基于扩散张量成像利用一阶有限元方法计算脑白质各向异性电导率对脑电位分布的影响

目的 研究脑白质各向异性电导率对头皮电位分布的影响.方法 把由扩散张量成像得到的水分子扩散张量采用体积约束规则计算出白质组织的电导率张量,并根据成像的空间信息建立了包括头皮、颅骨、灰质、脑脊液及白质5种组织的真实头的有限元模型.基于此模型,推导了各向异性电导率脑电正问题的一阶有限元数值算法.最后,采用电流偶极子模型计算头皮电位分布.结果 脑白质各向异性电导率对头皮电位分布有一定的影响,径切比越大,影响越大;左右分布偶极子比上下分布的影响要大.结论 脑电研究中,白质电导率的各向异性为一不可忽略的因素.     

从头皮电位分布进行大脑皮层电位成像技术的仿真研究

用仿真的方法研究了根据头皮脑电分布重建大脑皮层电位分布的可行性。该方法采用三层媒质球作头的容积导体模型，利用最小范数的最小二乘法则估计头内一闭合的球型偶极层上偶极上的分布，然后根据估计结果重建大脑皮层电位分布。     

大鼠皮层脑电和海马电位的复杂度和功率谱分析

目的：分析大鼠皮层脑电和海马电位在觉醒和睡眠各期的复杂度和功率谱特性，以及两种电信号的复杂度值与功率谱分布之间的关系。方法：通过慢性埋植电极采集自由活动大鼠皮层脑电和海马电位，计算其算法复杂度Kc值，近似熵ApEn值，以及功率谱和功率谱重心频率，结果：海马电位在任何时期的复杂度值都显著高于皮层电位；皮层电位的复杂度值在非块动眼（NREM）睡眠期最小，海马电位的复杂度值在清醒期最大，两电位的总功率在NREM睡眠期最大，结论：两种复杂度值Kc和ApEN与脑电功率谱的频率分布具有密切关系，功率谱中具有明显的高峰时，信号复杂度降低，复杂度指标可以区分大鼠皮层和海马电位之间的差别，以及同一电位在不同行为状态下的较大差别。     

双层头模型以及从单次诱发响应确定脑电发生源的研究

从头皮记录到的脑电分布推算脑电活动源的脑电逆问题是脑电信号处理的主要方向之一。其中关键问题之一是关于头的容积导体模型的建立。本文为克服传统的均匀介质球、同心球及其有实际形状的均匀媒质模型的不足,利用CT 扫描图象建立了头的双层边界模型,并运用边界元方法进行正问题的求解。由于颅骨的电导率与脑组织导电率的比值很低,脑电正问题的解决存在很大的数值计算误差,本文提出了一种两步法有效地解决了这一问题。观测信号中的噪声是影响脑电求逆的一个主要因素。特别是对于脑电诱发响应,由于其信噪比很低,传统的方法是先利用相干平均或时频域的;滤波处理提取出诱发响应,然后再进行求逆研究。为了利用单次诱发信号直接进行求逆计算,本文提出了一种独特的处理方法,运用噪声相关抵消与奇异值分解技术,使得在信噪比较低的情况下仍能进行准确的定位(求逆)。仿真研究表明,利用这种方法,信噪比可以降到—10dB;同时,求逆的结果有助于诱发信号的单次提取。     

事件相关电位快速提取方法研究进展

空间飞行条件下,失重可能对航天员脑高级功能产生不可预期的影响,而事件相关电位(event-related potentials,ERPs)是探索这种影响的重要手段。ERPs总是淹没在自发脑电中,信噪比很低。少次甚至单次提取ERPs,可减少刺激次数,同时可观察每次实验ERPs之间的变异性,具有重要意义。本文主要从时域、变换域和空间域来述快速提取ERPs的方法,并对这3种典型方法进行仿真比较。     

