-guided amplitude adjustments during concurrent TMS/fMRI.

Methodology

We utilize concurrent TMS/fMRI with neuronavigation tools, ensuring precise coil positioning, to validate a novel method for adjusting the amplitude of the stimulating TMS signal inside the scanner based on the distance between the cortical surface of the ROI and the TMS coil. All measurements are performed in a 3 Tesla MR scanner. 3D-printed optical trackers are placed on the TMS coil and the subjects' maxilla (upper jaw), allowing for continuous tracking and measurement of the CCD throughout an fMRI session. Python-based scripts are used to adjust TMS intensity based on measured distances to account for head motion [2]. After testing the setup in phantoms (objects designed to mimic the properties of human tissue), experiments will be performed on 10 healthy participants. TMS will be delivered in single pulses to the right-hand region of the primary motor cortex during continuous fMRI imaging. We chose the motor cortex area because it allows us to visually validate the TMS effects through finger twitching. Furthermore, the required stimulation intensity can be easily defined by defining the minimum intensity to create finger twitches (motor threshold). Finally, the TMS-evoked response in the stimulated brain ROI will be evaluated and changes in signal will be compared between runs with and without CCD-guided adjustments. We expect to see more consistent neural activation during the continuous CCD-guided adjustments of TMS amplitude than during the control condition.

References

[1] M. A. Stokes et al., “Simple Metric For Scaling Motor Threshold Based on Scalp-Cortex Distance: Application to Studies Using Transcranial Magnetic Stimulation,” Journal of Neurophysiology, vol. 94, no. 6, pp. 4520–4527, Dec. 2005, doi: https://doi.org/10.1152/jn.00067.2005.

[2] S. Grosshagauer, M. Woletz, M. Vasileiadi, M. Tik, D. Linhardt, and C. Windischberger, “Motion tracking for real-time measurements of coil-cortex distance: A novel approach for improving dose consistency in concurrent TMS/fMRI,” Brain Stimulation, vol. 16, no. 1, p. 323, Jan. 2023, doi: https://doi.org/10.1016/j.brs.2023.01.600.