Sergei Turovets

• PhD in Physics
• Senior Researcher at University of Oregon

It is shown that the systems of equations describing the propagation of elliptic Gaussian beams in a nonlinear medium are Ermakov Hamiltonian systems. It is demonstrated that these systems are completely integrable. The condition under which any Ermakov system is also a Hamiltonian system is determined.

Results of an experimental study tracing the nonlinear dynamics of a CW CO2 laser with an acoustooptic loss modulator are presented. The resonance structure of the nonlinear response for various choices of the modulation frequency with respect to the frequency of relaxation oscillations is investigated. The correlation dimension and entropy for the...

The basic properties of high-current-density Josephson tunnel junctions are calculated from the microscopic theory taking into account a finite width 2δ of the Riedel peak. The calculated I-V curves for the finite values of the normalized capacitance β c , the Riedel peak halfwidth δ, and the parameter α of pair current suppression are in good agre...

A key challenge in multi-electrode transcranial electrical stimulation (TES) or transcranial direct current stimulation (tDCS) is to find a current injection pattern that delivers the necessary current density at a target and minimizes it in the rest of the head, which is mathematically modeled as an optimization problem. Such an optimization with...

The transmission and reflection of a thin film of inverted two-level atoms is investigated numerically and analytically. The case of normal incidence of rectangular and secanthyperbolic-shape pulses is considered. It is shown, that the induced superradiance effect occurs due to the initial excited-state atoms. The time dependence of the transmitted...

Objective. Inclusion of individualised electrical conductivities of head tissues is crucial for the accuracy of electrical source imaging techniques based on electro/magnetoencephalography and the efficacy of transcranial electrical stimulation. Parametric electrical impedance tomography (pEIT) is a method to cheaply and non-invasively estimate the...

Purpose To assess the 6-month visual and refractive outcomes of keratorefractive lenticule extraction (KLEx) as compared to laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) in a military population at a Joint Warfighter Refractive Surgery Center. Setting Joint Warfighter Refractive Surgery Center (JWRSC), Lackland Air For...

Objective: Inclusion of individualised electrical conductivities of head tissues is crucial for the accuracy of electrical source imaging techniques based on electro/magnetoencephalography and the efficacy of transcranial electrical stimulation. Parametric electrical impedance tomography (pEIT) is a method to cheaply and non-invasively estimate the...

Background, Motivation and Objective Transcranial Focused ultrasound (FUS) is currently under investigation as a potential non-invasive method of neuromodulation and stimulation. While early studies have shown some promise, focal specificity and pressure are compromised by the skull. The skull attenuates and refracts the FUS beam which has been s...

Electrical Impedance Tomography (EIT) can be used to estimate the electrical properties of the head tissues in a parametric approach. This modality is called parametric EIT or bounded EIT (bEIT). Typical bEIT protocols alternate between several current injection patterns with two current injection electrodes each: one source and one sink ("1-to-1")...

It turns out that TES can achieve larger normal-to-cortex dose on sulci walls rather than on top of giri. • Some deep sources can be targeted with good intensity compared to shallow ROIs, but the focality decreases more strongly for deep targets. • The focality vs intensity curves can be used to compare the inherent difficulty due to the brain geom...

One of the major questions in high-density transcranial electrical stimulation (TES) is: given a region of interest (ROI) and electric current limits for safety, how much current should be delivered by each electrode for optimal targeting of the ROI? Several solutions, apparently unrelated, have been independently proposed depending on how “optimal...

Computed tomography (CT) is a widely used imaging modality for medical diagnosis and treatment. In electroencephalography (EEG), CT imaging is used for co-registering with magnetic resonance imaging (MRI) to create more accurate head models for the brain electrical activity due to better representation of bone anatomy. Unfortunately , CT imaging ex...

The paper is about change of a dose and optimal pattern with the scalp and skull bEIT realistic estimates versus literature default specs in TES when using the reciprocity intensity maximizing optimization.