成人烟雾病脑血流重建手术中不同亚型患者皮层脑电信号特征差异分析

目的寻找反映烟雾病脑血流重建手术前后血流变化情况的信号特征,分析其在出血亚型与缺血亚型重建手术中的分布差异性。方法回顾性分析70例成人烟雾病患者在脑血流重建手术过程中的皮层脑电信号,提取不同频率段内能表征脑电信号特点的多类特征来反映信号幅度、离散程度、复杂度等方面的特性,筛选出其中对于临床分析有价值的特征,统计比较出血型和缺血型烟雾病患者脑电信号特征之间的差异及其在手术不同阶段的变化情况。结果筛选出了能够反映手术前后血流变化情况的脑电特征,包括幅度均值、幅度标准差、最大最小距离、Petrosian分形维数、对数差分均方根和归一化线段长度,这些特征在出血型和缺血型烟雾病患者中的分布具有显著性差异(P<0.05)。结论在烟雾病脑血流重建手术中,皮层脑电信号的多种统计特征可以用来反映血流变化情况。对于出血亚型和缺血亚型,这些特征的分布存在显著性差异,因此在基于脑电的术中监测和预后判断等应用中需要有针对性地加以分析。     

介绍一种飞行中脑电检测系统

飞行中脑电检测系统是同经过改装的、安装有脑电电极的飞行头盔及置于救生装备内的记录系统两大部分组成。 头盔按被试者头型铸造,衬里为可拆装式,根据所要安装电极数,并按国际10/20制在头盔相对于头皮的部位打孔。电极为针状、针杆上覆有镀金电极头。其上覆有特制泡沫接头。脑电通道则根据国际10/20制     

一种基于二代小波变换与盲信号分离的脑电信号处理方法

目的研究对混杂有眼电和心电干扰脑电信号的处理方法。方法首先用二代小波硬/软阈值、折衷阈值、μ律阈值方法对脑电信号消噪,然后运用FastICA算法对消噪后仍含眼电和心电的脑电信号进行盲信号分离。结果二代小波μ律阈值方法对脑电信号有较好的消噪效果,FastICA算法能成功分离出脑电中眼电和心电的干扰。结论运用二代小波μ律阈值法对脑电消噪后再用FastICA算法对独立源产生的干扰进行分离是一种有效的预处理方法。     

磁敏感加权成像相位图对脑内顺磁性物质与逆磁性物质的鉴别诊断

目的 探讨脑内顺磁性和逆磁件物质在磁敏感加权成像(SWI)上的表现、成像原理以及SWI对于两者鉴别诊断的价值.方法 制作逆磁性和顺磁性物质模型并行SWI.回顾性分析28例脑内钙化灶和21例腩内出血灶的cT和MR影像资料,分析钙化和出血的SWI相位图表现,并与CT和常规MRI进行对比.结果 体外模型实验中在相位图上逆磁性物质在X-Y轴平面中间层面中心以高信号为主,最外层为低信号,Z轴前后两极层面呈高信号改变;顺磁件物质中间层面中心以低信号为主,最外层为高信号,Z轴前后两极层面呈低信号改变.4例脑内球形或类球形钙化灶(均为脉络丛钙化)和9例球形或类球形出血灶(2例外伤、1例高血压基底节血肿、4例转移瘤和2例海绵状血管瘤)的SWI相位图表现分别与逆磁性物质和顺磁性物质模型表现一致,4例基底节钙化灶为高低混杂信号,7例幕底节外不规则形钙化灶(均见于肿瘤内钙化)表现复杂,以高信号为主,其他基底节外球形或类球形钙化(14例松果体钙化、4例脉络丛钙化、2例脑囊虫钙化和1例肿瘤内钙化)均表现为明显高信号.9例腩内叶性血肿(2例高血压基底节血肿、4例不明原因血肿及3例海绵状血管瘤)SWI相位图表现为多环样高低混杂信号,3例脑内血肿表现复杂,以低信号为主.10例腩内钙化灶在T1 WI和T2 WI上表现无特异性.结论 SWI相位图有助于鉴别脑内逆磁性物质和顺磁性物质,且对脑内钙化灶的显示要优于普通T1 WI和T2WI.     

