TAP: targeting and analysis pipeline for optimization and verification of coil placement in transcranial magnetic stimulation

Abstract

Objective.Transcranial magnetic stimulation (TMS) can modulate brain function via an electric field (E-field) induced in a brain region of interest (ROI). The ROI E-field can be computationally maximized and set to match a specific reference using individualized head models to find the optimal coil placement and stimulus intensity. However, the available software lacks many practical features for prospective planning of TMS interventions and retrospective evaluation of the experimental targeting accuracy.Approach.The TMS targeting and analysis pipeline (TAP) software uses an MRI/fMRI-derived brain target to optimize coil placement considering experimental parameters such as the subject's hair thickness and coil placement restrictions. The coil placement optimization is implemented in SimNIBS 3.2, for which an additional graphical user interface (TargetingNavigator) is provided to visualize/adjust procedural parameters. The coil optimization process also computes the E-field at the target, allowing the selection of the TMS device intensity setting to achieve specific E-field strengths. The optimized coil placement information is prepared for neuronavigation software, which supports targeting during the TMS procedure. The neuronavigation system can record the coil placement during the experiment, and these data can be processed in TAP to quantify the accuracy of the experimental TMS coil placement and induced E-field.Main results.TAP was demonstrated in a study consisting of three repetitive TMS sessions in five subjects. TMS was delivered by an experienced operator under neuronavigation with the computationally optimized coil placement. Analysis of the experimental accuracy from the recorded neuronavigation data indicated coil location and orientation deviations up to about 2 mm and 2°, respectively, resulting in an 8% median decrease in the target E-field magnitude compared to the optimal placement.Significance.TAP supports navigated TMS with a variety of features for rigorous and reproducible stimulation delivery, including planning and evaluation of coil placement and intensity selection for E-field-based dosing.

Keywords: MRI; TMS; coil; electric field; model; neuronavigation; targeting.

© 2022 IOP Publishing Ltd.

Figures

Figure 1.

(A) Diagram of the TMS targeting and analysis pipeline (TAP) integrating different hardware and software components for prospective targeting. The dashed arrows indicate optional steps that are not required for prospective targeting. (B) SimNIBS software coordinate convention (green) with origin at the volumetric center, X′ and Y′ axes flipped with respect to the MRI RAI convention (X, Y, Z, in red), and a TMS coil coordinate system (X′′′, Y′′′, Z′′′). (C) Neuronavigation (Brainsight) coordinate convention (blue) with pitch X′′′ = −X′′ and yaw Z′′′ = −Z′′ flipped compared to the SimNIBS convention. The nasion and right/left periauricular points (RPA/LPA) are typical registration points for neuronavigation. (D) A screenshot of the TargetingNavigator GUI: a MATLAB-based SimNIBS 3.2 add-on to adjust and visualize simulation parameters in prospective and retrospective TMS analysis.

Figure 2.

(A) Diagram of TAP as used for retrospective dosing analysis based on TMS coil scalp placement data recorded via neuronavigation (Brainsight). Dashed arrows indicate optional steps that are not required for retrospective TMS analysis. (B)–(G) Violin plots and medians of the differences in coil placement and induced E-field between prospectively-optimized and neuronavigation-recorded targeting in 5 subjects for 3 rTMS sessions of 4 blocks each. Deviation distance of (B) the coil center along the coil normal (yaw axis) direction (positive/negative values represent coil surface above/below the scalp surface) and (C) the coil center in the plane tangential to the scalp. Angular deviation of (D) the coil normal (yaw axis) and I the coil orientation about its yaw axis with clockwise as the positive direction. (F) Magnitude and (G) angular deviation of the E-field vectors for each finite element in the ROI. The violin plots show the distribution of the data, with thin black and thicker cyan vertical lines representing the 95% confidence interval and the interquartile range, respectively [33]. The plots exclude samples that have normal or tangential deviation distance of more than ± 10 mm, which were considered outliers that likely occurred due to brief disruptions in the coil tracking and comprised only 0.069% of the data.