Measurement of thoracic current flow in pigs for the study of defibrillation and cardioversion

Although defibrillation has been in clinical use for more than 50 years, the complete current flow distribution inside the body during a defibrillation procedure has never been directly measured. This is due to the lack of appropriate imaging technology to noninvasively monitor the current flow inside the body. The current density imaging (CDI) technique, using a magnetic resonance (MR) imager, provides a new approach to this problem [Scott et al. (1991)]. CDI measures the local magnetic field generated by the current and calculates the current density by computing its curl. In this study, CDI was used to measure current density at all points within a postmortem pig torso during an electrical current application through defibrillation electrodes. Furthermore, current flow information was visualized along the chest wall and within the chest cavity using streamline analysis. As expected, some of the highest current densities were observed in the chest wall. However, current density distribution varied significantly from one region to another, possibly reflecting underlying heterogeneous tissue conductivity and anisotropy. Moreover, the current flow analysis revealed many complex and unexpected current flow patterns that have never been observed before. This study has, for the first time, noninvasively measured the volume current measurement inside the pig torso.     

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.     

Local Harmonic $B_z$ Algorithm With Domain Decomposition in MREIT: Computer Simulation Study

Magnetic resonance electrical impedance tomography (MREIT) attempts to provide conductivity images of an electrically conducting object with a high spatial resolution. When we inject current into the object, it produces internal distributions of current density ${bf J}$ and magnetic flux density ${bf B}=(B_x,B_y,B_z)$. By using a magnetic resonance imaging (MRI) scanner, we can measure $B_z$ data where $z$ is the direction of the main magnetic field of the scanner. Conductivity images are reconstructed based on the relation between the injection current and $B_z$ data. The harmonic $B_z$ algorithm was the first constructive MREIT imaging method and it has been quite successful in previous numerical and experimental studies. Its performance is, however, degraded when the imaging object contains low-conductivity regions such as bones and lungs. To overcome this difficulty, we carefully analyzed the structure of a current density distribution near such problematic regions and proposed a new technique, called the local harmonic $B_z$ algorithm. We first reconstruct conductivity values in local regions with a low conductivity contrast, separated from those problematic regions. Then, the method of characteristics is employed to find conductivity values in the problematic regions. One of the most interesting observations of the new algorithm is that it can provide a scaled conductivity image in a local region without knowing conductivity values outside the region. We present the performance of the new algorithm by using computer simulation methods.      

相干衍射成像研究进展:叠层扫描相干衍射成像和相干调制成像

相干衍射成像技术是一种无透镜计算成像技术。该技术通过迭代算法求解相位问题,直接从衍射强度数据重构物体的振幅和相位图像信息,可获得由照明光源波长和数据记录有效数值孔径决定的衍射极限分辨率。由于相干衍射成像技术不依赖于高质量光学成像元件的使用,因而适合用于深紫外、X射线以及电子束等诸多很难制作高性能成像器件的辐射源。过去20年来新型光源（冷致电子枪、第三代和第四代同步辐射光源、自由电子激光器）、单粒子计数高灵敏度、宽动态范围面阵检测器的迅猛发展,也极大地促进了相干衍射成像技术的发展。目前相干衍射成像在材料科学和生物学等热门学科方向上已经显示出了相较传统方法的独特优势,在部分应用上已经逐渐成为主流技术。文中概略回顾相干衍射成像的发展历史。重点介绍叠层扫描相干衍射成像和相干调制成像这两种快速发展的技术。     

Reconstruction of conductivity and current density images using only one component of magnetic field measurements

Magnetic resonance current density imaging (MRCDI) is to provide current density images of a subject using a magnetic resonance imaging (MRI) scanner with a current injection apparatus. The injection current generates a magnetic field that we can measure from MR phase images. We obtain internal current density images from the measured magnetic flux densities via Ampere's law. However, we must rotate the subject to acquire all of the three components of the induced magnetic flux density. This subject rotation is impractical in clinical MRI scanners when the subject is a human body. In this paper, we propose a way to eliminate the requirement of subject rotation by careful mathematical analysis of the MRCDI problem. In our new MRCDI technique, we need to measure only one component of the induced magnetic flux density and reconstruct both cross-sectional conductivity and current density images without any subject rotation.     