脑电比功谱及其计算方法

针对目前脑电功谱计算中因方法不同而造成的彼此间结果难以比较的状况，本文提出了脑电比功谱的概念和计算方法。脑电比功谱即脑电信号的功率在单位频率（每赫）上的分布。计算方法包括如下步骤：（１）脑电幅值的规范化处理；（２）用快速哈特莱或富氏变换求出与所选样本长度有关的各频率节点上的功率值；（３）通过积分处理求出单位频率上的功率；（４）进行对数变换将功率值以ｄＢ表示；（５）校正平均功率水平。对模拟白噪信号及两类典型脑电信号的比功谱计算结果表明，本文提出的方法切实可行，为不同条件下的脑电功谱的比较提供了一个可靠的手段。     

Manipulation of image intensity distribution at 7.0 T: passive RF shimming and focusing with dielectric materials

PURPOSE: To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach. MATERIALS AND METHODS: The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B(1) field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations. RESULTS: MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain. CONCLUSION: The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B(1) field inside the human head of a given RF coil can be adjusted to reduce the B(1) field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing).     

Observation of changes in neural activity due to the static magnetic field of an MRI scanner

PURPOSE: To clarify whether a strong static magnetic field affects brain activity such as arousal level. MATERIALS AND METHODS: We compared the electroencephalography (EEG) inside an MRI scanner in the presence/absence of the static magnetic field in two different arousal levels of task and rest conditions. Cardiac-related pulsations of head and blood flow induce an electric voltage at each EEG electrode in a static magnetic field. This induced voltage overlaps with the intrinsic EEG signal and becomes a large confounding factor. To extract the information of the intrinsic EEG from the contaminated EEG data measured in a static magnetic field, we developed a new analysis method. RESULTS: No significant difference was observed in the intrinsic EEG in the absence of a magnetic field, whereas in the presence of the static magnetic field, the theta frequency band of the intrinsic EEG increased, especially during the task condition, but other frequency bands did not change. CONCLUSION: Our results demonstrate that a static magnetic field affects brain activity.     

六针状竖直电极电流场的积分方程法计算机模拟

目的 计算医用多针状电极系的场势分布.方法 根据针状电极上的电势,建立由针状电极流入导电介质(生物组织)中电流强度为未知量的离散方程,计算针状电极系上线电流密度分布,由针状电极系上线电流密度计算导电介质中任意一点的场分布.结果 实现了利用积分方程法对6针状电极系电流场分布的数值计算,计算结果与实验测量值符合的非常好,验证了方法的可靠性.结论 该方法可用于计算多针状电极系的电流场分布,为在医学上的进一步应用提供了一种快速、有效、准确的计算方法.     

Subject‐specific models of susceptibility‐induced B0 field variations in breast MRI

Purpose:

To rapidly calculate and validate subject‐specific field maps based on the three‐dimensional shape of the bilateral breast volume.

Materials and Methods:

Ten healthy female volunteers were scanned at 3 Tesla using a multi‐echo sequence that provides water, fat, in‐phase, out‐of‐phase, and field map images. A shape‐specific binary mask was automatically generated to calculate a computed field map using a dipole field model. The measured and computed field maps were compared by visualizing the spatial distribution of the difference field map, the mean absolute error, and the 80% distribution widths of frequency histograms.

Results:

The 10 computed field maps had a mean absolute error of 38 Hz (0.29 ppm) compared with the measured field maps. The average 80% distribution widths for the histograms of all of the computed, measured, and difference field maps are 205 Hz, 233 Hz, and 120 Hz, respectively.

Conclusion:

The computed field maps had substantial overall agreement with the measured field maps, indicating that breast MRI field maps can be computed based on the air–tissue interfaces. These estimates may provide a predictive model for field variations and thus have the potential to improve applications in breast MRI. J. Magn. Reson. Imaging 2013;37:227–232. © 2012 Wiley Periodicals, Inc.     

