Magnetic stimulation for non-homogeneous biological structures

BACKGROUND: Magnetic stimulation has gained relatively wide application in studying nervous system structures. This technology has the advantage of reduced excitation of sensory nerve endings, and hence results in quasi-painless action. It has become clinically accepted modality for brain stimulation. However, theoretical and practical solutions for assessment of induced current distribution need more detailed and accurate consideration. Some possible analyses are proposed for distribution of the current induced from excitation current contours of different shape and disposition. Relatively non-difficult solutions are shown, applicable for two- and three-dimensional analysis. METHODS: The boundary conditions for field analysis by the internal Dirichlet problem are introduced, based on the vector potential field excited by external current coils. The feedback from the induced eddy currents is neglected. Finite element modeling is applied for obtaining the electromagnetic fields distribution in a non-homogeneous domain. RESULTS: The distributions were obtained in a non-homogeneous structure comprised of homogeneous layers. A tendency was found of the induced currents to follow paths in lower resistivity layers, deviating from the expected theoretical course for a homogeneous domain. Current density concentrations occur at the boundary between layers, suggesting the possibility for focusing on, or predicting of, a zone of stimulation. CONCLUSION: The theoretical basis and simplified approach for generation of 3D FEM networks for magnetic stimulation analysis are presented, applicable in non-homogeneous and non-linear media. The inconveniences of introducing external excitation currents are avoided. Thus, the possibilities are improved for analysis of distributions induced by time-varying currents from contours of various geometry and position with respect to the medium

Effect of Contour Shape of Nervous System Electromagnetic Stimulation Coils on the Induced Electrical Field Distribution

BACKGROUND: Electromagnetic stimulation of the nervous system has the advantage of reduced discomfort in activating nerves. For brain structures stimulation, it has become a clinically accepted modality. Coil designs usually consider factors such as optimization of induced power, focussing, field shape etc. In this study we are attempting to find the effect of the coil contour shape on the electrical field distribution for magnetic stimulation. METHOD AND RESULTS: We use the maximum of the induced electric field stimulation in the region of interest as the optimization criterion. This choice required the application of the calculus of variation, with the contour perimeter taken as a pre-set condition. Four types of coils are studied and compared: circular, square, triangular and an 'optimally' shaped contour. The latter yields higher values of the induced electrical field in depths up to about 30 mm, but for depths around 100 mm, the circular shape has a slight advantage. The validity of the model results was checked by experimental measurements in a tank with saline solution, where differences of about 12% were found. In view the accuracy limitations of the computational and measurement methods used, such differences are considered acceptable. CONCLUSION: We applied an optimization approach, using the calculus of variation, which allows to obtain a coil contour shape corresponding to a selected criterion. In this case, the optimal contour showed higher intensities for a longer line along the depth-axis. The method allows modifying the induced field structure and focussing the field to a selected zone or line

Applications of RF/microwaves in medicine

Medical applications of RF/microwaves are highlighted in this paper. The emphasis is placed on newer emerging diagnostic and therapeutic applications, such as microwave breast cancer detection, and treatment with localized high power used in ablation of the heart, and liver, benign prostate hypertrophy, angioplasty, and others. A very brief outline of biological effects of RF/microwaves and associated issues is given as background to the applications

Human exposure assessment in the near field of GSM base station antennas using a hybrid Finite Element / Method of Moments technique

IngenieursweseElektriese En Elektroniese IngeniePlease help us populate SUNScholar with the post print version of this article. It can be e-mailed to: [email protected]

Do radiofrequency radiation affect the auditory system of people with occupational exposure?

The present study intended to investigate whether radiofrequency (RF) affects auditory system of people who are occupationally exposed to RF in terms of noise-induced hearing loss (NIHL) or not. The study was carried out on 31 men from 26 to 53 years of age. All of them have been working in the Diyarbakir Radio Broadcasting Station. On the other hand, the control group was based on 30 unexposed voluntary and healthy men. The range of age for control group was the same as in the exposed group (experimental group). The results of the present study showed that the incidence of NIHL in the exposed group is more and more than that of the unexposed (control) group. The incidence of NIHL was approximately found 70 % for exposed group and 6% for unexposed group. The difference of the incidence of NIHL between exposed and unexposed groups were statistically found significant for right and left ears at “4 kHz notch” (p < 0.001). Hearing loss was not observed below 1 kHz. The percentage difference in the incidence of hearing loss (decibel) between control and experimental group was found insignificant for frequency below 1 kHz (p > 0.05). The experimental group was also divided to three subgroups of technicians, officers and men staying in the quarters of radio broadcasting station. Three subgroups were compared with one another statistically. The incidence of NIHL for the right ears of technicians was found higher than the officer (p < 0.05). The incidence of NIHL for the left ears of men staying in quarters was found higher than officers (p < 0.05). The other results of comparison were found insignificant (p > 0.05). In conclusion, the incidence of NIHL at “4 kHz Notch” in the experimental group was found more common than the control group