Publications by authors named "Munish Chauhan"

31 Publications

Humoral Response to One and Two Doses of ChAdOx1-S Vaccine in Patients on Hemodialysis.

Clin J Am Soc Nephrol 2021 Sep 20. Epub 2021 Sep 20.

George Institute for Global Health, UNSW, New Delhi, India

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http://dx.doi.org/10.2215/CJN.10170721DOI Listing
September 2021

Magnetic-resonance-based measurement of electromagnetic fields and conductivity in vivo using single current administration-A machine learning approach.

PLoS One 2021 22;16(7):e0254690. Epub 2021 Jul 22.

School of Biological Health System Engineering, Arizona State University, Tempe, Arizona, United States of America.

Diffusion tensor magnetic resonance electrical impedance tomography (DT-MREIT) is a newly developed technique that combines MR-based measurements of magnetic flux density with diffusion tensor MRI (DT-MRI) data to reconstruct electrical conductivity tensor distributions. DT-MREIT techniques normally require injection of two independent current patterns for unique reconstruction of conductivity characteristics. In this paper, we demonstrate an algorithm that can be used to reconstruct the position dependent scale factor relating conductivity and diffusion tensors, using flux density data measured from only one current injection. We demonstrate how these images can also be used to reconstruct electric field and current density distributions. Reconstructions were performed using a mimetic algorithm and simulations of magnetic flux density from complementary electrode montages, combined with a small-scale machine learning approach. In a biological tissue phantom, we found that the method reduced relative errors between single-current and two-current DT-MREIT results to around 10%. For in vivo human experimental data the error was about 15%. These results suggest that incorporation of machine learning may make it easier to recover electrical conductivity tensors and electric field images during neuromodulation therapy without the need for multiple current administrations.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0254690PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8297925PMC
November 2021

A Case of Refractory Hypoxemic Respiratory Failure due to Antineutrophil Cytoplasmic Antibodies-associated Diffuse Alveolar Hemorrhage Rescued by Extracorporeal Membrane Oxygenation.

Indian J Crit Care Med 2020 Sep;24(9):879-881

Department of Critical Care Medicine, Fortis Memorial Research Institute, Gurugram, Haryana, India.

Diffuse alveolar hemorrhage (DAH) is a rare but life-threatening disease. Mortality is very high in those patients who require mechanical ventilation. Traditionally, active bleeding has been considered a contraindication for extracorporeal membrane oxygenation (ECMO) support. There is limited evidence for ECMO in DAH as rescue therapy. Herein, we describe a case of antineutrophil cytoplasmic antibodies-associated DAH with intractable hypoxemic respiratory failure. An appropriate ventilator strategy failed to improve her hypoxemia leading to imminent risk to her life. The patient was rescued with veno-venous ECMO targeting lower than usual range of anticoagulation. ECMO proved to be lifesaving in our patient who was initiated on prompt immunosuppressive therapy and plasmapheresis along with continuous veno-venous hemodiafiltration and hemodynamic support. We feel that ECMO could be considered as adjunctive therapy in severe hypoxemic respiratory failure associated with DAH after careful consideration of the risk of bleeding and a restrictive anticoagulation strategy. Goel MK, Chauhan M, Kumar A, Wadwa P, Maitra G, Talegaonkkar M, A Case of Refractory Hypoxemic Respiratory Failure due to Antineutrophil Cytoplasmic Antibodies-associated Diffuse Alveolar Hemorrhage Rescued by Extracorporeal Membrane Oxygenation. Indian J Crit Care Med 2020;24(9):879-881.
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http://dx.doi.org/10.5005/jp-journals-10071-23585DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7584836PMC
September 2020

Low frequency conductivity reconstruction based on a single current injection via MREIT.

Phys Med Biol 2020 11 17;65(22):225016. Epub 2020 Nov 17.

School of Mathematics and Statistics, Shandong Normal University, Jinan, Shandong, 250014, People's Republic of China. Center for Post-doctoral studies of Management Science and Engineering and also Institute of Data Science and Technology, Shandong Normal University, Jinan, Shandong, 250014, People's Republic of China.

Conventional magnetic resonance electrical impedance tomography (MREIT) reconstruction methods require administration of two linearly independent currents via at least two electrode pairs. This requires long scanning times and inhibits coordination of MREIT measurements with electrical neuromodulation strategies. We sought to develop an isotropic conductivity reconstruction algorithm in MREIT based on a single current injection, both to decrease scanning time by a factor of two and enable MREIT measurements to be conveniently adapted to general transcranial- or implanted-electrode neurostimulation protocols. In this work, we propose and demonstrate an iterative algorithm that extends previously published MREIT work using two-current administration approaches. The proposed algorithm is a single-current adaptation of the harmonic B algorithm. Forward modeling of electric potentials is used to capture changes of conductivity along current directions that would normally be invisible using data from a single-current administration. Computational and experimental results show that the reconstruction algorithm is capable of reconstructing isotropic conductivity images that agree well in terms of L error and structural similarity with exact conductivity distributions or two-current-based MREIT reconstructions. We conclude that it is possible to reconstruct high quality electrical conductivity images using MREIT techniques and one current injection only.
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http://dx.doi.org/10.1088/1361-6560/abbc4dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7870531PMC
November 2020

Development and testing of implanted carbon electrodes for electromagnetic field mapping during neuromodulation.

