Gyri-precise head model of transcranial direct current stimulation: improved spatial focality using a ring electrode versus conventional rectangular pad

Abstract

The spatial resolution of conventional transcranial direct current stimulation (tDCS) is considered to be relatively diffuse owing to skull dispersion. However, we show that electric fields may be clustered at distinct gyri/sulci sites because of details in tissue architecture/conductivity, notably cerebrospinal fluid (CSF). We calculated the cortical electric field/current density magnitude induced during tDCS using a high spatial resolution (1 mm3) magnetic resonance imaging (MRI)-derived finite element human head model; cortical gyri/sulci were resolved. The spatial focality of conventional rectangular-pad (7 x 5 cm2) and the ring (4 x 1) electrode configurations were compared. The rectangular-pad configuration resulted in diffuse (unfocal) modulation, with discrete clusters of electric field magnitude maxima. Peak-induced electric field magnitude was not observed directly underneath the pads, but at an intermediate lobe. The 4 x 1 ring resulted in enhanced spatial focality, with peak-induced electric field magnitude at the sulcus and adjacent gyri directly underneath the active electrode. Cortical structures may be focally targeted by using ring configurations. Anatomically accurate high-resolution MRI-based forward-models may guide the "rational" clinical design and optimization of tDCS.

Keywords: Finite Element Modeling; MRI Human Head Model; TES; focality; tDCS.

Figures

Figure 1

Finite element (FE) model of the conventional 7 × 5 cm2 rectangular-pad and 4 × 1 ring configurations. (A) Segmented compartments in the following order: Scalp, Skull, CSF and Brain. (B.1) FE model of the conventional rectangular-pad configuration and corresponding FE mesh (B.2). (C.1) FE model of the 4 × 1 ring electrode configuration and corresponding FE mesh (C.2). The two insets show the zoomed mesh images, highlighting finer detail. ‘Red’: Anode electrode; ‘Blue’: Cathode electrode(s); ‘Olive green’: sponge/gel.

Figure 2

Brain modulation during tDCS using conventional rectangular-pad (A.1) and the 4 × 1 ring electrode configuration (B.1). ‘Red’: Anode electrode; ‘Blue’: Cathode electrode(s); ‘Olive green’: sponge/gel. For each configuration, we calculated the induced cortical electric field (EF) magnitude. (A.2, B.2) Surface-magnitude plots of EF along the brain surface, same view as (A.1, B.1). The dashed region is expanded in inset. (A.3, B.3) Top view of the brain showing the induced surface-magnitude EF. The insets (A.4) and (B.4) show the zoomed surface-magnitude EF plots. These images are obtained with “lighting” on [COMSOL Multiphysics] to highlight gyri/sulci modulation. (A.5, B.5) Cross-section magnitude EF plotted on a series of successive cortical slices, same view as (A.1, B.1). The cross-section magnitude EF plots for two slices and their corresponding MRI scans are shown in (A.6), (B.6). One slice is chosen directly from underneath the rectangular-pad/active electrode of 4 × 1 ring configuration and another from the prefrontal area of the brain. The same slices are shown for each of the configurations. All plots are normalized between zero and the peak positive cortical EF magnitude: conventional rectangular-pad peak = 0.67 V/m (or 0.13 A/m2 current density peak); 4 × 1 ring peak = 0.44 V/m. (Note that for the rectangular-pad, the EF magnitude peak is observed between the pads and not underneath).