It is about making dry electrode to skin impedances smaller by injecting micro ampere direct currents

The paper deals with an impact of the skull and scalp conductivity specs on a TES dose

The paper is about detailed simulation of TEMPORAL Interference TES on the realistic human head geometry.

In this poster presentation we address a question : how the skull and the scalp conductivity specs impact the TES accuracy and dose. Results: Scalp and skull conductivity miss-specification has the larger impact on shallow targets than on deep targets, even in those spots where maximum possible current density on deep targets is of the order of the...

In this human pilot study, we show using Philips Neuro GTEN:100 system and a dry electrode head harness prototype by utlizing direct current (up to 50 uA, 30 seconds) to treat dry electrodes with nine 5 mm “feet” coated with Ag/AgCl, that the initial high impedances of 150 – 300 KOhms drop consistently by 30% -50% with a larger effect (by factor of...

One of the major questions in high-density transcranial electrical stimulation (TES) is: given a region of interest (ROI), and given electric current limits for safety, how much current should be delivered by each electrode for optimal targeting? Several solutions, apparently unrelated, have been independently proposed depending on how "optimality"...

Rationale Accurate cortical source estimation from magneto- and electroencephalography (MEG/EEG) ideally relies on subject specific forward head models that enable computation of lead fields to the scalp. With the widespread use of MRI and CT availability anatomically realistic head models can be used routinely, however there is still a lack of age...

Abstract MRI scans are utilized routinely in computational head models to represent subject specific geometry. However, an application of these models in high-density electroencephalography (hdEEG) and transcranial electrical stimulation (TES) is hindered by the lack of conductivity specifications of the head tissues, so researchers have to refer...

Introduction: Following the paper by Grossman et al [Grossman N. et al. Noninvasive deep brain stimulation via temporally interfering electric fields. Cell . 2017;169(6):1029-1041. ] there has been a renewal of interest to temporal interference (TI) transcranial electrical stimulation (TES) introduced clinically in the 1960s in context of electrosl...

Introduction: Head modeling is an important part of EEG source localization and optimization in TES allowing to see how much current is delivered to the brain target. Using MRI one can routinely create anatomically accurate individual head models in terms of geometrical precision. However, estimation of subject-specific electrical conductivity valu...

Does a Subject Posture Affect EEG and TES? A Study Using EIT and FEM Head Modeling

We investigate finite difference schemes and iterative methods for solving anisotropic diffusion problems governed by elliptic Partial Diffrential Equations (PDE) with mixed derivatives. On an example of a test problem with discontinuous coefficients, it is shown that the spectral characteristics of the finite difference problem and efficiency of t...

Objective: To estimate the scalp, skull, compact bone and marrow bone electrical conductivity values based on Electrical Impedance Tomography (EIT) measurements, and to determine the influence of the skull modeling details on the estimates. Methods: We collected EIT data with 62 current injection pairs and built five 6-8 million finite element (...

Direct and alternating current iontophoresis and electro-osmosis methodologies have provided new methods of transcutaneous drug delivery. A byproduct of such methods is lowering the electrical impedance of the electrode to skin contact, as conductive ions permeate the stratum corneum, the primary resistive layer of the skin. We developed a method f...

It is a poster presented at OHBM 2017

Method for transcranial neurostimulation. A target of neurons for transcranial stimulation or modulation is located within an impedance model of the brain, a model of a selected device to be used for producing an electric or magnetic field outside the brain is obtained and used, along with the impedance model, to compute respective virtual target s...

While electroencephalography (EEG) allows one to observe brain activity, transcranial electrical stimulation (TES), also known as transcranial direct and alternating current stimulation (tDCS and tACS) depending on the nature of the applied current, is a tool for intervention in brain processing. Even though the current levels are typically small (...