Magn Reson Med 2020 10 16;84(4):2103-2116. Epub 2020 Apr 16.

School of Biological and Health Systems Engineering, Arizona State University, Tempe, Arizona, USA.

Purpose: Deep brain stimulation electrodes composed of carbon fibers were tested as a means of administering and imaging magnetic resonance electrical impedance tomography (MREIT) currents. Artifacts and heating properties of custom carbon-fiber deep brain stimulation (DBS) electrodes were compared with those produced with standard DBS electrodes.

Methods: Electrodes were constructed from multiple strands of 7-μm carbon-fiber stock. The insulated carbon electrodes were matched to DBS electrode diameter and contact areas. Images of DBS and carbon electrodes were collected with and without current flow and were compared in terms of artifact and thermal effects in phantoms or tissue samples in 7T imaging conditions. Effects on magnetic flux density and current density distributions were also assessed.

Results: Carbon electrodes produced magnitude artifacts with smaller FWHM values compared to the magnitude artifacts around DBS electrodes in spin echo and gradient echo imaging protocols. DBS electrodes appeared 269% larger than actual size in gradient echo images, in sharp contrast to the negligible artifact observed in diameter-matched carbon electrodes. As expected, larger temperature changes were observed near DBS electrodes during extended RF excitations compared with carbon electrodes in the same phantom. Magnitudes and distribution of magnetic flux density and current density reconstructions were comparable for carbon and DBS electrodes.

Conclusion: Carbon electrodes may offer a safer, MR-compatible method for administering neuromodulation currents. Use of carbon-fiber electrodes should allow imaging of structures close to electrodes, potentially allowing better targeting, electrode position revision, and the facilitation of functional imaging near electrodes during neuromodulation.
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http://dx.doi.org/10.1002/mrm.28273DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8183461PMC
October 2020

Methods to Compare Predicted and Observed Phosphene Experience in tACS Subjects.

Neural Plast 2018 6;2018:8525706. Epub 2018 Dec 6.

School of Biological and Health Systems Engineering, Arizona State University, USA.

Background: Phosphene generation is an objective physical measure of potential transcranial alternating current stimulation (tACS) biological side effects. Interpretations from phosphene analysis can serve as a first step in understanding underlying mechanisms of tACS in healthy human subjects and assist validation of computational models.

Objective/hypothesis: This preliminary study introduces and tests methods to analyze predicted phosphene occurrence using computational head models constructed from tACS recipients against verbal testimonies of phosphene sensations. Predicted current densities in the eyes and the occipital lobe were also verified against previously published threshold values for phosphenes.

Methods: Six healthy subjects underwent 10 Hz tACS while being imaged in an MRI scanner. Two different electrode montages, T7-T8 and Fpz-Oz, were used. Subject ratings of phosphene experience were collected during tACS and compared against current density distributions predicted in eye and occipital lobe regions of interest (ROIs) determined for each subject. Calculated median current densities in each ROI were compared to minimum thresholds for phosphene generation.

Main Results: All subjects reported phosphenes, and predicted median current densities in ROIs exceeded minimum thresholds for phosphenes found in the literature. Higher current densities in the eyes were consistently associated with decreased phosphene generation for the Fpz-Oz montage. There was an overall positive association between phosphene perceptions and current densities in the occipital lobe.

Conclusions: These methods may have promise for predicting phosphene generation using data collected during in-scanner tACS sessions and may enable better understanding of phosphene origin. Additional empirical data in a larger cohort is required to fully test the robustness of the proposed methods. Future studies should include additional montages that could dissociate retinal and occipital stimulation.
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http://dx.doi.org/10.1155/2018/8525706DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6304915PMC
March 2019

Benchmarking transcranial electrical stimulation finite element models: a comparison study.

J Neural Eng 2019 04 3;16(2):026019. Epub 2019 Jan 3.

Department of Clinical and Health Psychology, Center for Cognitive Aging and Memory, McKnight Brain Institute, University of Florida, Gainesville, FL, United States of America.

Objective: To compare field measure differences in simulations of transcranial electrical stimulation (tES) generated by variations in finite element (FE) models due to boundary condition specification, use of tissue compartment smoothing filters, and use of free or structured tetrahedral meshes based on magnetic resonance imaging (MRI) data.

Approach: A structural MRI head volume was acquired at 1 mm resolution and segmented into ten tissue compartments. Predicted current densities and electric fields were computed in segmented models using modeling pipelines involving either an in-house (block) or a commercial platform commonly used in previous FE tES studies involving smoothed compartments and free meshing procedures (smooth). The same boundary conditions were used for both block and smooth pipelines. Differences caused by varying boundary conditions were examined using a simple geometry. Percentage differences of median current density values in five cortical structures were compared between the two pipelines for three electrode montages (F3-right supraorbital, T7-T8 and Cz-Oz).

Main Results: Use of boundary conditions commonly used in previous tES FE studies produced asymmetric current density profiles in the simple geometry. In head models, median current density differences produced by the two pipelines, using the same boundary conditions, were up to 6% (isotropic) and 18% (anisotropic) in structures targeted by each montage. Tangential electric field measures calculated via either pipeline were within the range of values reported in the literature, when averaged over cortical surface patches.

