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

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.

Keywords: Magnetic resonance imaging; Neuronavigation; Prefrontal cortex; Scalp; Transcranial magnetic stimulation.

Figure 1

Comparison of scalp sites for BeamF3 versus MRI-guided methods. Using the cardinal scalp measurements (obtained as in Fig. 3), the BeamF3 algorithm returned values for circumferential arc X and radial arc Y, thereby indicating a scalp location for F3, to be used in stimulating left DLPFC. The BeamF3-generated measurement for Arc X was first traced along the head circumference (A) and the image volume then resliced through the vertical plane from the vertex through point X. The BeamF3-generated measurement for Arc Y was then traced along the scalp in this plane (B) to locate the BeamF3 point. The distance between the BeamF3 scalp site and the scalp site at minimum distance from the MRI-guided coordinate was then measured (C) to quantify the discrepancy. Finally, the image volume was again resliced in the vertical plane from the vertex through the MRI-guided scalp site rather than the BeamF3 site, in order to measure empirical MRI-guided values for parameters X and Y for comparison to the BeamF3-generated values.

Figure 2

Determination of cardinal scalp measurements for BeamF3 localization of site for left DLPFC stimulation. Using the curvilinear measurement tool in Osirix 5.9 software, measurements were performed in each subject to determine (A) the length of the nasion-inion scalp surface line along the midline, as well as the positions for the vertex (Cz) at the midpoint of this line, and the points FPz and Oz at 10% of the nasion-inion distance from each end; (B) the head circumference in the horizontal plane passing through FPz and Oz; and (C) the length of the left tragus-right tragus scalp line along a plane through the vertex. These measurements served as inputs for the BeamF3 method for locating the scalp point for left DLPFC stimulation.

Figure 3

Approach for MRI-based localization of scalp site for left DLPFC stimulation. In each subject, the site of the MNI stereotaxic coordinate [X − 38 Y + 44 Z + 26] was first localized using the Visor 2.0 neuronavigation system (indicated as DLPFC in this figure). Next, the scalp site at minimum Euclidean distance from this coordinate was located in each subject (shown here in coronal, axial, sagittal, and surface-rendered views). Finally, this scalp site was marked and the image volume and marker exported in DICOM format for comparison to scalp-based localization methods as illustrated in Fig. 1.

Figure 4

Cumulative distributions for X and Y parameters using BeamF3 and MRI-guided approaches. Values for the X and Y localization parameters, expressed in terms relative to cardinal scalp measurements, are depicted here as cumulative distributions across the entire patient sample for the BeamF3 (A, B) versus the MRI-guided (C, D) scalp sites for left DLPFC stimulation.

Figure 5

Cumulative distribution for the absolute values of the distance between BeamF3 and MRI-guided scalp sites. Values for the distance between the scalp site localized using the BeamF3 approach versus the scalp site localized using the MRI-guided approach to left DLPFC stimulation, depicted here as a cumulative distribution across the entire patient sample.