Transcranial Magnetic Stimulation (TMS) is a non-invasive brain stimulation approach for neurological and psychiatric conditions. The TMS coil delivers magnetic pulses to interact with brain tissue. This article explains the coil’s design, function, and applications.
Understanding the TMS Coil
A TMS coil is a device made from insulated wire, usually in a plastic housing. Its primary purpose is to generate a powerful, rapidly changing magnetic field when an electrical current passes through its windings. The coil’s shape and size influence the magnetic field and how it interacts with the brain.
How TMS Coils Stimulate the Brain
When a high-current electrical pulse enters the TMS coil, it generates an intense magnetic field. This field passes through the scalp and skull. As this magnetic field rapidly changes, it induces an electrical current within the underlying brain tissue. These induced electrical currents cause depolarization of neurons in the targeted brain region, leading to either excitation or inhibition of their activity.
The induced current flow in the brain is typically in the order of microamperes to milliamperes, sufficient to alter the resting membrane potential of neurons. By controlling the magnetic pulse’s intensity, frequency, and duration, TMS coils modulate neural circuits, influencing brain function and behavior. The depth and spread of this induced current are directly related to the coil’s design and the strength of the magnetic field it generates.
Medical Applications of TMS Coils
TMS coils are integral to therapies for several neurological and psychiatric conditions by modulating specific brain regions. One of the most common applications is in treating major depressive disorder, particularly for individuals who have not responded to antidepressant medications. TMS targets the dorsolateral prefrontal cortex, a brain area implicated in mood regulation, to normalize its activity.
The coils are also used in the treatment of obsessive-compulsive disorder (OCD), where they stimulate areas like the medial prefrontal cortex and anterior cingulate cortex. For migraine headaches, TMS coils deliver single-pulse or repetitive magnetic stimulation to the occipital cortex, helping to interrupt the neurological processes associated with migraine auras and pain. These applications leverage the coil’s ability to non-invasively alter neural excitability in dysfunctional brain circuits, offering a therapeutic alternative.
Varieties of TMS Coils and Their Specific Uses
TMS coils come in various shapes, each designed to produce a distinct magnetic field pattern and stimulate different brain regions. The figure-8 coil, for instance, is widely used due to its focused stimulation. Its design creates two opposing magnetic fields that converge to produce a highly localized and precise electric field at their intersection, making it suitable for targeting superficial cortical areas with accuracy. This precision is beneficial for conditions requiring specific brain region modulation, such as the dorsolateral prefrontal cortex in depression.
Circular coils, while less commonly used for therapeutic applications today, produce a broader and less focused magnetic field. This design leads to a wider area of stimulation but with less depth and specificity compared to the figure-8 coil. For deeper brain structures, specialized H-coils have been developed. These coils feature a unique configuration of wire windings that allows the magnetic field to penetrate several centimeters deeper into the brain, reaching areas like the anterior cingulate cortex or insula that are inaccessible with standard coils. This deeper penetration is particularly relevant for conditions involving subcortical structures.