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

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 <ref name=":0">Barker, A.T., Jalinous, R., Freeston, I.L. Non-invasive magnetic stimulation of human motor cortex. ''Lancet'', 1, 1106-1107, 1985.</ref>.  

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 <ref>Jalinous, R., Technical and practical aspects of magnetic stimulation. ''J. Clin. Neurophysiol.'', 8, 10-25, 1991.</ref>. 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:

In practice, it is possible to obtain a maximum induced electric field only in regions close to the coil’s edge. Furthermore, as the depth of the measurement plane increases, the maximum field value tends to decrease and spread into increasingly wider circles. Additionally, for circular coils with different radii, the maximum field value is always found at a distance from the axis approximately equal to the coil’s radius.

[[File:Figure 3 (Ravazzani).jpg|center|thumb|500x500px|'''Figure 3:''' Isoline plots of the electric field produced by a circular coil in an infinite medium, at different distances from the coil plane (1, 2, 4 cm).]]

In clinical practice, there are three common configurations: i) edge tangential coil (ET), ii) orthogonal longitudinal coil (OL), and iii) two adjacent butterfly-shaped coils (BT). Focusing capacity can be defined as:

The field generated by current coil configurations decreases as the distance from the coil increases. For this reason, stimulating deep tissue with an induced electric field <math>E</math> necessarily involves generating larger electric fields in the more superficial tissues between the coil and the stimulated point. Additionally, the focusing capacity of the coils decreases at greater depths.<ref name=":1" /> These considerations mean that stimulating deep brain tissue leads to the activation of large areas of the motor cortex, which control the muscles of extensive body regions, thus resulting in generalized stimulation and diagnostic results of limited value.

==Conclusions==