Significance: Apparently equivalent boundary settings may affect predicted current density outcomes and care must be taken in their specification. Smoothing FE model compartments may not be necessary, and directly translated, voxellated tissue boundaries at 1 mm resolution may be sufficient for use in tES FE studies, greatly reducing processing times. The findings here may be used to inform future current density modeling studies.
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http://dx.doi.org/10.1088/1741-2552/aafbbdDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6748834PMC
April 2019

Evaluation of magnetohydrodynamic effects in magnetic resonance electrical impedance tomography at ultra-high magnetic fields.

Magn Reson Med 2019 04 19;81(4):2264-2276. Epub 2018 Nov 19.

School of Biological and Health Systems Engineering, Ira A. Fulton Schools of Engineering, Arizona State University, Tempe, Arizona.

Purpose: Artifacts observed in experimental magnetic resonance electrical impedance tomography images were hypothesized to be because of magnetohydrodynamic (MHD) effects.

Theory And Methods: Simulations of MREIT acquisition in the presence of MHD and electrical current flow were performed to confirm findings. Laminar flow and (electrostatic) electrical conduction equations were bidirectionally coupled via Lorentz force equations, and finite element simulations were performed to predict flow velocity as a function of time. Gradient sequences used in spin-echo and gradient echo acquisitions were used to calculate overall effects on MR phase images for different electrical current application or phase-encoding directions.

Results: Calculated and experimental phase images agreed relatively well, both qualitatively and quantitatively, with some exceptions. Refocusing pulses in spin echo sequences did not appear to affect experimental phase images.

Conclusion: MHD effects were confirmed as the cause of observed experimental phase changes in MREIT images obtained at high fields. These findings may have implications for quantitative measurement of viscosity using MRI techniques. Methods developed here may be also important in studies of safety and in vivo artifacts observed in high field MRI systems.
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http://dx.doi.org/10.1002/mrm.27534DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6373455PMC
April 2019

Analytic modeling of conductively anisotropic neural tissue.

J Appl Phys 2018 Aug 10;124(6):064701. Epub 2018 Aug 10.

School of Biological and Health Systems Engineering, Arizona State University, Tempe, Arizona 85287-9709, USA.

The abdominal ganglion of the is an established model for studying neuroelectric behavior in the presence of an applied electrical current and recently used in studies of magnetic resonance electrical impedance tomography (MREIT) which allows for quantitative visualization of spatially distributed current and magnetic flux densities. Understanding the impact the geometry and anisotropic conductivity have on applied electromagnetic fields is central to intepreting and refining MREIT data and protocols, respectively. Here we present a simplified bidomain model of an experimental preparation of the abdominal ganglion, describing the tissue as a radially anisotropic sphere with equal anisotropy ratios, i.e., where radial conductivities in both intra- and extra-cellular regions are ten times that of their polar and azimuthal conductivities. The fully three dimensional problem is validated through comparisons with limiting examples of 2D isotropic analyses. Results may be useful in validating finite element models of MREIT experiments and have broader relevance to analysis of MREIT data obtained from complex neural architecture in the human brain.
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http://dx.doi.org/10.1063/1.5036659DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6086692PMC
August 2018

Functional magnetic resonance electrical impedance tomography (fMREIT) sensitivity analysis using an active bidomain finite-element model of neural tissue.

Magn Reson Med 2019 01 16;81(1):602-614. Epub 2018 May 16.

School of Biological and Health Systems Engineering, Arizona State University, Tempe, Arizona.

Purpose: A direct method of imaging neural activity was simulated to determine typical signal sizes.

Methods: An active bidomain finite-element model was used to estimate approximate perturbations in MR phase data as a result of neural tissue activity, and when an external MR electrical impedance tomography imaging current was added to the region containing neural current sources.

Results: Modeling-predicted, activity-related conductivity changes should produce measurable differential phase signals in practical MR electrical impedance tomography experiments conducted at moderate resolution at noise levels typical of high field systems. The primary dependence of MR electrical impedance tomography phase contrast on membrane conductivity changes, and not source strength, was demonstrated.

Conclusion: Because the injected imaging current may also affect the level of activity in the tissue of interest, this technique can be used synergistically with neuromodulation techniques such as deep brain stimulation, to examine mechanisms of action.
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http://dx.doi.org/10.1002/mrm.27351DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6239993PMC
January 2019

The effect of potassium chloride on abdominal ganglion activity.

Biomed Phys Eng Express 2018 May 11;4(3). Epub 2018 Apr 11.

School of Biological and Health Systems Engineering, Arizona State University, Tempe, AZ 85287, USA.

Objective: Spontaneous activity in the abdominal ganglion of can be used as a convenient bioelectricity source in tests of novel MRI-based functional imaging methods, such as functional Magnetic Resonance Electrical Impedance Tomography (fMREIT). In these tests, it is necessary to find a consistent treatment that modulates neural activity, so that these results can be compared with control data. Effects of MREIT imaging currents combined with treatment were also of interest.

Approach: Potassium chloride (KCl) was employed as a rhythm modulator. In a series of experiments, effects of adding different volumes of KCl solution were tested and compared with experiments on control groups that had artificial sea water administered. In all cases, neuronal activity was measured with micro electrode arrays.

Main Results: It was possible to reversibly stop spontaneous activity in ganglia by increasing the extracellular potassium chloride concentration to 89 mM. There was no effect on experimental outcomes when current was administered to the sample chamber between recordings.

