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.