Post Hoc Localization of Beam F3 Stimulation Targets: An MRI-Derived Geodesic Approach for Refined TMS E-Field Simulations
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Background
Transcranial magnetic stimulation (TMS) targeting the left dorsolateral prefrontal cortex (dlPFC) is an established treatment option in major depressive disorder. One of the most common approaches for targeting the dlPFC is the Beam F3 method, which determines the stimulation site (F3 Beam ) as a function of external cranial measurements. Precise knowledge of the individual stimulation site is essential for imaging-based analyses of TMS effects. However, due to the method’s reliance on individual anatomy, retrospective identification of F3 Beam targets across cohorts is challenging, limiting the analysis of existing datasets. We developed a scalable method to reconstruct subject-specific F3 Beam target locations for e-field simulations based on structural imaging.
Methods
High-resolution three-dimensional (3D) T1-weighted MRI was used to generate individual scalp meshes via the “Simulation of Non-Invasive Brain Stimulation” (SimNIBS) software. Subject-specific anatomical distances and coordinates of interest were measured geodesically using a Python-based script to reconstruct the individual F3 Beam targets. Validation included a retrospective comparison between digital geodesic measurements and manual cranial measurements in 20 patients and a prospective comparison with MR-visible scalp markers in 2 healthy controls. To assess the impact of our targeting algorithm on e-field simulations, volumetric e-field maps based on three potential targets (F3 Beam, F3 MNI , F3 Geo ) were generated in SimNIBS and compared using voxel-wise statistics in SPM12.
Results
Retrospective analysis revealed a systematic bias towards higher in vivo measurements compared to digital geodesic measurements, though deviations in the final distances determining F3 Beam (x Beam and y Beam ) were minimal (Λx Beam : 0.11 ± 0.08 cm; Λy Beam : 0.14 ± 0.21 cm). Prospective validation demonstrated that F3 Beam coordinates better matched in vivo coil positions than group-template-derived targets (F3 MNI ). Group-level analysis showed method-dependent clustering of coil positions with corresponding voxel-wise e-field differences.
Conclusions
Individualized geodesic measurements may enable accurate, scalable and retrospective identification of Beam F3 targets and coil orientations. This approach may yield more accurate e-field simulations than group-template based targeting and provides a practical method for retrospective analysis of existing TMS treatment cohorts. This could be leveraged to identify response predictors or imaging-based biomarkers of treatment response.