Significance: KCl can be used as a reversible neural modulator for testing neural detection methods.
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http://dx.doi.org/10.1088/2057-1976/aab72eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6668993PMC
May 2018

Low-Frequency Conductivity Tensor Imaging of the Human Head In Vivo Using DT-MREIT: First Study.

IEEE Trans Med Imaging 2018 04;37(4):966-976

We present the first in vivo images of anisotropic conductivity distribution in the human head, measured at a frequency of approximately 10 Hz. We used magnetic resonance electrical impedance tomography techniques to encode phase changes caused by current flow within the head via two independent electrode pairs. These results were then combined with diffusion tensor imaging data to reconstruct full anisotropic conductivity distributions in 5-mm-thick slices of the brains of two participants. Conductivity values recovered in this paper were broadly consistent with literature values. We anticipate that this technique will be of use in many areas of neuroscience, most importantly in functional imaging via inverse electroencephalogram. Future studies will involve pulse sequence acceleration to maximize brain coverage and resolution.
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http://dx.doi.org/10.1109/TMI.2017.2783348DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5963516PMC
April 2018

Accelerating acquisition strategies for low-frequency conductivity imaging using MREIT.

Phys Med Biol 2018 02 13;63(4):045011. Epub 2018 Feb 13.

School of Mathematics and Statistics, Institute of Data Science and Technology, Shandong Normal University, Jinan, Shandong, 250014, People's Republic of China.

We sought to improve efficiency of magnetic resonance electrical impedance tomography data acquisition so that fast conductivity changes or electric field variations could be monitored. Undersampling of k-space was used to decrease acquisition times in spin-echo-based sequences by a factor of two. Full MREIT data were reconstructed using continuity assumptions and preliminary scans gathered without current. We found that phase data were reconstructed faithfully from undersampled data. Conductivity reconstructions of phantom data were also possible. Therefore, undersampled k-space methods can potentially be used to accelerate MREIT acquisition. This method could be an advantage in imaging real-time conductivity changes with MREIT.
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http://dx.doi.org/10.1088/1361-6560/aaa8d2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5851300PMC
February 2018

Projected current density comparison in tDCS block and smooth FE modeling.

Annu Int Conf IEEE Eng Med Biol Soc 2016 Aug;2016:4079-4082

Current density distribution and projected current density calculation following transcranial direct current stimulation (tDCS) forward model in a human head were compared between two modeling pipelines: block and smooth. Block model was directly constructed from MRI voxel resolution and simulated in C. Smooth models underwent a boundary smoothing process by applying recursive Gaussian filters and simulated in COMSOL. Three smoothing levels were added to determine their effects on current density distribution compared to block models. Median current density percentage differences were calculated in anterior superior temporal gyrus (ASTG), hippocampus (HIP), inferior frontal gyrus (IFG), occipital lobes (OCC) and precentral gyrus (PRC) and normalized against a baseline value. A maximum of + 20% difference in median current density was found for three standard electrode montages: F3-RS, T7-T8 and Cz-Oz. Furthermore, median current density percentage differences in each montage target brain structures were found to be within + 7%. Higher levels of smoothing increased median current density percentage differences in T7-T8 and Cz-Oz target structures. However, while demonstrating similar trends in each montage, additional smoothing levels showed no clear relationship between their smoothing effects and calculated median current density in the five cortical structures. Finally, relative L2 error in reconstructed projected current density was found to be 17% and 21% for block and smooth pipelines, respectively. Overall, a block model workflow may be a more attractive alternative for simulating tDCS stimulation because involves a shorter modeling time and independence from commercial modeling platforms.
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http://dx.doi.org/10.1109/EMBC.2016.7591623DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5929137PMC
August 2016

Multishot echo-planar MREIT for fast imaging of conductivity, current density, and electric field distributions.

Magn Reson Med 2018 Jan 16;79(1):71-82. Epub 2017 Feb 16.

School of Biological and Health Systems Engineering, Arizona State University, Tempe, Arizona, USA.

Purpose: Magnetic resonance electrical impedance tomography (MREIT) sequences typically use conventional spin or gradient echo-based acquisition methods for reconstruction of conductivity and current density maps. Use of MREIT in functional and electroporation studies requires higher temporal resolution and faster sequences. Here, single and multishot echo planar imaging (EPI) based MREIT sequences were evaluated to see whether high-quality MREIT phase data could be obtained for rapid reconstruction of current density, conductivity, and electric fields.

Methods: A gel phantom with an insulating inclusion was used as a test object. Ghost artifact, geometric distortion, and MREIT correction algorithms were applied to the data. The EPI-MREIT-derived phase-projected current density and conductivity images were compared with simulations and spin-echo images as a function of EPI shot number.

Results: Good agreement among measures in simulated, spin echo, and EPI data was achieved. Current density errors were stable and below 9% as the shot number decreased from 64 to 2, but increased for single-shot images. Conductivity reconstruction relative contrast ratios were stable as the shot number decreased. The derived electric fields also agreed with the simulated data.

Conclusions: The EPI methods can be combined successfully with MREIT reconstruction algorithms to achieve fast imaging of current density, conductivity, and electric field. Magn Reson Med 79:71-82, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
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http://dx.doi.org/10.1002/mrm.26638DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5559352PMC
January 2018

Changing head model extent affects finite element predictions of transcranial direct current stimulation distributions.

J Neural Eng 2016 12 5;13(6):066006. Epub 2016 Oct 5.

School of Biological and Health Systems Engineering, Arizona State University, Box 879709, Tempe AZ, USA.

