Technical Overview

TMS – Butterfly Coils

TMS uses electromagnetic induction to generate an electric current across the scalp and skull without physical contact. A plastic-enclosed coil of wire is held next to the skull and when activated, produces a magnetic field oriented orthogonally to the plane of the coil. The magnetic field passes unimpeded through the skin and skull, inducing an oppositely directed current in the brain that activates nearby nerve cells in much the same way as currents applied directly to the cortical surface.
The path of this current is difficult to model because the brain is irregularly shaped and electricity and magnetism are not conducted uniformly throughout its tissues. The magnetic field is about the same strength as an MRI, and the pulse generally reaches no more than 5 centimeters into the brain.

Coil types

The design of transcranial magnetic stimulation coils used in either treatment or diagnostic/experimental studies may differ in a variety of ways. These differences should be considered in the interpretation of any study result, and the type of coil used should be specified in the study methods for any published reports.
The most important considerations include:

The type of material used to construct the core of the coil
The geometry of the coil configuration
the biophysical characteristics of the pulse produced by the coil.

With regard to coil composition, the core material may be either a magnetically inert substrate (i.e., the so-called ‘air-core’coil design), or possess a solid, ferromagnetically active material (ie, the so-called ‘solid-core’ design). Solid core coil design result in a more efficient transfer of electrical energy into a magnetic field, with a substantially reduced amount of energy dissipated as heat, and so can be operated under more aggressive duty cycles often mandated in therapeutic protocols, without treatment interruption due to heat accumulation, or the use of an accessory method of cooling the coil during operation. Varying the geometric shape of the coil itself may also result in variations in the focality, shape, and depth of cortical penetration of the magnetic field. Differences in the coil substance as well as the electronic operation of the power supply to the coil may also result in variations in the biophysical characteristics of the resulting magnetic pulse (e.g., width or duration of the magnetic field pulse). All of these features should be considered when comparing results obtained from different studies, with respect to both safety and efficacy.

A number of different types of coils exist, each of which produce different magnetic field patterns. Some examples:

Round coil: the original type of TMS coil
Figure-eight coil (i.e. butterfly coil): results in a more focal pattern of activation
Double-cone coil: conforms to shape of head, useful for deeper stimulation
Four-leaf coil: for focal stimulation of peripheral nerves

Design variations in the shape of the TMS coils allow much deeper penetration of the brain than the standard depth of 1.5 cm. Circular, H-shaped, double cone coils and other experimental variations can induce excitation or inhibition of neurons deeper in the brain including activation of motor neurons for the cerebellum, legs and pelvic floor. Though able to penetrate deeper in the brain, they are less able to produced a focused, localized response and are relatively non-focal.

Research Overview

Neuronetics has developed the NeuroStar TMS Therapy® system, the first and only non-systemic and non-invasive TMS device cleared by the FDA for the treatment of patients with major depression who have not benefited from prior antidepressant treatment.* NeuroStar has a reliable safety record, with over 10,000 active treatments demonstrating its safe across all NeuroStar clinical trials.1 Treatment with NeuroStar TMS therapy significantly improved patient scores on the Montgomery Asberg Depression Rating Scale (MADRS) and Hamilton Depression Rating Scale (HAMD-24) scale.2 View the research.

Through 23 premier research institutions internationally, Neuronetics completed the largest clinical trial ever conducted evaluating the efficacy and safety of TMS Therapy in the treatment of major depression.

Three Premier International Depression Research Centers:

• Centre for Addiction and Mental Health, Toronto, Canada
• Black Dog Institute, Sydney, Australia
• Alfred Psychiatry Research Centre, Melbourne, Australia


1. Janicak, P, et al. Transcranial Magnetic Stimulation (TMS) in the Treatment of Major Depression: A Comprehensive Summary of Safety Experience from Acute Exposure, Extended Exposure and During Reintroduction Treatment.
Journal of Clinical Psychiatry, 2008; Vol 69:222-232.

2. Demitrack MA, Thase ME, Clinical Significance of Transcranial Magnetic Stimulation in the treatment of depression: Synthesis of Recent Data. Psychopharmacology Bulletin 2009; 42(2):5-38.

* NeuroStar TMS Therapy® is indicated for the treatment of Major Depressive Disorder in adult patients who have failed to achieve satisfactory improvement from one prior antidepressant medication at or above the minimal effective dose and duration in the current episode.

Click here for full prescribing and safety information.

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