Electrical and Magnetic Stimulation of the Central and Peripheral Nervous System:Modeling of Generated Fields and Data Interpretation

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Electrical and Magnetic Stimulation of the Central and Peripheral Nervous System:Modeling of Generated Fields and Data Interpretation

Article by Paolo Ravazzani · Gabriella Tognola · Marta Parazzini · Vincenzo Raschellà · Ferdinando Grandori

Magnetic stimulation of the central and peripheral nervous system can now be considered a common method in clinical neurophysiology for assessing the conduction status of motor efferent pathways and peripheral nerves. Introduced in the mid-1980s as an improvement over direct electrical stimulation, it is based on the application of rapidly changing and high-intensity magnetic fields (up to 2 T), which induce an electric field in brain and nerve tissues through electromagnetic induction .

In a short time, this technique has spread widely, becoming an excellent clinical tool for evaluating the functionality of motor efferent pathways and diagnosing central nervous system dysfunctions. Despite this, the level of empiricism in the entire method remains significant, and many areas of investigation remain open, both in the technological, neurophysiological, and clinical fields.

The first scientific studies documenting the behavior of the induced electric field $E$ by different types of stimulators appeared in the literature around 10-15 years ago, with the development of numerous mathematical models, both analytical and numerical .

Despite these studies, technological progress related to the improvement and innovation of stimulators and stimulation coils has been scarce and often limited to changes in coil shape. In recent years, stimulation devices have not seen significant progress. In particular, there have been few improvements in focusing and controlling the induced electric field, while only systems for rapid repetitive stimulation have seen some innovations.

The ability to control the focusing of the induced electric field by a coil system would greatly expand the application areas of magnetic stimulation. For example, one could consider the potential offered by stimulating nerve centers responsible for controlling respiratory muscles (for patients with descending tract lesions), studying new methods of ventricular defibrillation, and developing non-invasive temporary pacemaking techniques.

For these interesting research prospects, it is essential, in addition to optimizing the construction of the coils and associated equipment, to search for new configurations with greater field focusing capabilities. In fact, the ability to focus the induced electric field in arbitrarily small regions, and consequently to concentrate induced currents in these regions, remains a decisive aspect for a qualitative leap in the use of magnetic stimulation.

This contribution aims to provide a brief overview of the current state of magnetic stimulation from a methodological and technological perspective and to introduce and discuss some innovative approaches that could contribute to the future development of the method and its use in new application fields.

Focusing and Control of Induced Fields: A stimulation coil is made up of one or more well-insulated copper windings, along with other electronic devices such as temperature sensors and safety switches . Circular coils are the simplest and most straightforward design, and to this day, they are the most commonly used for stimulating the central nervous system (CNS) or spinal nerve roots. In clinical neurology, the CNS is stimulated with a circular coil of relatively large diameter, and the coil's position and angle relative to the subject’s head are adjusted to obtain the desired motor response.

The expression for the induced electric field inside a spherical medium has the following form:

$E(x,y,z)=\left({\frac {di(t)}{dt}}\right)\cdot G(x,y,z)$ $(1)$

Conclusions: Focusing and controlling the fields induced within biological tissues through magnetic stimulation must undoubtedly be considered priority aspects in improving the technique. Improving these characteristics, along with optimizing the equipment to reduce the power used, should be seen as a fundamental step towards extending the technique to other biomedical applications.

Regarding existing devices, the focusing capacity provided by the double coil (BF) seems entirely insufficient for using the technique in deep nerve structures, such as the trigeminal nerve roots in gnathology. A configuration capable of achieving, under certain conditions, volume focusing is the one proposed by Edrich and Zhang. It consists of two mutually orthogonal coils that can produce the desired focusing effect within tissues, provided that the radii and currents are appropriately selected. By exploiting the mutual surface cancellation effect of the fields induced by each coil, relative maxima can be achieved at depth, along particular directions. Specifically, along the vertical axis passing through the point where the two coils meet, the field induced by the horizontal coil cancels out the field generated by the vertical coil on the surface. In this way, the induced field achieves an internal maximum at a depth of about 3 cm from the surface of the head. This maximum is due to the different spatial distribution, within the sphere, of the field induced by the horizontal coil and that induced by the vertical coil, which combine to produce the surface cancellation effect and volume focusing.

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