Concordance Between BeamF3 and MRI-neuronavigated Target Sites for Repetitive Transcranial Magnetic Stimulation of the Left Dorsolateral Prefrontal Cortex

Highlights

• Comparison of scalp versus MRI-based localization of DLPFC for rTMS treatment.

• Data drawn from MRIs in 100 patients scheduled to undergo DLPFC-rTMS in a clinical trial.

• BeamF3 algorithm had <6.5 mm discrepancy from MRI navigation in >50% of patients.

• BeamF3 radial measurement fell significantly short of MRI coordinate by 3.5 mm.

• Adjusted BeamF3 method may acceptably approximate neuronavigation if MRI unavailable.

Abstract

Background

The dorsolateral prefrontal cortex (DLPFC) is a common target for repetitive transcranial magnetic stimulation (rTMS) in major depression, but the conventional “5 cm rule” misses DLPFC in >1/3 cases. Another heuristic, BeamF3, locates the F3 EEG site from scalp measurements. MRI-guided neuronavigation is more onerous, but can target a specific DLPFC stereotaxic coordinate directly. The concordance between these two approaches has not previously been assessed.

Objective

To quantify the discrepancy in scalp site between BeamF3 versus MRI-guided neuronavigation for left DLPFC.

Methods

Using 100 pre-treatment MRIs from subjects undergoing left DLPFC-rTMS, we localized the scalp site at minimum Euclidean distance from a target MNI coordinate (X − 38 Y + 44 Z + 26) derived from our previous work. We performed nasion-inion, tragus–tragus, and head-circumference measurements on the same subjects' MRIs, and applied the BeamF3 heuristic. We then compared the distance between BeamF3 and MRI-guided scalp sites.

Results

BeamF3-to-MRI-guided discrepancies were <0.65 cm in 50% of subjects, <0.99 cm in 75% of subjects, and <1.36 cm in 95% of subjects. The angle from midline to the scalp site did not differ significantly using MRI-guided versus BeamF3 methods. However, the length of the radial arc from vertex to target site was slightly but significantly longer (mean 0.35 cm) with MRI-guidance versus BeamF3.

Conclusions

The BeamF3 heuristic may provide a reasonable approximation to MRI-guided neuronavigation for locating left DLPFC in a majority of subjects. A minor optimization of the heuristic may yield additional concordance.