Current Density Imaging Research Articles

Reverse saturation current density imaging of highly doped regions in silicon: A photoluminescence approach

We present a camera-based technique for the local determination of reverse saturation current densities J0 of highly doped regions in silicon wafers utilizing photoconductance calibrated photoluminescence imaging (PC-PLI). We apply this approach to 12.5×12.5cm² float zone silicon samples with textured surfaces and a homogeneous phosphorous diffusion with sheet resistances between 24 and 230Ω/□. We find enhanced photoluminescence emission at metallized regions of a sample due to reflection of long-wavelength light at the rear side of the sample. Our measurement setup comprises an optical short pass filter in front of the camera effectively blocking wavelengths above 970nm and therefore ensuring a correct calibration of the PL signal in terms of excess charge carrier density Δn. We analyze two sets of samples comprising metal contacts to highly doped regions prepared by Laser Transfer Doping (LTD) as well as standard tube furnace phosphorus diffusion. We find a considerably smaller J0 value of 370fA/cm² for the LTD approach compared to a standard diffusion process resulting in J0=570fA/cm². On the basis of these results we demonstrate that J0 imaging is a powerful analysis technique for process optimization.

High resolution saturation current density imaging at grain boundaries by lock-in thermography

The electronic properties of multicrystalline solar silicon materials are dominated by low-lifetime defect regions containing recombination-active grain boundaries and dislocations. Besides reducing the carrier collection probability, these regions increase the dark saturation current density J01, which governs the open circuit voltage. By applying lock-in thermography with spatial deconvolution it is shown that the dominant contribution to J01 comes from recombination-active grain boundaries and to a lower degree from intra-grain defects like dislocations.

Magnetic resonance current density imaging using one component of magnetic flux density

Current density distribution generated inside a volume conductor by externally applied currents can be calculated by using spatial distribution of its magnetic flux density, . The imaging modality used to reconstruct images of the current density distribution is known as magnetic resonance current density imaging (MRCDI). In MRCDI, spatial distribution of the current‐induced magnetic flux density is measured on a magnetic resonance imaging (MRI) platform. Calculation of current density distribution from magnetic flux density measurements requires all three components of . As only the component of which is in parallel with the main field of the MRI system can be measured, rotation of the subject is necessary to be able to acquire all three components of for a given current injection pattern. Rotating the subject inside the magnet is not trivial even for small‐sized objects and remains as a strong limitation to clinical applicability of the technique. In this study, a novel MRCDI reconstruction algorithm using only one component of is proposed to eliminate the need for subject rotation. Reconstruction performance of the proposed algorithm is evaluated on numerical and experimental models. Images obtained by the proposed and the conventional MRCDI algorithms, which utilizes three components of , are compared.

Simulation in the Front Region of the Earth’s Magnetosphere

A laboratory experiment was conducted to investigate the interaction between a plasma beam and a magnetic dipole, simulating the interaction between the solar wind and magnetized planets. The emphasis in this paper is on the laboratory simulation in the front region of the Earth’s magnetosphere and their variation under different solar wind conditions. The boundary in the front region of the magnetosphere is observed in a space simulation laboratory and the magnetospheric structure is produced by a super-Alfvenic and collisionless plasma beam interacting with terrella field. The boundary of the magnetosphere is determined by the factors that include solar wind parameters, such as magnetic fields, ion current density and magnetospheric structure images. It is interesting to compare the results of laboratory simulations with the empirical model by Shue et al. (1997) and the theoretical model by Cheng (1998) as well for the prediction of magnetopause locations under any solar wind condition. The comparisons show that for the northward IMF, magnetopause locations in the laboratory simulation are consistent with the theoretical model. As the magnitude of northward IMF Bz becomes higher, the subsolar distance and the flank position in laboratory simulations are consistent with the empirical model as well. For a lower southward IMF Bz, magnetopause locations in laboratory simulations are consistent with both the empirical and theoretical models. As the magnitude of the southward IMF Bz becomes higher, the subsolar distance and the flank position in laboratory simulations seem closer to the theoretical model than the empirical model.

Four-dimensional ultrasound current source density imaging of a dipole field

Ultrasound current source density imaging (UCSDI) potentially transforms conventional electrical mapping of excitable organs, such as the brain and heart. For this study, we demonstrate volume imaging of a time-varying current field by scanning a focused ultrasound beam and detecting the acoustoelectric (AE) interaction signal. A pair of electrodes produced an alternating current distribution in a special imaging chamber filled with a 0.9% NaCl solution. A pulsed 1 MHz ultrasound beam was scanned near the source and sink, while the AE signal was detected on remote recording electrodes, resulting in time-lapsed volume movies of the alternating current distribution.

