Three-Dimensional Distribution of the Electric Field Induced in the Brain by Transcranial Magnetic Stimulation Using Figure-8 and Deep H-Coils

TLDR

H‑coils are a novel transcranial magnetic stimulation design that can stimulate deep neuronal regions without inducing intolerably high cortical fields, thereby broadening TMS’s potential for research and treatment of neurological disorders. The study aimed to compare the electric‑field distribution of two H‑coil variants, H1 and H2, with that of a standard figure‑of‑eight coil. Three‑dimensional electric‑field distributions for the H1, H2, and figure‑of‑eight coils were measured in a saline‑filled head model. The H1 coil produced suprathreshold fields in lateral and medial frontal areas up to 4–5 cm deep, the H2 coil reached medial prefrontal depths of 2–3 cm and lateral frontal depths of 5–6 cm, while the figure‑8 coil generated a focal suprathreshold field only up to 1.5 cm, illustrating that H‑coils enable deeper, broader, and multi‑region stimulation at the expense of field focality, with depth adjustable by output or coil‑skull distance.

Abstract

The H-coils are a novel development in transcranial magnetic stimulation (TMS), designed to achieve effective stimulation of deep neuronal regions without inducing unbearable fields cortically, thus broadly expanding the potential feasibility of TMS for research and for treating various neurologic disorders. This study compared the field distribution of two H-coil versions, termed H1 and H2, and of a standard figure-of-eight coil. Three-dimensional electrical field distributions of the H1 and H2-coils, designed for effective stimulation of prefrontal regions, and of a standard figure-8 coil, were measured in a head model filled with physiologic saline solution. With stimulator output at 120% of the hand motor threshold, suprathreshold field is induced by the H1-coil at lateral and medial frontal regions at depths of up to 4 to 5 cm, and by the H2-coil at medial prefrontal regions up to 2 to 3 cm, and at lateral frontal regions up to 5 to 6 cm. The figure-8 coil induced suprathreshold field focally under the coil's central segment, at depths of up to 1.5 cm. The ability of the H-coils to stimulate effectively deeper neuronal structures is obtained at the cost of a wider electrical field distribution in the brain. However, the H-coils enable simultaneous stimulation of several brain regions, whereas the depth penetration in each region can be controlled either by adjusting the stimulator output, and/or by varying the distance between various coil elements and the skull.