The electric field induced in the brain by magnetic stimulation: a 3-D finite-element analysis of the effect of tissue heterogeneity and anisotropy

We investigate the effect of tissue heterogeneity and anisotropy on the electric field and current density distribution induced in the brain during magnetic stimulation. Validation of the finite-element (FE) calculations in a homogeneous isotropic sphere showed that the magnitude of the total electric field can be calculated to within an error of approximately 5% in the region of interest, even in the presence of a significant surface charge contribution. We used a high conductivity inclusion within a sphere of lower conductivity to simulate a lesion due to an infarct. Its effect is to increase the electric field induced in the surrounding low conductivity region. This boost is greatest in the vicinity of interfaces that lie perpendicular to the current flow. For physiological values of the conductivity distribution, it can reach a factor of 1.6 and extend many millimeters from the interface. We also show that anisotropy can significantly alter the electric field and current density distributions. Either heterogeneity or anisotropy can introduce a radial electric field component, not present in a homogeneous isotropic conductor. Heterogeneity and anisotropy are predicted to significantly affect the distribution of the electric field induced in the brain. It is, therefore, expected that anatomically faithful FE models of individual brains which incorporate conductivity tensor data derived from diffusion tensor measurements, will provide a better understanding of the location of possible stimulation sites in the brain.     

Current density impedance imaging

Current density impedance imaging (CDII) is a new impedance imaging technique that can noninvasively measure the conductivity distribution inside a medium. It utilizes current density vector measurements which can be made using a magnetic resonance imager (MRI) (Scott , 1991). CDII is based on a simple mathematical expression for $nablasigma/sigma=nablalnsigma$, the gradient of the logarithm of the conductivity $sigma$ , at each point in a region where two current density vectors ${bf J}_{1}$ and ${bf J}_{2}$ have been measured and ${bf J}_{1}times{bf J}_{2}neq 0$. From the calculated $nablalnsigma$ and a priori knowledge of the conductivity at the boundary, the logarithm of the conductivity $lnsigma$ is integrated by two different methods to produce an image of the conductivity $sigma$ in the region of interest. The CDII technique was tested on three different conductivity phantoms. Much emphasis has been placed on the experimental validation of CDII results against direct bench measurements by commercial LCR meters before and after CDII was performed.      

Functional Imaging of Spinal Cord Electrical Activity From Its Evoked Magnetic Field

This paper investigates dynamic source imaging of the spinal cord electrophysiological activity from its evoked magnetic field by applying the spatial filter version of standardized low-resolution brain electromagnetic tomography (sLORETA). Our computer simulation shows that the sLORETA-based spatial filter can reconstruct the four current sources typically associated with the elicitation of the spinal cord evoked magnetic field (SCEF). The results from animal experiments show that significant changes in the latency and intensity of the reconstructed volume current arise near the location of the artificial incomplete conduction block. The results from the human SCEF show that the SCEF source imaging can visualize the dynamics of the volume currents and other nerve electrical activity propagating along the human spinal cord. These experimental results demonstrate the potential of SCEF source imaging as a future clinical tool for diagnosing cervical spinal cord disorders.     

Noninvasive imaging of bioimpedance distribution by means of current reconstruction magnetic resonance electrical impedance tomography

We have developed a novel magnetic resonance electrical impedance tomography (MREIT) algorithm-current reconstruction MREIT algorithm-for noninvasive imaging of electrical impedance distribution of a biological system using only one component of magnetic flux density. The newly proposed algorithm uses the inverse of Biot-Savart Law to reconstruct the current density distribution, and then, uses a modified J-substitution algorithm to reconstruct the conductivity image. A series of computer simulations has been conducted to evaluate the performance of the proposed current reconstruction MREIT algorithm with simulation settings for breast cancer imaging applications, with consideration of measurement noise, current injection strength, size of simulated tumors, spatial resolution, and position dependency. The present simulation results are highly promising, demonstrating the high spatial resolution, high accuracy in conductivity reconstruction, and robustness against noise of the proposed algorithm for imaging electrical impedance of a biological system. The present MREIT method may have potential applications to breast cancer imaging and imaging of other organs.     

Distinguishability in impedance imaging

Impedance imaging systems apply currents to the surface of a body, measure the induced voltages on the surface, and from this information, reconstruct an approximation to the electrical conductivity in the interior. This paper gives a detailed discussion of several ways to measure the ability of such a system to distinguish between two different conductivity distributions. The subtle differences between these related measures are discussed, and examples are provided to show that these different measures can give rise to different answers to various practical questions about system design.     

