Page 229 - Computational Modeling in Biomedical Engineering and Medical Physics

218 Computational Modeling in Biomedical Engineering and Medical Physics

and in diagnosis and may help identify and repair damaged transmission pathways in
the central and peripheral nervous system or can be used to map the motor cortex
Davey et al. (1994).
In medical practice, MS is accepted for its noninvasive and painless nature. It produces
minimal discomfort to the subject because the pain sensors (sensitive cells) in the skin do
not react to a magnetic field, while the induced electric field is too low to be perceived as
pain. MS also has the advantage of the accessibility of the equipment, which is not sophis-
ticated in principle. Lately, efficient technical solutions have been designed for applicator
coils and control electronics. Although the physics are straightforward, stimulation by an
induced electric field is a medically sensitive task, as it requires precision to locate the target
inside the body, good focus and adequate intensity of the stimulus, while minimizing side
effects. Mathematical simulation of the distribution of stimuli inside the body correlated
with the optimization of technical characteristics of magnetic applicators helps the progress
of this medical procedure, by enhancing its precision.
MS is characterized by a vast palette of medical and clinical applications, starting
from noninvasive nerve stimulation, TMS, motor-evoked potentials to neuropsychiat-
ric applications, drug-resistant depression, obsessive compulsive disorders, and many
others. One of the first applications of MS, widely spread nowadays, is the TMS, a
procedure based on a high-frequency magnetic field generated using magnetic coils
positioned in the proximity of the cranium, able to induce stimulating electrical cur-
rents (Chokroverty, 1990; Hallet, 2007; Rossini and Rossi, 2007; Babbs, 2014). This
technique easily became an alternative to the painful transcranial electrical stimulation
(TES) (Berényi et al., 2012; Paulus, 2011).
TMS is currently used to cure neuropsychiatric diseases, depression, migraine, and
to understand different neural processes, for example, Baxendale (2009). It can act
either localized or at reasonably distant regions of interest, influencing the neuronal
activity as modulated by the stimulating magnetic field frequency (Wasserman and
Linsaby, 2001). Also TMS is used in brain mapping (cognitive functions, sensorial pro-
cesses, motor cortex mapping, cranial nerve muscles mapping, etc.) by activating or
inhibiting brain regions.
The increasing interest in TMS, TES, and their associated clinical applications led
to the development of numerical models, especially targeted at the magnetic field
source analysis and optimization. One of the TES approaches is presented in a finite
element method (FEM) model based on the computational domain of the head seg-
mented out of an MRI image set (Datta et al., 2013). The numerical simulation results
in presenting voltage maps of the scalp, generated by the applied stimulation electrical
pulses and using different electrode setups.
Repetitive TMS (rTMS) proves to be highly efficient in the treatment of depression.
For patients suffering from epilepsy, for whom the temporal lobectomy (Novelly
et al., 1984; Meyer et al., 1986; Sperling et al., 1996) is meant to improve the quality