Objective: In this study, we determined efficient head model sizes relative to predicted current densities in transcranial direct current stimulation (tDCS).

Approach: Efficiency measures were defined based on a finite element (FE) simulations performed using nine human head models derived from a single MRI data set, having extents varying from 60%-100% of the original axial range. Eleven tissue types, including anisotropic white matter, and three electrode montages (T7-T8, F3-right supraorbital, Cz-Oz) were used in the models.

Main Results: Reducing head volume extent from 100% to 60%, that is, varying the model's axial range from between the apex and C3 vertebra to one encompassing only apex to the superior cerebellum, was found to decrease the total modeling time by up to half. Differences between current density predictions in each model were quantified by using a relative difference measure (RDM). Our simulation results showed that [Formula: see text] was the least affected (a maximum of 10% error) for head volumes modeled from the apex to the base of the skull (60%-75% volume).

Significance: This finding suggested that the bone could act as a bioelectricity boundary and thus performing FE simulations of tDCS on the human head with models extending beyond the inferior skull may not be necessary in most cases to obtain reasonable precision in current density results.
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http://dx.doi.org/10.1088/1741-2560/13/6/066006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5361410PMC
December 2016

Analytic Modeling of Neural Tissue: I. A Spherical Bidomain.

J Math Neurosci 2016 Dec 9;6(1). Epub 2016 Sep 9.

School of Biological and Health Systems Engineering, Arizona State University, 501 E Tyler Mall, Tempe, AZ, 85287-9709, USA.

Presented here is a model of neural tissue in a conductive medium stimulated by externally injected currents. The tissue is described as a conductively isotropic bidomain, i.e. comprised of intra and extracellular regions that occupy the same space, as well as the membrane that divides them, and the injection currents are described as a pair of source and sink points. The problem is solved in three spatial dimensions and defined in spherical coordinates [Formula: see text]. The system of coupled partial differential equations is solved by recasting the problem to be in terms of the membrane and a monodomain, interpreted as a weighted average of the intra and extracellular domains. The membrane and monodomain are defined by the scalar Helmholtz and Laplace equations, respectively, which are both separable in spherical coordinates. Product solutions are thus assumed and given through certain transcendental functions. From these electrical potentials, analytic expressions for current density are derived and from those fields the magnetic flux density is calculated. Numerical examples are considered wherein the interstitial conductivity is varied, as well as the limiting case of the problem simplifying to two dimensions due to azimuthal independence. Finally, future modeling work is discussed.
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http://dx.doi.org/10.1186/s13408-016-0041-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5018001PMC
December 2016

Fast conductivity imaging in magnetic resonance electrical impedance tomography (MREIT) for RF ablation monitoring.

Int J Hyperthermia 2014 Nov 20;30(7):447-55. Epub 2014 Oct 20.

Department of Mathematics, Konkuk University , Seoul , Korea and.

Purpose: This study shows the potential of magnetic resonance electrical impedance tomography (MREIT) as a non-invasive RF ablation monitoring technique.

Materials And Methods: We prepared bovine muscle tissue with a pair of needle electrodes for RF ablation, a temperature sensor, and two pairs of surface electrodes for conductivity image reconstructions. We used the injected current non-linear encoding with multi-echo gradient recalled echo (ICNE-MGRE) pulse sequence in a series of MREIT scans for conductivity imaging. We acquired magnetic flux density data induced by externally injected currents, while suppressing other phase artefacts. We used an 8-channel RF head coil and 8 echoes to improve the signal-to-noise ratio (SNR) in measured magnetic flux density data. Using the measured data, we reconstructed a time series of 180 conductivity images at every 10.24 s during and after RF ablation.

Results: Tissue conductivity values in the lesion increased with temperature during RF ablation. After reaching 60 °C, a steep increase in tissue conductivity values occurred with relatively little temperature increase. After RF ablation, tissue conductivity values in the lesion decreased with temperature, but to values different from those before ablation due to permanent structural changes of tissue by RF ablation.

Conclusion: We could monitor temperature and also structural changes in tissue during RF ablation by producing spatio-temporal maps of tissue conductivity values using a fast MREIT conductivity imaging method. We expect that the new monitoring method could be used to estimate lesions during RF ablation and improve the efficacy of the treatment.
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http://dx.doi.org/10.3109/02656736.2014.966337DOI Listing
November 2014

Frequency-dependent conductivity contrast for tissue characterization using a dual-frequency range conductivity mapping magnetic resonance method.

IEEE Trans Med Imaging 2015 Feb 7;34(2):507-13. Epub 2014 Oct 7.

