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Three-dimensional distribution of the electric field induced in the brain by transcranial magnetic stimulation using figure-8 and deep H-coils. J Clin Neurophysiol 24(1):31-38, Epub 2007 Feb 06

Advanced Technology Center, Sheba Medical Center, Tel-Hashomert, Israel.
Journal of Clinical Neurophysiology (Impact Factor: 1.6). 03/2007; 24(1):31-8. DOI: 10.1097/WNP.0b013e31802fa393
Source: PubMed

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.

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    • "For dTMS sessions we used Brainsway's H1 coil deep TMS System (Brainsway, Har Hotzvim, Jerusalem, Israel). The H1 coil is designed to elicit neuronal activation in medial and lateral prefrontal regions, including the orbitofrontal cortex, with a preference for the left hemisphere (Roth et al., 2007). H1 coils were positioned over patient's scalp. "
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    • "This is an advantage in the context of magnetic seizure therapy, but in subconvulsive applications, this is a significant source of risk. A family of dTMS coil designs called Hesed (H) coils has been developed with the goal of effective stimulation of deep brain structures (Roth et al., 2002; Zangen et al., 2005; Roth et al., 2007a,b). More than twenty different types of H coils have been designed and manufactured for various applications (Roth et al., 2013). "
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    ABSTRACT: To explore the field characteristics and design tradeoffs of coils for deep transcranial magnetic stimulation (dTMS). We simulated parametrically two dTMS coil designs on a spherical head model using the finite element method, and compare them with five commercial TMS coils, including two that are FDA approved for the treatment of depression (ferromagnetic-core figure-8 and H1 coil). Smaller coils have a focality advantage over larger coils; however, this advantage diminishes with increasing target depth. Smaller coils have the disadvantage of producing stronger field in the superficial cortex and requiring more energy. When the coil dimensions are large relative to the head size, the electric field decay in depth becomes linear, indicating that, at best, the electric field attenuation is directly proportional to the depth of the target. Ferromagnetic cores improve electrical efficiency for targeting superficial brain areas; however magnetic saturation reduces the effectiveness of the core for deeper targets, especially for highly focal coils. Distancing winding segments from the head, as in the H1 coil, increases the required stimulation energy. Among standard commercial coils, the double cone coil offers high energy efficiency and balance between stimulated volume and superficial field strength. Direct TMS of targets at depths of ∼4cm or more results in superficial stimulation strength that exceeds the upper limit in current rTMS safety guidelines. Approaching depths of ∼6cm is almost certainly unsafe considering the excessive superficial stimulation strength and activated brain volume. Coil design limitations and tradeoffs are important for rational and safe exploration of dTMS.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 12/2013; 125(6). DOI:10.1016/j.clinph.2013.11.038 · 2.98 Impact Factor
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    • "Finally, as the ability of the H-coil to modulate deeper neuronal structures is obtained at the cost of less focality (Roth et al., 2007; Zangen, Roth, Voller, & Hallett, 2005), it is possible that the clinical improvement observed by Mrs A could have resulted from the larger brain volume receiving direct stimulation rather than from its depth. Thus, because of its more widespread effects, one hypothesis is that the H-coil might able to modulate the DLPFC more consistently/broadly as compared to the standard figure-of-eight coil, which, by being relatively focal, might instead preferentially target specific anatomical parts of the DLPFC in each individual. "
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    Neurocase 12/2013; 21(1). DOI:10.1080/13554794.2013.860173 · 1.38 Impact Factor
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