Author
Rosalind J. Sadleir
Other affiliations: Johns Hopkins University, Kyung Hee University, University of Western Australia ...read more
Bio: Rosalind J. Sadleir is an academic researcher from Arizona State University. The author has contributed to research in topics: Electrical impedance tomography & Medicine. The author has an hindex of 19, co-authored 64 publications receiving 1402 citations. Previous affiliations of Rosalind J. Sadleir include Johns Hopkins University & Kyung Hee University.
Papers published on a yearly basis
Papers
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TL;DR: The authors' simulations established that high current densities were found directly under the stimulating and reference electrodes, but values of the same order of magnitude occurred in other structures, and many areas of the brain that might be behaviorally active were also subjected to what may be substantial amounts of current.
245 citations
TL;DR: The complex geometry of different tissue compartments in the head and their contrasting resistivities may jointly determine the strength and location of current densities in the brain after transcranial stimulation and this might be predictable with FEM incorporating white matter anisotropies.
141 citations
TL;DR: A current source suitable for Electrical Impedance Tomography (EIT) was simulated using a SPICE model, and built to verify stable operation, which provides 80 dB precision for this EIT application.
Abstract: The Howland current pump is a popular bioelectrical circuit, useful for delivering precise electrical currents. In applications requiring high precision delivery of alternating current to biological loads, the output impedance of the Howland is a critical figure of merit that limits the precision of the delivered current when the load changes. We explain the minimum operational amplifier requirements to meet a target precision over a wide bandwidth. We also discuss effective compensation strategies for achieving stability without sacrificing high frequency output impedance. A current source suitable for Electrical Impedance Tomography (EIT) was simulated using a SPICE model, and built to verify stable operation. This current source design had stable output impedance of 3.3 MΩ up to 200 kHz, which provides 80 dB precision for our EIT application. We conclude by noting the difficulty in measuring the output impedance, and advise verifying the plausibility of measurements against theoretical limitations.
99 citations
TL;DR: It is concluded that a similar method using multiple electrodes may be a useful means of directing current toward or away from specific brain regions and also of reducing tDCS side effects.
Abstract: Transcranial direct current stimulation (tDCS) is an emerging neuromodulation therapy that has been experimentally determined to affect a wide range of behaviors and diseases ranging from motor, cognitive, and memory processes to depression and pain syndromes. The effects of tDCS may be inhibitory or excitatory, depending on the relative polarities of electrodes and their proximity to different brain structures. This distinction is believed to relate to the interaction of current flow with activation thresholds of different neural complexes. tDCS currents are typically applied via a single pair of large electrodes, with one (the active electrode) sited close to brain structures associated with targeted processes. To efficiently direct current toward the areas presumed related to these effects, we devised a method of steering current toward a selected area by reference to a 19-electrode montage applied to a high-resolution finite element model of the head. We used a non-linear optimization procedure to maximize mean current densities inside the left inferior frontal gyrus (IFG), while simultaneously restricting overall current, and median current densities within the accumbens. We found that a distributed current pattern could be found that would indeed direct current toward the IFG in this way, and compared it to other candidate 2-electrode configurations. Further, we found a combination of four anterior-posterior electrodes could direct current densities to the accumbens. We conclude that a similar method using multiple electrodes may be a useful means of directing current toward or away from specific brain regions and also of reducing tDCS side effects.
92 citations
TL;DR: Four-electrode measurements of conductivities and their relation to tissue anisotropy ratio (ratio of tangential to radial conductivity) in layered or anisotropic biological samples similar to bone are investigated and it is shown that typical values for the thicknesses and radial conductivities of individual skull layers produce tissue with much smaller an isotropy ratios than 10.
Abstract: Accurate representations and measurements of skull electrical conductivity are essential in developing appropriate forward models for applications such as inverse EEG or Electrical Impedance Tomography of the head Because of its layered structure, it is often assumed that skull is anisotropic, with an anisotropy ratio around 10 However, no detailed investigation of skull anisotropy has been performed In this paper we investigate four-electrode measurements of conductivities and their relation to tissue anisotropy ratio (ratio of tangential to radial conductivity) in layered or anisotropic biological samples similar to bone It is shown here that typical values for the thicknesses and radial conductivities of individual skull layers produce tissue with much smaller anisotropy ratios than 10 Moreover, we show that there are very significant differences between the field patterns formed in a three-layered isotropic structure plausible for bone, and those formed assuming that bone is homogeneous and anisotropic We performed a measurement of conductivity using an electrode configuration sensitive to the distinction between three-layered and homogeneous anisotropic composition and found results consistent with the sample being three-layered We recommend that the skull be more appropriately represented as three isotropic layers than as homogeneous and anisotropic
89 citations
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University of São Paulo1, University of Göttingen2, University of Milano-Bicocca3, City University of New York4, Massachusetts Institute of Technology5, Harvard University6, Boston University7, Beth Israel Deaconess Medical Center8, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico9, Mackenzie Investments10, Spaulding Rehabilitation Hospital11
TL;DR: A workgroup of researchers in the field was convened to review, discuss, and provide updates and key challenges of tDCS use in clinical research, and some alternative methods to facilitate clinical research on tDCS are proposed.
1,186 citations
TL;DR: It is shown that electric fields may be clustered at distinct gyri/sulci sites because of details in tissue architecture/conductivity, notably cerebrospinal fluid (CSF).
1,071 citations
Paris 12 Val de Marne University1, University of Göttingen2, University Hospital of Lausanne3, French Institute of Health and Medical Research4, University of Milan5, University of Otago6, University of Regensburg7, University of Marburg8, Ruhr University Bochum9, Ludwig Maximilian University of Munich10, University of Siena11, University of Texas at Dallas12, University of Tübingen13
TL;DR: It remains to be clarified whether the probable or possible therapeutic effects of tDCS are clinically meaningful and how to optimally perform tDCS in a therapeutic setting.
1,062 citations
City College of New York1, Mackenzie Presbyterian University2, University of São Paulo3, New York University4, Beth Israel Deaconess Medical Center5, University Medical Center Freiburg6, University of Minnesota7, University of Pennsylvania8, University of Michigan9, Air Force Research Laboratory10, University of Calgary11, Albert Einstein College of Medicine12, University of Göttingen13, University of New South Wales14, University of Freiburg15, University of South Carolina16, MedStar National Rehabilitation Hospital17, University of Florida18
TL;DR: Evidence from relevant animal models indicates that brain injury by Direct Current Stimulation (DCS) occurs at predicted brain current densities that are over an order of magnitude above those produced by conventional tDCS.
874 citations
TL;DR: The history and current applications of noninvasive brain stimulation, stimulation device design principles, the electromagnetic and physical foundations of the techniques, and the current knowledge about the electrophysiologic basis of the effects are reviewed.
Abstract: Noninvasive brain stimulation with transcranial magnetic stimulation (TMS) or transcranial direct current stimulation (tDCS) is valuable in research and has potential therapeutic applications in cognitive neuroscience, neurophysiology, psychiatry, and neurology. TMS allows neurostimulation and neuromodulation, while tDCS is a purely neuromodulatory application. TMS and tDCS allow diagnostic and interventional neurophysiology applications, and focal neuropharmacology delivery. However, the physics and basic mechanisms of action remain incompletely explored. Following an overview of the history and current applications of noninvasive brain stimulation, we review stimulation device design principles, the electromagnetic and physical foundations of the techniques, and the current knowledge about the electrophysiologic basis of the effects. Finally, we discuss potential biomedical and electrical engineering developments that could lead to more effective stimulation devices, better suited for the specif...
834 citations