Electrical conductivities of biological tissues show frequency-dependent behaviors, and these values at different frequencies may provide clinically useful diagnostic information. MR-based tissue property mapping techniques such as magnetic resonance electrical impedance tomography (MREIT) and magnetic resonance electrical property tomography (MREPT) are widely used and provide unique conductivity contrast information over different frequency ranges. Recently, a new method for data acquisition and reconstruction for low- and high-frequency conductivity images from a single MR scan was proposed. In this study, we applied this simultaneous dual-frequency range conductivity mapping MR method to evaluate its utility in a designed phantom and two in vivo animal disease models. Magnetic flux density and B(1)(+) phase map for dual-frequency conductivity images were acquired using a modified spin-echo pulse sequence. Low-frequency conductivity was reconstructed from MREIT data by the projected current density method, while high-frequency conductivity was reconstructed from MREPT data by B(1)(+) mapping. Two different conductivity phantoms comprising varying ion concentrations separated by insulating films with or without holes were used to study the contrast mechanism of the frequency-dependent conductivities related to ion concentration and mobility. Canine brain abscess and ischemia were used as in vivo models to evaluate the capability of the proposed method to identify new electrical properties-based contrast at two different frequencies. The simultaneous dual-frequency range conductivity mapping MR method provides unique contrast information related to the concentration and mobility of ions inside tissues. This method has potential to monitor dynamic changes of the state of disease.
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http://dx.doi.org/10.1109/TMI.2014.2361689DOI Listing
February 2015

Optimization of magnetic flux density measurement using multiple RF receiver coils and multi-echo in MREIT.

Phys Med Biol 2014 Sep 6;59(17):4827-44. Epub 2014 Aug 6.

Department of Biomedical Engineering, Kyung Hee University, Dongdaemun-gu, Seoul, Korea.

Magnetic Resonance Electrical Impedance Tomography (MREIT) is an MRI method that enables mapping of internal conductivity and/or current density via measurements of magnetic flux density signals. The MREIT measures only the z-component of the induced magnetic flux density B = (Bx, By, Bz) by external current injection. The measured noise of Bz complicates recovery of magnetic flux density maps, resulting in lower quality conductivity and current-density maps. We present a new method for more accurate measurement of the spatial gradient of the magnetic flux density gradient (∇ Bz). The method relies on the use of multiple radio-frequency receiver coils and an interleaved multi-echo pulse sequence that acquires multiple sampling points within each repetition time. The noise level of the measured magnetic flux density Bz depends on the decay rate of the signal magnitude, the injection current duration, and the coil sensitivity map. The proposed method uses three key steps. The first step is to determine a representative magnetic flux density gradient from multiple receiver coils by using a weighted combination and by denoising the measured noisy data. The second step is to optimize the magnetic flux density gradient by using multi-echo magnetic flux densities at each pixel in order to reduce the noise level of ∇ Bz and the third step is to remove a random noise component from the recovered ∇ Bz by solving an elliptic partial differential equation in a region of interest. Numerical simulation experiments using a cylindrical phantom model with included regions of low MRI signal to noise ('defects') verified the proposed method. Experimental results using a real phantom experiment, that included three different kinds of anomalies, demonstrated that the proposed method reduced the noise level of the measured magnetic flux density. The quality of the recovered conductivity maps using denoised ∇ Bz data showed that the proposed method reduced the conductivity noise level up to 3-4 times at each anomaly region in comparison to the conventional method.
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http://dx.doi.org/10.1088/0031-9155/59/17/4827DOI Listing
September 2014

Predictors of mortality and length of stay in hospitalized cases of 2009 influenza A (H1N1): Experiences of a tertiary care center.

Indian J Crit Care Med 2013 Sep;17(5):275-82

Department of Respiratory, Sleep and Critical Care Medicine, Indraprastha Apollo Hospital, Sarita Vihar, New Delhi, India.

Aim: To study the clinical characteristics and outcome of admitted patients of H1N1 (hemagglutinin -H neuraminidase -N) influenza in a tertiary level hospital, from Oct 2009 to Dec 2010.

Materials And Methods: A retrospective analysis of 77 confirmed patients admitted in this unit with H1N1 infection.

Results: Of the 77 patients studied, 33 (42.8%) were female. Mean age was 40.88 ± 13.45 years, majority (70.13%) being less than 50 years. Thirty eight (49.3%) patients had at least one co-morbidity, diabetes mellitus being the most common (n = 15, 19.5%). The most common presenting symptom was fever in 75 (97.4%) patients, cough in 67 (87%) and dyspnoea in 59 (76.6%) patients. At admission, mean PaO2/FiO2 ratio was 213.16 ± 132.75 mmHg (n = 60) while mean PaCO2 was 40.14 ± 14.86 mmHg. One or more organ failure was present in 45 (58.4%) patients. Nineteen (24.60%) patients required invasive mechanical ventilation. Circulatory failure was observed in 10 (13%) patients while 2 patients required hemodialysis. Overall, 13% mortality (n = 10) was observed. PaCO2 level at admission (OR 1.093; 95% confidence interval: 1.002-1.193; P = 0.044) and number of organ failure (OR 8.089; 95% confidence interval: 1.133-57.778; P = 0.037) were identified as independent risk- factors for mortality.

Conclusion: Increased duration of dyspnoea prior to admission, pneumonia, low PaO2/FiO2 ratio at admission and 24 hours later, higher PaCO2 values on admission, higher O2 requirement, number of organ failures and use of corticosteroids and delay in specialized treatment were associated with a poorer outcome.
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http://dx.doi.org/10.4103/0972-5229.120318DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841489PMC
September 2013

Radiofrequency ablation lesion detection using MR-based electrical conductivity imaging: a feasibility study of ex vivo liver experiments.

Int J Hyperthermia 2013 Nov;29(7):643-52

Department of Biomedical Engineering, Kyung Hee University , Yongin , Korea and.

Purpose: The aim of this study was to show the potential of magnetic resonance electrical impedance tomography (MREIT) conductivity imaging in terms of its capability to detect ablated lesions and differentiate tissue conditions in liver radiofrequency (RF) ablation.