3D current source density imaging based on the acoustoelectric effect: a simulation study using unipolar pulses

It is of importance to image electrical activity and properties of biological tissues. Recently hybrid imaging modality combing ultrasound scanning and source imaging through the acoustoelectric (AE) effect has generated considerable interest. Such modality has the potential to provide high spatial resolution current density imaging by utilizing the pressure-induced AE resistivity change confined at the ultrasound focus. In this study, we investigate a novel three-dimensional (3D) ultrasound current source density imaging approach using unipolar ultrasound pulses. Utilizing specially designed unipolar ultrasound pulses and by combining AE signals associated to the local resistivity changes at the focusing point, we are able to reconstruct the 3D current density distribution with the boundary voltage measurements obtained while performing a 3D ultrasound scan. We have shown in computer simulation that using the present method it is feasible to image with high spatial resolution an arbitrary 3D current density distribution in an inhomogeneous conductive media.

LORETA analysis of three-dimensional distribution of delta band activity in schizophrenia: Relation to negative symptoms

We sought to determine if altered electroencephalography (EEG) activities, such as delta band activity, in specific brain regions are associated with psychotic symptoms. Data were obtained from 17 neuroleptic-naive patients with schizophrenia and age- and sex-matched 17 healthy control subjects. Low Resolution Brain Electromagnetic Tomography (LORETA) was used to generate current source density images of delta, theta, alpha, and beta activities. Localization of the difference in EEG activity between the two groups was assessed by voxel-by-voxel non-paired t-test of the LORETA images. Spearman's correlation coefficient was obtained to relate LORETA values of EEG current density in brain regions showing a significant between-group difference and psychopathology scores. Delta band activity, represented by LORETA current density, was greater for patients in the following areas; the left inferior temporal gyrus, right middle frontal gyrus, right superior frontal gyrus, right inferior frontal gyrus, and right parahippocampal gyrus. LORETA values for delta band activity in the above five brain regions were negatively correlated with negative, but not positive symptoms. The results of this study suggest the role for electrophysiological changes in some of the brain regions, e.g. prefrontal cortex, in the manifestation of negative symptoms.

Scanning Hall-probe microscopy system for two-dimensional imaging of critical current density in RE-123 coated conductors

We have developed a characterization method for two-dimensional imaging of critical current density in coated conductors (CCs) based on scanning Hall-probe microscopy (SHPM). The distributions of the magnetic field around a sample were measured for several different conditions of external magnetic fields, and then were converted to those of the sheet current density which flowed to shield the external magnetic field or to trap the penetrated magnetic field. As a result, it was found that the amplitude of the sheet current density corresponded to that of critical current density almost in all the area of the sample except for the region where current direction changed. This indicates that we could obtain an in-plane distribution of the critical current density with a spatial resolution of around 100μm in non-destructive manner by this method. We believe that this measurement will be a multifunctional and comprehensive characterization method for coated conductors.

Magnetic Resonance Electrical Impedance Tomography for Monitoring Electric Field Distribution During Tissue Electroporation

Electroporation is a phenomenon caused by externally applied electric field of an adequate strength and duration to cells that results in the increase of cell membrane permeability to various molecules, which otherwise are deprived of transport mechanism. As accurate coverage of the tissue with a sufficiently large electric field presents one of the most important conditions for successful electroporation, applications based on electroporation would greatly benefit with a method of monitoring the electric field, especially if it could be done during the treatment. As the membrane electroporation is a consequence of an induced transmembrane potential which is directly proportional to the local electric field, we propose current density imaging (CDI) and magnetic resonance electrical impedance tomography (MREIT) techniques to measure the electric field distribution during electroporation. The experimental part of the study employs CDI with short high-voltage pulses, while the theoretical part of the study is based on numerical simulations of MREIT. A good agreement between experimental and numerical results was obtained, suggesting that CDI and MREIT can be used to determine the electric field during electric pulse delivery and that both of the methods can be of significant help in planning and monitoring of future electroporation based clinical applications.

