Focality Assessment in Transcranial Magnetic Stimulation With Double and Cone Coils

Abstract

To evaluate the performance with respect to selectivity of the effect of the wings bending in the cone coil relative to the double coil in transcranial magnetic stimulation. The focal area and the width vector of the central lobe of the induced electrical field distributed along an elliptic surface approximating the cortex were computed for four coil models. The models represented the real coils, the double B70 and the cone B80 Medtronic, and their corresponding simulated flat coils (B70flat and B80flat). A response function was evaluated in 10 subjects for distal and proximal muscles of the upper limb by stimulation of the motor cortex along a line approximating the central sulcus. The width of the response function, at the level of the center of gravity, provided a quantitative measure for coil focality. The focal area for B70, B70flat, B80, and B80flat calculated from the model was 31.4, 32.2, 94.4, and 50.6 cm2, respectively. The width of the central lobe along the stimulation line was: 36.2, 37, 46, and 48.6 mm, respectively. Mean values of focality measure obtained experimentally were in distal muscles, 5.06 RPU (relative position units) for B70 and 5.99 RPU for B80; in proximal muscles, 4.11 RPU for B70 and 5.13 RPU for B80, with a mean RPU value of 11.13 mm. The difference, a 19% focality measure increase in B80 relative to B70 in distal muscles, was statistically significant (P < 0.001). The focality was demonstrated to be highest for the double coil. The width of the central lobe of the induced electrical field distribution is well reflected in the width of the response function. The increase in B80 is mainly due to wing geometry and relative placement of wings and is not due to the wing bending. The width of the central lobe characterizes the spread of the induced current below the wing junction, and it is a better focality estimator than the focal area for cone coils.

Comparison of the induced fields using different coil configurations during deep transcranial magnetic stimulation

“…The double cone coil can be considered as a larger Fo8 coil with an fixed angle of about 95 degree between the two wings [18]. Stimulation of regions at depth of 3–4 cm, such as the leg motor area, can be achieved using the double cone coil [19].…”

Coil design considerations for deep transcranial magnetic stimulation

“…The double cone coil—formed by two adjacent, 110 mm diameter, circular windings fixed at a 100° angle—induces a more deeply penetrating and less focal electric field compared to a planar, 70 mm winding diameter figure-8 coil (Deng et al, 2013; Lontis et al, 2006). The double cone coil has been used for direct activation of the pelvic floor and lower limb motor representation at the interhemispheric fissure (Terao et al, 1994) as well as for transsynaptic activation of the anterior cingulate cortex via stimulation of the medial frontal cortex (Hayward et al, 2007).…”

Magnetic Stimulation of Neural Tissue: Techniques and System Design

“…In a perpendicular orientation, however, a comparative application of an electrically induced dE e /dx can be challenging. Other coil shapes such as the cone [63], crown [64], HESED [65], and Slinky [66] coils used primarily to improve the depth of effect in transcranial magnetic stimulation systems present similar challenges to simple first order modeling. With FEA modeling design verification and careful manufacture, there is no reason alternate coil topologies could not be applied to an in vitro magnetic stimulation system.…”