In pain management as well as other clinical applications of neuromodulation, it is important to consider the timing parameters influencing activity-dependent plasticity, including pulsed versus sustained currents, as well as the spatial action of electrical currents as they polarize the complex convolutions of the cortical mantle. These factors ar...

text: Introduction: One application of EIT is the detection and monitoring of acute strokes, such as intraventricular hemorrhages (IVH) in premature neonates [1]. Experiments with piglets provide a test bench for validation. Geometrically realistic head models would improve the sensitivity in the EIT and concurrent EEG approach. Most image processi...

Targeting deep regions in the brain is a key challenge in noninvasive transcranial electrical neuromodulation. We explore this problem by means of computer simulations within a detailed seven-tissue finite element head model (2 millions tetrahedrons) constructed from high resolution MRI and CT volumes. We solve the forward electrical stimulation an...

Intraventricular hemorrhage (IVH) is a common occurrence in the days immediately after premature birth. It has been correlated with outcomes such as periventricular leukomalacia (PVL), cerebral palsy and developmental delay. The causes and evolution of IVH are unclear; it has been associated with fluctuations in blood pressure, damage to the subven...

Current injection patterns for the 128 electrode sensor net simulations, showing the electric current injected by each electrode [mA]. Each row corresponds, from top to bottom, to each of the different trial targets from T1 to T4.

Current injection patterns for the 256 electrode sensor net simulations, showing the electric current injected by each electrode [mA]. Each row corresponds, from top to bottom, to each of the different trial targets from T1 to T4.

Current density vs. skull conductivity. The four lines depict the total current density at the four targets for the reciprocity “one-source” approach using the 128 sensor net.

Current density coronal slice computed in the finite element model with a single source near the vertex (top) and multiple sinks at all other electrodes (“one-source” configuration in the reciprocity method). The gray circles show high current density at narrow CSF regions, and the black circles show regions of the cortex that are strongly stimulat...

Objective. Transcranial direct current stimulation (tDCS) aims to alter brain function non-invasively via electrodes placed on the scalp. Conventional tDCS uses two relatively large patch electrodes to deliver electrical current to the brain region of interest (ROI). Recent studies have shown that using dense arrays containing up to 512 smaller ele...

Dense array transcranial direct current stimulation (tDCS) has become of increasing interest as a noninvasive modality to modulate brain function. To target a particular brain region of interest (ROI), using a dense electrode array placed on the scalp, the current injection pattern can be appropriately optimized. Previous optimization methods have...

Preferred systems can include an electrical impedance tomography apparatus electrically connectable to an object; an ultra low field magnetic resonance imaging apparatus including a plurality of field directions and disposable about the object; a controller connected to the ultra low field magnetic resonance imaging apparatus and configured to impl...

We estimate the scalp and skull conductivities in two healthy adults, based on bounded (or parametric) Electri- cal Impedance Tomography (bEIT) measurements, and using current injection pairs of a high dense 128 sensor array. We compare the estimates obtained with three different electrode models: point-wise, volumetric, and the Complete Electrode...

Advances in human brain neuroimaging for high-temporal and high-spatial resolution will depend on localization of Electroencephalography (EEG) signals to their cortex sources. The source localization inverse problem is inherently ill-posed and depends critically on the modeling of human head electromagnetics. We present a systematic methodology to...

Electrical Source Imaging (ESI) estimates the cortical sources responsible for generating the potentials at the head surface as measured with EEG sensors. To solve this inverse problem-mapping the sensor potentials to appropriate cortical sources-requires a physical model of the forward problem-describing how electricity propagates from the cortex...

Computational modeling and optimization of transcranial brain stimulation (both tDCS/tACS and TMS) is increasingly important as stimulation systems become more complex and applications become more diverse. However the field lacks sophisticated, flexible, useable, and extensible software tools for modeling and optimization of stimulation. To fill th...

Abstract Background The accuracy of EEG source localization depends on a sufficient sampling of the surface potential field, an accurate conducting volume estimation (head model), and a suitable and well-understood inverse technique. The goal of the present study is to examine the effect of sampling density and coverage on the ability to accuratel...