Sparse cortical current density imaging in motor potentials induced by finger movement

Predominant components in electro- or magneto-encephalography (EEG/MEG) are scalp projections of synchronized neuronal electrical activity distributed over cortical structures. Reconstruction of cortical sources underlying EEG/MEG can thus be achieved with the use of the cortical current density (CCD) model. We have developed a sparse electromagnetic source imaging method based on the CCD model, named as the variation-based cortical current density (VB-SCCD) algorithm, and have shown that it has much enhanced performance in reconstructing extended cortical sources in simulations (Ding 2009 Phys. Med. Biol. 54 2683–97). The present study aims to evaluate the performance of VB-SCCD, for the first time, using experimental data obtained from six participants. The results indicate that the VB-SCCD algorithm is able to successfully reveal spatially distributed cortical sources behind motor potentials induced by visually cued repetitive finger movements, and their dynamic patterns, with millisecond resolution. These findings of motor sources and cortical systems are supported by the physiological knowledge of motor control and evidence from various neuroimaging studies with similar experiments. Furthermore, our present results indicate the improvement of cortical source resolvability of VB-SCCD, as compared with two other classical algorithms. The proposed solver embedded in VB-SCCD is able to handle large-scale computational problems, which makes the use of high-density CCD models possible and, thus, reduces model misspecifications. The present results suggest that VB-SCCD provides high resolution source reconstruction capability and is a promising tool for studying complicated dynamic systems of brain activity for basic neuroscience and clinical neuropsychiatric research.

Radio-Frequency Current Density Imaging Based on a 180$^\circ$ Sample Rotation With Feasibility Study of Full Current Density Vector Reconstruction

Radio-frequency current density imaging (RF-CDI) is a technique that noninvasively measures current density distributions at the Larmor frequency utilizing magnetic resonance imaging. Previously implemented RF-CDI methods reconstruct the applied current density component J(z) along the static magnetic field of the imager [(B)\vec](0) (the z direction) based on the assumption that the z-directional change of the magnetic field component H(z) can be ignored compared to J(z). However, this condition may be easily violated in biomedical applications. We propose a new reconstruction method for RF-CDI, which does not rely on the aforementioned assumption. Instead, the sample is rotated by 180 (°) in the horizontal plane to collect magnetic resonance data from two opposite positions. Using simulations and experiments, we have verified that this approach can fully recover one component of current density. Furthermore, this approach can be extended to measure three dimensional current density vectors by one additional sample orientation in the horizontal plane. We have therefore demonstrated for the first time the feasibility of imaging the magnitude and phase of all components of a radio-frequency current density vector field.

Neural circuits and temporal plasticity in hindlimb representation of rat primary somatosensory cortex: revisited by multi-electrode array on brain slices

The well-established planar multi-electrode array recording technique was used to investigate neural circuits and temporal plasticity in the hindlimb representation of the rat primary somatosensory cortex (S1 area). Freshly dissociated acute brain slices of rats were subject to constant perfusion with oxygenated artificial cerebrospinal fluid (95% O(2) and 5% CO(2)), and were mounted on a Med64 probe (64 electrodes, 8x8 array) for simultaneous multi-site electrophysiological recordings. Current sources and sinks across all the 64 electrodes were transformed into two-dimensional current source density images by bilinear interpolation at each point of the 64 electrodes. The local intracortical connection, which is involved in mediation of downward information flow across layers II-VI, was identified by electrical stimulation (ES) at layers II-III. The thalamocortical connection, which is mainly involved in mediation of upward information flow across layers II-IV, was also characterized by ES at layer IV. The thalamocortical afferent projections were likely to make more synaptic contacts with S1 neurons than the intracortical connections did. Moreover, the S1 area was shown to be more easily activated and more intensively innervated by the thalamocortical afferent projections than by the intracortical connections. Finally, bursting conditioning stimulus (CS) applied within layer IV of the S1 area could successfully induce long-term potentiation (LTP) in 5 of the 6 slices (83.3%), while the same CS application at layers II-III induced no LTP in any of the 6 tested slices. The rat hindlimb representation of S1 area is likely to have at least 2 patterns of neural circuits on brain slices: one is the intracortical circuit (ICC) formed by interlaminar connections from layers II-III, and the other is the thalamocortical circuit (TCC) mediated by afferent connections from layer IV. Besides, ICC of the S1 area is spatially limited, with less plasticity, while TCC is spatially extensive and exhibits a better plasticity in response to somatosensory afferent stimulation. The present data provide a useful experimental model for further studying microcircuit properties in S1 cortex at the network level in vitro.

