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Electrical and Computer Engineering, Materials Science and Engineering, Ames Laboratory

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Accepted Manuscript

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IEEE Transactions on Magnetics




Transcranial Magnetic Stimulation (TMS) is a well-established neurostimulation technique that regulates the synaptic activity of neurons to treat several neurological conditions and psychiatric disorders. During TMS, electric fields (E-fields) are induced within the brain through the application of time-varying magnetic fields, using coils located outside the head. Different definitions in the literature calculate the focality of the E-field estimating the stimulated area during TMS. Most of them differ in measurement methodologies, showing lack of homogeneity and diverse criteria toward a unique conception, without evaluating possible scenarios of under-stimulation of the target and overstimulation of adjacent biological tissues. Another common fact in the field is the use of “focality” definitions without reference to a target area or focal distance, parameters intrinsically related and required for comparison between coils. Furthermore, to evaluate the degree and quality of stimulation, a criterion indicating how well the stimulated area covers the target area is required. This work proposes a generalized function and methodology for the focality quantification in TMS coils, considering target area, focal distance, maximum magnitude of the E-field and stimulation threshold. New definitions such as “specific focality (sf)” and “focality form factor (dn)” are introduced to describe the suitability of coils for specific stimulation applications. The proposed definitions also offer a general framework to compare coils using homogeneous methodology, parameters and nomenclature, with potential impact on the standardization of measurement methods in the industry of TMS devices. Using finite element simulation tool, ANSYS Maxwell 3D, we calculated the specific focality over an MRI-based realistic head model of an adult healthy human subject. The proposed definition assigned dimensionless values to three coils under study, compared to results in surface units of pre-existing definitions. The coils studied are: a figure-of-eight (Fo8) coil, a quadruple butterfly coil (QBC) and a quintuple AISI 1010 carbon steel core coil (QCC). The results confirm the suitability of the method to evaluate contexts of specific target area and focal distance, and identify scenarios of focal stimulation, under-stimulation or overstimulation with respect to the target size.


This is a manuscript of an article published as Carmona, Ivan C., Oluwaponmile F. Afuwape, David C. Jiles, and Ravi L. Hadimani. "Estimation of the Focality of Coils and Quality of Stimulation of Biological Tissues During Transcranial Magnetic Stimulation." IEEE Transactions on Magnetics (2021). DOI: 10.1109/TMAG.2021.3082853. Posted with permission.


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