老龄大鼠脑缺血再灌注海马神经元iNOS的表达及超微结构改变

目的：观察老年大鼠脑缺血再灌注后海马神经元诱导型一氧化氮合酶（induced nitric oxide synthase iNOS）的表达及超微结构变化。方法：建立老年大鼠不完全性全脑缺血动物模型，应用免疫组织化学染色和透射电镜，观察海马神经元iNOS的表达及超微结构变化。结果：缺血30min后再灌注24h组海马神经元iNOS活性显著升高；缺血30min再灌注21h和48h组中量表达；假手术组、缺血30min即刻取材、再灌注1h、6h、96h组iNOS几乎无表达；再灌注超过48h组海马神经元损伤较重。结论：NO是脑缺血后神经元迟发性死亡的重要因素之一。     

The boundary shift integral: an accurate and robust measure ofcerebral volume changes from registered repeat MRI

We propose the boundary shift integral (BSI) as a measure of cerebral volume changes derived from registered repeat three-dimensional (3-D) magnetic resonance (MR) [3D MR] scans. The BSI determines the total volume through which the boundaries of a given cerebral structure have moved and, hence, the volume change, directly from voxel intensities. We found brain and ventricular BSI's correlated tightly (r=1.000 and r=0.999) with simulated volumes of change. Applied to 21 control scan pairs and 11 scan pairs from Alzheimer's disease (AD) patients (mean interval 386 days) the BSI yielded mean brain volume loss of 1.8 cc (controls) and 34.7 cc (AD); the control group was tightly bunched (SD=3.8 cc) and there was wide group separation, the group means differing by 8.7 control group standard deviations (SDs). A measure based on the same segmentation used by the BSI yielded similar group means, but wide spread in the control group (SD=13.4 cc) and group overlap, the group means differing by 2.8 control group SDs. The BSI yielded mean ventricular volume losses of 0.4 cc (controls) and 10.1 cc (AD). Good linear correlation (r=0.997) was obtained between the ventricular BSI and the difference in their segmented volumes. We conclude the BSI is an accurate and robust measure of regional and global cerebral volume changes     

光声技术在脑组织成像中的应用（特邀）

基于激光诱导超声机制的光声成像技术结合了光学成像的高对比度和超声成像的深穿透性,能无标记、非侵入反映生命体内源性吸收物质的分布,适合啮齿类动物模型全脑的即时成像。为了证明光声技术在脑科学研究和脑疾病监测中的应用,搭建了光声显微成像系统,其空间分辨率可达几十微米,有效成像深度可达1 mm以上,并以APP/PS1转基因阿尔茨海默症（Alzheimer’s disease, AD）模型小鼠和野生型WT小鼠为研究对象,从脑组织切片、离体全脑和活体全脑三个层面探究了光声成像在表征AD鼠和WT鼠脑结构变化和血管网络的能力,证明了光声技术在研究脑疾病发展过程中监控脑结构变化和脑血管网络特征的巨大潜力,可以为诸多脑科学研究和神经退行性疾病发展机制提供更深入的见解。     

Toward a direct measure of video quality perception using EEG

An approach to the direct measurement of perception of video quality change using electroencephalography (EEG) is presented. Subjects viewed 8-s video clips while their brain activity was registered using EEG. The video signal was either uncompressed at full length or changed from uncompressed to a lower quality level at a random time point. The distortions were introduced by a hybrid video codec. Subjects had to indicate whether they had perceived a quality change. In response to a quality change, a positive voltage change in EEG (the so-called P3 component) was observed at latency of about 400-600 ms for all subjects. The voltage change positively correlated with the magnitude of the video quality change, substantiating the P3 component as a graded neural index of the perception of video quality change within the presented paradigm. By applying machine learning techniques, we could classify on a single-trial basis whether a subject perceived a quality change. Interestingly, some video clips wherein changes were missed (i.e., not reported) by the subject were classified as quality changes, suggesting that the brain detected a change, although the subject did not press a button. In conclusion, abrupt changes of video quality give rise to specific components in the EEG that can be detected on a single-trial basis. Potentially, a neurotechnological approach to video assessment could lead to a more objective quantification of quality change detection, overcoming the limitations of subjective approaches (such as subjective bias and the requirement of an overt response). Furthermore, it allows for real-time applications wherein the brain response to a video clip is monitored while it is being viewed.     

Influence of oxygen on semi-transparent organic solar cells with gas permeable electrodes

Semi-transparent P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) solar cells with high conductivity PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) top electrodes are used to study degradation. Due to the gas permeability of this type of electrode, the simultaneous exposure to oxygen and light leads to a strong decrease of the short circuit current on the timescale of minutes. The losses are not due to a change in the conductivity of the PEDOT:PSS layer, and the short circuit current can be partially recovered by a heating step, indicating that the observed degradation is taking place in the photo-active layer of the cell.     