The electrical impedance tomortaphy (EIT) problems in anisotropic inhomogeneous media like head tissues belongs to the class of the three-dimensional boundary value problems for elliptic equations with mixed derivatives. The efficiency of the most discussed and usable in practice numerical methods in context of modeling EIT problems is reviewed in...

By utilizing direct current (DC) iontophoretic treatment on electrodes with high impedance we show the effectiveness of iontophoresis as a driving force for permeation of ionic electrolyte into the skin barrier. After a 60 second DC treatment at 50μA or 100μA amplitudes on either saline, Nihon Kohden Elefix, or Agar based electrolytes we saw an imm...

The accuracy of EEG source localization depends on a sufficient sampling of the surface potential field, an accurate conducting volume estimation (head model), and a suitable and well-understood inverse technique. The goal of the present study is to examine the effect of sampling density and coverage on the ability to accurately localize sources, u...

The Electrical Impedance Tomography (EIT) and electroencephalography (EEG) forward problems in anisotropic inhomogeneous media like the human head belongs to the class of the three-dimensional boundary value problems for elliptic equations with mixed derivatives. We introduce and explore the performance of several new promising numerical techniques...

Electroencephalography (EEG) is a brain imaging technology that is noninvasive, cost effective, and provides millisecond temporal resolution. Improved spatial resolution of EEG measures can benefit multiple clinical and research applications, including the assessment of Traumatic Brain Injury (TBI), stroke, and neurodevelopmental disorders. Recent...

Normative values of pediatric skull circumference, cranial index, and braincase volume would inform multiple disciplines including neurosurgery, plastic surgery and anthropology. Semi-automated methods exist for obtaining these data but are time consuming and require expertise. We report on a new method for automated extraction of in vivo measures...

Advances in human brain neuroimaging to achieve high-temporal and high-spatial resolution will depend on computational approaches to localize EEG signals to their sources in the cortex. The source localization inverse problem is inherently ill-posed and depends critically on the modeling of human head electromagnetics. In this paper we present a sy...

Children differ from adults in head size, skull morphology, and tissue conductivity. We conducted a simulation to examine the error of source localization when a rescaled adult head model and different skull conductivities are used for EEG source localization in children. We have proven by simulation that source localization accuracy is the best wi...

Skull thickness and density measures of normal pediatric crania would inform multiple disciplines including neurosurgery, optical and magnetoelectrophysiological imaging, and biomechanical modeling of head trauma. We report on a new method for automated extraction of in vivo skull thickness and density measures of pediatric crania based on x-ray co...

Understanding the milliscale (temporal and spatial) dynamics of the human brain activity requires high-resolution modeling of head electromagnetics and source localization of EEG data. We have developed an automated environment to construct individualized computational head models from image segmentation and to estimate conductivity parameters usin...

Understanding the milliscale (temporal and spatial) dynamics of the human brain activity requires high-resolution modeling of head electromagnetics and source localization of EEG data. We have developed an automated environment to construct individualized computational head models from image segmentation and to estimate conductivity parameters usin...

Introduction. Inverse problems in neuroscience such as electroencephalography (EEG) and magnetoencephalograpy (MEG) source localization, electrical impedance tomography (EIT) or Magnetic Resonance EIT require high accuracy in measurements and PDE modeling. The emerging new reconstruction algorithms usually need a laboratory experiment on physical p...

We installed an MREIT system from Kyung Hee University at the Siemens Allegra 3T MRI scanner at the University of Oregon neuroimaging center and performed the first MREIT tests. The setup including the EGI fMRI/EEG acquisition system, the 128 EEG/MREIT electrodes phantom with solution of saline and copper sulfate (to shorten T2 time) targeting 1 S/...

We describe an efficient numerical method for solving the isotropic inhomogeneous 3D Poisson equation in cylindrical coordinates as applied to analysis of EIT phantom experimental systems. The proposed approach is based on the second order accuracy finite-difference scheme and the BiCG iterative method with the FFT preconditioner. Extensive validat...