Materials And Methods: RF ablation procedures were performed in bovine livers using a LeVeen RF needle electrode. Ablation lesions were created using a power-controlled mode at 30, 50, and 70 W for 1, 3, and 5 min of exposure time, respectively. After the ablation, the liver was cut into several blocks including the ablated lesion, and positioned inside a phantom filled with agarose gel. Electrodes were attached on the side of the phantom and it was placed inside the MRI bore. For MREIT imaging, multi-spin-echo pulse sequence was used to obtain the magnetic flux density data according to the injection currents.

Results: The conductivity of ablation lesions was significantly changed with the increase of exposure time (pKW < 0.01, Kruskal-Wallis test). With RF powers of 30 and 50 W, significant differences between the coagulation necrosis and hyperaemic rim were observed for more than 5 min and 3 min, respectively (pMW < 0.01, Mann-Whitney test). At 70 W, all cases showed significant differences except 3 min (pMW < 0.01). The positive correlation between the exposure time and tissue conductivity was observed in both two ablation areas (pSC < 0.01, Spearman correlation).

Conclusions: This ex vivo feasibility study demonstrates that current MREIT conductivity imaging can detect liver RF ablation lesions without using any contrast media or additional MR scan.
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http://dx.doi.org/10.3109/02656736.2013.842265DOI Listing
November 2013

Hyperammonemic coma in a post-partum patient with undiagnosed urea cycle defect.

Indian J Crit Care Med 2013 Mar;17(2):107-10

Department of Critical Care Medicine, Indraprastha Apollo Hospital, New Delhi, India.

Urea cycle disorders (UCD) are common during neonatal period, and it is rarely reported in adults. We are reporting a patient presenting with post-partum neuropsychiatric symptoms rapidly progressing to coma. Markedly raised serum ammonia level on presentation with an initial normal magnetic resonance imaging (MRI) of brain and normal liver function tests led to the suspicion of UCD, which was confirmed on the basis of urine orotic acid and elevated serum amino acid levels. We had to resort to hemodialysis to correct the hyperammonemic coma, which was unresponsive to conventional anti-ammonia measures. She exhibited remarkable improvement with a progressive decline in serum ammonia with repeated hemodialysis and made a full recovery. Timely diagnosis and early institution of hemodialysis in the setting of a poor neurological status maybe considered a suitable treatment option.
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http://dx.doi.org/10.4103/0972-5229.114816DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3752861PMC
March 2013

Optimization of magnetic flux density for fast MREIT conductivity imaging using multi-echo interleaved partial fourier acquisitions.

Biomed Eng Online 2013 Aug 27;12:82. Epub 2013 Aug 27.

Department of Biomedical Engineering, Kyung Hee University, Yongin, Korea.

Background: Magnetic resonance electrical impedance tomography (MREIT) has been introduced as a non-invasive method for visualizing the internal conductivity and/or current density of an electrically conductive object by externally injected currents. The injected current through a pair of surface electrodes induces a magnetic flux density distribution inside the imaging object, which results in additional magnetic flux density. To measure the magnetic flux density signal in MREIT, the phase difference approach in an interleaved encoding scheme cancels out the systematic artifacts accumulated in phase signals and also reduces the random noise effect by doubling the measured magnetic flux density signal. For practical applications of in vivo MREIT, it is essential to reduce the scan duration maintaining spatial-resolution and sufficient contrast. In this paper, we optimize the magnetic flux density by using a fast gradient multi-echo MR pulse sequence. To recover the one component of magnetic flux density Bz, we use a coupled partial Fourier acquisitions in the interleaved sense.

Methods: To prove the proposed algorithm, we performed numerical simulations using a two-dimensional finite-element model. For a real experiment, we designed a phantom filled with a calibrated saline solution and located a rubber balloon inside the phantom. The rubber balloon was inflated by injecting the same saline solution during the MREIT imaging. We used the multi-echo fast low angle shot (FLASH) MR pulse sequence for MRI scan, which allows the reduction of measuring time without a substantial loss in image quality.

Results: Under the assumption of a priori phase artifact map from a reference scan, we rigorously investigated the convergence ratio of the proposed method, which was closely related with the number of measured phase encode set and the frequency range of the background field inhomogeneity. In the phantom experiment with a partial Fourier acquisition, the total scan time was less than 6 seconds to measure the magnetic flux density Bz data with 128×128 spacial matrix size, where it required 10.24 seconds to fill the complete k-space region.

Conclusion: Numerical simulation and experimental results demonstrated that the proposed method reduces the scanning time and provides the recovered Bz data comparable to what we obtained by measuring complete k-space data.
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http://dx.doi.org/10.1186/1475-925X-12-82DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3766253PMC
August 2013

Numerical simulations of MREIT conductivity imaging for brain tumor detection.

Comput Math Methods Med 2013 29;2013:704829. Epub 2013 Apr 29.

Department of Biomedical Engineering, Impedance Imaging Research Center (IIRC), Kyung Hee University, Yongin, Republic of Korea.