Fourier transform magnetic resonance current density imaging (FT-MRCDI) from one component of magnetic flux density

Fourier transform (FT)-based algorithms for magnetic resonance current density imaging (MRCDI) from one component of magnetic flux density have been developed for 2D and 3D problems. For 2D problems, where current is confined to the xy-plane and z-component of the magnetic flux density is measured also on the xy-plane inside the object, an iterative FT-MRCDI algorithm is developed by which both the current distribution inside the object and the z-component of the magnetic flux density on the xy-plane outside the object are reconstructed. The method is applied to simulated as well as actual data from phantoms. The effect of measurement error on the spatial resolution of the current density reconstruction is also investigated. For 3D objects an iterative FT-based algorithm is developed whereby the projected current is reconstructed on any slice using as data the Laplacian of the z-component of magnetic flux density measured for that slice. In an injected current MRCDI scenario, the current is not divergence free on the boundary of the object. The method developed in this study also handles this situation.

Simulating the effects of time-varying magnetic fields with a realistic simulated scanner

Transient magnetic fields induce changes in magnetic resonance (MR) images ranging from small, visually undetectable effects (caused, for instance, by neuronal currents) to more significant ones, such as those created by the gradient fields and eddy currents. Accurately simulating these effects may assist in correcting or optimising MR imaging for many applications (e.g., diffusion imaging, current density imaging, use of magnetic contrast agents, neuronal current imaging, etc.). Here we have extended an existing MR simulator (POSSUM) with a model for changing magnetic fields at a very high-resolution time-scale. This simulator captures a realistic range of scanner and physiological artifacts by modeling the scanner environment, pulse sequence details and subject properties (e.g., brain geometry and air-tissue boundaries). The simulations were validated by using previously published experimental data sets. In the first dataset a transient magnetic field was produced by a single conducting wire with varying current amplitude (between 17 μA and 765 μA). The second was identical except that current amplitude was fixed (at 7.8 mA) and current timing varied. A very close match between simulated images and experimental data was observed. In addition, these validation results led to the observation that the current-induced effects included ringing in the image, which extended away from the conductor, primarily in the phase-encode direction. This effect had previously not been noticed in the noisy, experimentally-acquired images, demonstrating one way in which simulated images can provide potential insight into imaging experiments.

Enhanced sensitivity current density imaging

One of the major weaknesses of current density imaging (CDI) is its poor sensitivity and therefore a need for the use of high voltage in CDI. In this work, a new CDI technique with enhanced sensitivity (ES-CDI) is presented. The ES-CDI sequence overcomes the sensitivity problem in samples with a long T 2 relaxation time that allows the use of a long current encoding period. As successful CDI detection is conditioned by a sufficiently large product of current and its application time a longer current encoding period enables the use of lower current and also lower voltage therefore significantly reducing any sample damage. In addition, the ES-CDI sequence also uses fast image signal acquisition and so enables heavy signal averaging and with it associated additional CDI sensitivity increase within the experiment time of the conventional CDI experiment. The feasibility of the ES-CDI sequence was tested on a model sample filled with physiological solution. Voltage of just 1 V and current application time of 800 ms were sufficient to detect current density of 20 A/m 2 with a detection limit of 0.7 A/m 2.

Visible Human Utilization to Render Induced Electric Field and Current Density Images Inside the Human

The external and internal exposure to power frequency electromagnetic field, generated by high-voltage power transmission lines, raises serious safety concerns. Since we cannot measure the induced electric fields and current densities inside the human body, we used the Visible Human (VH) to investigate the induced electric fields and currents in human body tissues and organs of a worker standing 2 m away from conductor phase C of a double-circuit 132 kV transmission line. The double circuit 132 kV 60 Hz transmission line has a power rating of 293 MVA and a maximum recorded peak load current of 603 A. Charge simulation method and the Biot-Savart law have been used for computation of external electric and magnetic fields. Finite-difference time-domain technique was used to calculate the organs' internal induced electric field and circulating current densities in more than 40 different tissues of the VH with 3 mm voxel size. The simulation indicates that, at 2 m away from a 132 kV transmission line, the computed external electric field is 6.485 kV/m and the external magnetic field is 66.4 muT, which are below the limits set by the IEEE standards for external exposure for live-line workers. The maximum induced electric fields in the brain and heart are 23 and 14 mV/m, respectively. These values are below the IEEE standard recommended limits of 53 mV/m for the brain and 943 mV/m for the heart. The VH data allowed us to obtain two- and three-dimensional images of the induced electric field and current density distribution in different organs, tissues, and cross-sections of the human body.