Electric Dipole Tracing in the Brain by Means of the Boundary Element Method and Its Accuracy

A method of localizing an electrical dipole in the brain from the scalp potential distribution has been developed with the aid of the boundary element method, in which a real geometry of the head is exactly taken into account and homogeneous electrical conductivity is assumed. Accuracy of the method was evaluated through animal experiments with a cat in which a current dipole was artificially generated in the brain. Deviation of the estimated dipole location from the true one was not random, but rather systematic (probably due to in-homogeneous conductivity distribution). It is numerically found that cavities in the skull disturb the inverse solution especially when the dipole is oriented toward the cavities. In vivo tests of the method were also done for primary somatosensory evoked potentials as a response to median nerve stimulation of a cat and myoclonic EEG. Although the homogeneous approximation was made, it does not change the significance of the results obtained by the present method.     

The effect of growth rate variation on the conductivity and morphology of polypyrrole thin films

This study reports the growth conditions and resulting surface and electrical properties of BF_-4 doped polypyrrole films for possible electronic device applications. Growth rate has been observed to significantly affect both the conductivity and morphology of the polypyrrole films grown by a constant current electrochemical polymerization technique. A peak in the conductivity occurs at a growth current density of 0.75 mA/cm² (anode voltage of 0.54 V) coincident with a break in the linearity of the current density-anode voltage plot suggesting a possible change in reaction kinetics. Surface morphology also showed a strong dependence on film thickness as well as growth rate. Electronic conductivities in excess of 200 (Ω-cm)¹ have been obtained but tend to deteriorate with time. Thin films are most affected showing a conductivity decrease of three orders of magnitude in two months, and results suggest changes are limited to the top 1–2 μm of the film.     

Adaptive estimation of latency changes in evoked potentials

Changes in latency of evoked potentials (EP) may indicate clinically and diagnostically important changes in the status of the nervous system. A low signal-to-noise ratio of the EP signal makes it difficult to estimate small, transient, time-varying changes in latency, or delays. Here, the authors present an adaptive algorithm that estimates small delay (latency change) values even when EP signal amplitudes are time-varying. When the delay is time invariant, the adaptive algorithm produces an unbiased estimate with delay estimation error less than half of the sampling interval. A lower estimation error variance is obtained when, in a pair of signals, the adaptive algorithm delays the signal with the higher SNR. The adaptive delay estimation algorithm was tested on intra-operative recordings of somatosensory EP, and analysis of those recordings reveals that the anesthetic etomidate produces a step change in the amplitude and latency of the EP signals     

带有肖特基漏极和场板结构的反向阻断垂直MOSFET研究

提出了一种采用肖特基漏极(SD)与场板相结合、实现硅基垂直MOSFET器件反向阻断应用的技术。基于该技术,采用二维仿真提出并研究了两种新型垂直MOSFET器件,即带有垂直场板(VFP)的SD-VFP-MOS器件和带有倾斜场板(SFP)的SD-SFP-MOS器件。相比采用肖特基漏极的MOSFET (SD-MOS)和采用超结和肖特基漏极的MOSFET(SD-SJ-MOS),所提出的SD-VFP-MOS,尤其是SD-SFP-MOS,反向击穿电压有显著提高,且几乎不影响导通特性。开展了器件的开态电流密度、关态电势分布、关态电流密度和电场分布分析,揭示了VFP和SFP提高器件反向阻断能力的内在机理。详细讨论了场板结构参数对器件反向击穿电压和场板效率的影响,研究结果对于SD-VFP-MOS和SD-SFP-MOS的设计具有重要意义。     

LED-Based Optical Device for Chronic In Vivo Cerebral Blood Volume Measurement

We demonstrate a reflectivity-based cerebral blood volume sensor comprised of surface-mount light-emitting diodes on a flexible substrate with integrated photodetectors in a form factor suitable for direct brain contact and chronic implantation. This reflectivity monitor is able to measure blood flow through the change of the surface reflectivity and, through this mechanism, detect the cerebral-blood-volume changes associated with epileptic seizures with a signal-to-noise (SNR) response of 42 dB. The device is tested in an in vivo model confirming its compatibility and sensitivity. The data taken demonstrate that placing the sensor into direct brain contact improves the SNR by more than four orders of magnitude over current noncontact technologies.