Magnetic resonance electrical impedance tomography (MREIT) is a new modality capable of imaging the electrical properties of human body using MRI phase information in conjunction with external current injection. Recent in vivo animal and human MREIT studies have revealed unique conductivity contrasts related to different physiological and pathological conditions of tissues or organs. When performing in vivo brain imaging, small imaging currents must be injected so as not to stimulate peripheral nerves in the skin, while delivery of imaging currents to the brain is relatively small due to the skull's low conductivity. As a result, injected imaging currents may induce small phase signals and the overall low phase SNR in brain tissues. In this study, we present numerical simulation results of the use of head MREIT for brain tumor detection. We used a realistic three-dimensional head model to compute signal levels produced as a consequence of a predicted doubling of conductivity occurring within simulated tumorous brain tissues. We determined the feasibility of measuring these changes in a time acceptable to human subjects by adding realistic noise levels measured from a candidate 3 T system. We also reconstructed conductivity contrast images, showing that such conductivity differences can be both detected and imaged.
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http://dx.doi.org/10.1155/2013/704829DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3657440PMC
January 2014

Detection of temperature distribution via recovering electrical conductivity in MREIT.

Phys Med Biol 2013 Apr 3;58(8):2697-711. Epub 2013 Apr 3.

Impedance Imaging Research Center and Department of Biomedical Engineering, Kyung Hee University, Yongin, Korea.

In radiofrequency (RF) ablation or hyperthermia, internal temperature measurements and tissue property imaging are important to control their outputs and assess the treatment effect. Recently, magnetic resonance electrical impedance tomography (MREIT), as a non-invasive imaging method of internal conductivity distribution using an MR scanner, has been developed. Its reconstruction algorithm uses measured magnetic flux density induced by injected currents. The MREIT technique has the potential to visualize electrical conductivity of tissue with high spatial resolution and measure relative conductivity variation according to the internal temperature change based on the fact that the electrical conductivity of biological tissues is sensitive to the internal temperature distribution. In this paper, we propose a method to provide a non-invasive alternative to monitor the internal temperature distribution by recovering the electrical conductivity distribution using the MREIT technique. To validate the proposed method, we design a phantom with saline solution and a thin transparency film in a form of a hollow cylinder with holes to create anomalies with different electrical and thermal conductivities controlled by morphological structure. We first prove the temperature maps with respect to spatial and time resolution by solving the thermal conductivity partial differential equation with the real phantom experimental environment. The measured magnetic flux density and the reconstructed conductivity distributions using the phantom experiments were compared to the simulated temperature distribution. The relative temperature variation of two testing objects with respect to the background saline was determined by the relative conductivity contrast ratio (rCCR,%). The relation between the temperature and conductivity measurements using MREIT was approximately linear with better accuracy than 0.22 °C.
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http://dx.doi.org/10.1088/0031-9155/58/8/2697DOI Listing
April 2013

Effect of short-term propofol administration on pancreatic enzymes and lipid biochemistry in children between 1 month and 36 months.

Paediatr Anaesth 2013 Apr 9;23(4):355-9. Epub 2012 Nov 9.

Department of Anaesthesiology, SMS Medical College, Jaipur, Rajasthan, India.

Background: Use of propofol in pediatric age group has been marred by reports of its adverse effects like hypertriglyceridemia and acute pancreatitis, although a causal relation has not yet been established.

Objectives: This prospective, clinical trial was carried out to evaluate the effects of short-term propofol administration on serum lipid profile and serum pancreatic enzymes in children of ASA physical status I and II aged between 1 month and 36 months.

Methods: Anesthesia was induced with Propofol (1%) in the dose of 3 mg·kg(-1) intravenously and was maintained by propofol infusion (0.5%) at the rate of 12 mg·kg(-1·) h(-1) for the first 20 min and at 8 mg·kg(-1·) h(-1) thereafter. The mean dose of propofol administered was 12.02 ± 2.75 mg·kg(-1) (fat load of 120.2 ± 27.5 mg·kg(-1) ). Lipid profile, serum amylase, and lipase were measured before induction of anesthesia, at 90 min, 4 h, and finally 24 h after induction.

Results: Serum lipase levels (P < 0.05), serum triglyceride levels (P < 0.05), and serum very low-density lipoproteins VLDL levels (P < 0.05) were raised significantly after propofol administration from baseline although remained within normal limits. Serum cholesterol levels and serum low-density lipoproteins LDL levels showed a statistically significant fall over 24 h. No significant changes in serum pancreatic amylase levels were seen (P > 0.05). None of the patients developed any clinical features of pancreatitis in the postoperative period.

Conclusion: We conclude that despite a small, transient increase in serum triglycerides and pancreatic enzymes, short-term propofol administration in recommended dosages in children of ASA status I and II aged between 1 month and 36 months does not produce any clinically significant effect on serum lipids and pancreatic enzymes.
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http://dx.doi.org/10.1111/pan.12072DOI Listing
April 2013

Tumor lysis syndrome developing intraoperatively.

J Anaesthesiol Clin Pharmacol 2011 Oct;27(4):561-3

Department of Anaesthesiology, Dharmshila Cancer Hospital and Research Centre, Vasundhara Enclave, Shahdara, India.

Tumor lysis syndrome is a potentially life threatening condition which is most commonly encountered in patients being treated with chemotherapy. We report a case of spontaneous tumor lysis syndrome that developed intraoperatively in a patient with undiagnosed Burkitt's lymphoma. Characteristic electrolyte disturbances and white emulsion like urine following laparotomy and tumor handling intraoperatively suggested the diagnosis. This is a rare perioperative complication and the report emphasizes the importance of being vigilant in recognizing the same.
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http://dx.doi.org/10.4103/0970-9185.86611DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3214572PMC
October 2011
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