TU-D-210A-03: Where Sound Meets Electricity: The Acoustoelectric Effect in Biomedicine

The acoustoelectric (AE) effect is a well-documented interaction between local pressure and electric resistivity. In recent years, the phenomenon has been revived for applications in biomedical imaging and therapy. My laboratory is developing two new approaches based on this concept: 1) ultrasonic imaging of current flow and 2) novel detection and imaging of an acoustic beam. Ultrasound Current Source Density Imaging (UCSDI) is a new modality for mapping electrical current deep into tissue. This approach combines moderate acoustic pressure with recording electrode technology to directly image current densities. We have demonstrated feasibility of UCSDI in saline, tissue-equivalent phantoms, neural tissue and direct mapping of the cardiac activation wave in the live rabbit heart. Although potential applications of UCSDI are diverse, we are focusing on enhancing electrical cardiac mapping during ablation treatment of arrhythmias. There are several potential advantages of UCSDI over conventional electrophysiology and electrical imaging, highlighted by 1) enhanced spatial resolution determined by the size of the ultrasound focus (<1 mm); and 2) automatic co-registration of UCSDI with pulse echo ultrasound, depicting current density maps superimposed on heart structure and motion. In addition to UCSDI, we are also developing the AE hydrophone as a new device for detecting pressure and imaging an ultrasound beam. As clinical applications for ultrasound therapy continue to proliferate—from lithotripsy to ablation treatment of uterine fibroids and cancer—the need for simple, rapid, and accurate estimates of the acoustic field become increasingly important. The AE hydrophone does not depend on a piezoelectric material; instead, a small region with high current density is used as a gain mechanism for detecting ultrasonic waves. I will present early results from initial prototypes and compare with conventional hydrophones, as well as simulations. The AE hydrophone has attractive attributes not typically seen with other devices, including simple construction, low cost, decent sensitivity, and resistant to damage at high intensity ultrasonic fields.

Reconstructing cortical current density by exploring sparseness in the transform domain

In the present study, we have developed a novel electromagnetic source imaging approach to reconstruct extended cortical sources by means of cortical current density (CCD) modeling and a novel EEG imaging algorithm which explores sparseness in cortical source representations through the use of L1-norm in objective functions. The new sparse cortical current density (SCCD) imaging algorithm is unique since it reconstructs cortical sources by attaining sparseness in a transform domain (the variation map of cortical source distributions). While large variations are expected to occur along boundaries (sparseness) between active and inactive cortical regions, cortical sources can be reconstructed and their spatial extents can be estimated by locating these boundaries. We studied the SCCD algorithm using numerous simulations to investigate its capability in reconstructing cortical sources with different extents and in reconstructing multiple cortical sources with different extent contrasts. The SCCD algorithm was compared with two L2-norm solutions, i.e. weighted minimum norm estimate (wMNE) and cortical LORETA. Our simulation data from the comparison study show that the proposed sparse source imaging algorithm is able to accurately and efficiently recover extended cortical sources and is promising to provide high-accuracy estimation of cortical source extents.

Cardiac activation mapping using ultrasound current source density imaging (UCSDI).

We describe the first mapping of biological current in a live heart using ultrasound current source density imaging (UCSDI). Ablation procedures that treat severe heart arrhythmias require detailed maps of the cardiac activation wave. The conventional procedure is time-consuming and limited by its poor spatial resolution (5-10 mm). UCSDI can potentially improve on existing mapping procedures. It is based on a pressure-induced change in resistivity known as the acousto-electric (AE) effect, which is spatially confined to the ultrasound focus. Data from 2 experiments are presented. A 540 kHz ultrasonic transducer (f/# = 1, focal length = 90 mm, pulse repetition frequency = 1600 Hz) was scanned over an isolated rabbit heart perfused with an excitation-contraction decoupler to reduce motion significantly while retaining electric function. Tungsten electrodes inserted in the left ventricle recorded simultaneously the AE signal and the low-frequency electrocardiogram (ECG). UCSDI displayed spatial and temporal patterns consistent with the spreading activation wave. The propagation velocity estimated from UCSDI was 0.25 +/- 0.05 mm/ms, comparable to the values obtained with the ECG signals. The maximum AE signal-to-noise ratio after filtering was 18 dB, with an equivalent detection threshold of 0.1 mA/ cm(2). This study demonstrates that UCSDI is a potentially powerful technique for mapping current flow and biopotentials in the heart.

Multislice Radio-Frequency Current Density Imaging

Radio-frequency current density imaging (RF-CDI) is an imaging technique that noninvasively measures current density distribution at the Larmor frequency utilizing magnetic resonance imaging (MRI). Previously implemented RF-CDI techniques were only able to image a single slice transverse to the static magnetic field B(0) . This paper describes the first realization of a multislice RF-CDI sequence on a 1.5 T clinical imager. Multislice RF current density images have been reconstructed for two phantoms. The influence of MRI random noise on the sensitivity of the multislice RF-CDI measurement has also been studied by theoretical analysis, simulation and phantom experiments.