36 research outputs found
A diffusion tensor-based computational model for transcranial magnetic stimulation : from macroscopic fields to neuronal membrane potentials
No full textAn efficient 3-D eddy-current solver using an independent impedance method for transcranial magnetic stimulation
No full textIn many important bioelectromagnetic problem settings, eddy-current simulations are required. Examples are the reduction of eddy-current artifacts in magnetic resonance imaging and techniques, whereby the eddy currents interact with the biological system, like the alteration of the neurophysiology due to transcranial magnetic stimulation (TMS). TMS has become an important tool for the diagnosis and treatment of neurological diseases and psychiatric disorders. A widely applied method for simulating the eddy currents is the impedance method (IM). However, this method has to contend with an ill conditioned problem and consequently a long convergence time.When dealing with optimal design problems and sensitivity control, the convergence rate becomes even more crucial since the eddy-current solver needs to be evaluated in an iterative loop. Therefore, we introduce an independent IM (IIM), which improves the conditionality and speeds up the numerical convergence. This paper shows how IIM is based on IM and what are the advantages. Moreover, the method is applied to the efficient simulation of TMS. The proposed IIM achieves superior convergence properties with high time efficiency, compared to the traditional IM and is therefore a useful tool for accurate and fast TMS simulations
The effect of inaccurate targeting of the left dorsolateral prefrontal cortex on TMS response
No full textTranscranial magnetic stimulation (TMS) of the dorsolateral prefrontal cortex (DLPFC) is increasingly used as a non-invasive treatment for medication-resistant depression. However, this target site is commonly localized using an inaccurate standard procedure, which uses a fixed distance with respect to the motor cortical site for optimal stimulation of a hand muscle. Several studies have suggested this 5-cm distance is in average too short to reliably target the DLPFC, while others just found the opposite. Nevertheless, they all agree on the fact that the interindividual neuroanatomical variability cannot be neglected. We simulated the effect of this inaccurate targeting on the electromagnetic and neurophysiological response to TMS by displacing the stimulation coil to more rostral or dorsal areas with respect to the correct reference position. The induced electric field distribution in the brain and the spatio-temporal distribution of the membrane potentials along a traced nerve bundle, localized in the targeted brain region of interest, are computed. Our results show that the number of spikes initiated and propagating towards deeper limbic regions is highly variable, even for coil shifts of only 2 mm whereas the electric field changes are moderate. Moreover, they illustrate the increased (decreased) generation of spikes in case of targeting more dorsal (rostral) regions. This confirms that an accurate patient-specific determination of the stimulation target in combination with a neuronavigation system is mandatory to perform reliable TMS studies
Eddy-current simulations using an independent impedance method in anisotropic biological tissues
No full textA widely applied method for simulating the induced eddy currents in biological tissues is the Impedance Method (IM). This method has recently been extended to the Independent Impedance Method (IIM), which identifies and solves a linear system of independent equations, resulting in acceleration of computations. Both IM and IIM assume isotropic material properties, even though some tissues have directionally dependent characteristics. Therefore, we formulate IIM towards spacedependent anisotropic material properties. In this paper, the method is applied to the simulation of Transcranial Magnetic Stimulation (TMS) for both isotropic and anisotropic spherical head models. The numerical results show that anisotropy has a non-negligible effect on the induced currents, yielding maximum differences of about 40% in the skull and 19% in gray matter. The method is validated by comparing the results with Finite Element Method (FEM) solutions. This study leads to a more realistic numerical modeling of the eddy currents due to the incorporation of anisotropy
Numerical study on IC-MREIT with multiple gradient echo sequences for the conductivity estimation of brain tissues
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Modeling the active neurodynamic behavior of rTMS in the prefrontal cortex of a realistic human head
No full textConductivity estimation of the human brain in the low frequency domain using IC-MREIT with multi gradient echo sequences
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A numerical study on conductivity estimation of the human head in the low frequency domain using induced current MR phase imaging EIT with multiple gradients
No full textAn accurate conductivity estimation of the human brain tissues is important for the correct diagnosis and therapy of neurological diseases. These values are patient-specific and vary naturally with the frequency. Nevertheless, they are often approximated by a constant value. Induced current magnetic resonance - electrical impedance tomography (ICMR-EIT) is a possible technique for non-invasive conductivity reconstruction in the low frequency domain. This paper presents a novel ICMR-EIT based method that uses the difference of two MR phase images. These images are obtained by gradient echo sequences with and without switching an eddy-current induction gradient. We propose the use of multiple gradients with different time periods, so that only a single parameter per tissue needs to be estimated with conservation of the frequency dependence. A numerical study on a spherical head model with four tissues investigates the feasibility of estimating conductivity values and shows that the proposed technique can successfully reconstruct these conductivity values. Furthermore, the influence of the material model and the number of harmonics associated to the gradient is investigated, together with performance of the proposed technique in the presence of noise