Reduction of implant RF heating through modification of transmit coil electric field

In this work, we demonstrate the possibility to modify the electric‐field distribution of a radio frequency (RF) coil to generate electric field‐free zones in the body without significantly altering the transmit sensitivity. Because implant heating is directly related to the electric‐field distribution, implant‐friendly RF transmit coils can be obtained by this approach. We propose a linear birdcage transmit coil with a zero electric‐field plane as an example of such implant‐friendly coils. When the zero electric‐field plane coincides with the implant position, implant heating is reduced, as we demonstrated by the phantom experiments. By feeding RF pulses with identical phases and shapes but different amplitudes to the two orthogonal ports of the coil, the position of the zero electric‐field plane can also be adjusted. Although implant heating is reduced with this method, a linear birdcage coil results in a whole‐volume average specific absorption rate that is twice that of a quadrature birdcage coil. To solve this issue, we propose alternative methods to design implant‐friendly RF coils with optimized electromagnetic fields and reduced whole‐volume average specific absorption rate. With these methods, the transmit field was modified to reduce RF heating of implants and obtain uniform transmit sensitivity . Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

体外除颤时胸腔电场分布的有限元集成仿真研究

目的 探讨求解体外除颤时胸腔电场分布的集成仿真分析方法,为生物电场理论的临床应用研究提供参考.方法 分析人体胸腔的生物电容积导体属性,给出描述心律失常的体外电除颤时胸腔电场的数理方程;进而采用SCIRun有限元分析工具对胸腔电场分布进行集成仿真求解.结果 通过数值计算求解,获得了放电过程中胸腔各处的电势、电压梯度线、等势面,及心脏区域电压梯度值等多种电场分布数据.结论 应用有限元集成求解方法分析体外除颤时人体胸腔复杂电场分布是可行的,对除颤技术的临床研究和方法改进具有一定的指导意义.     

Induced magnetic field gradients and forces in the human head in MRI

PURPOSE: To map the induced magnetic field gradients and estimate the magnetic force in the human head during magnetic resonance imaging at 4 Tesla (T). MATERIALS AND METHODS: The magnetic field distribution in the human head was measured using two gradient-echo experiments with different echo times. The phase of the complex image ratio removed the wrapping artifact, characteristic of phase images, and was used to map the magnetic field distribution and calculate the accurate maps of the magnetic field gradients in the human head. RESULTS: The time-independent gradient fields induced by air/tissue interfaces in the head can be 50 times larger than those resulting from the magnetic field inhomogeneity of the MRI magnet. However, the associated magnetic force in the brain is by far smaller than the gravitational force. CONCLUSION: The induced gradient fields increase the magnetic force on tissues. However, even for tissue components with large magnetic susceptibility such as iron-containing proteins, this force is negligible compared with the gravitational force. Therefore, this study suggests that static and uniform magnetic fields do not have a significant risk for the tissues in the head.     

Quantitative conductivity and permittivity imaging of the human brain using electric properties tomography

The electric properties of human tissue can potentially be used as an additional diagnostic parameter, e.g., in tumor diagnosis. In the framework of radiofrequency safety, the electric conductivity of tissue is needed to correctly estimate the local specific absorption rate distribution during MR measurements. In this study, a recently developed approach, called electric properties tomography (EPT) is adapted for and applied to in vivo imaging. It derives the patient's electric conductivity and permittivity from the spatial sensitivity distributions of the applied radiofrequency coils. In contrast to other methods to measure the patient's electric properties, EPT does not apply externally mounted electrodes, currents, or radiofrequency probes, which enhances the practicability of the approach. This work shows that conductivity distributions can be reconstructed from phase images and permittivity distributions can be reconstructed from magnitude images of the radiofrequency transmit field. Corresponding numerical simulations using finite‐difference time‐domain methods support the feasibility of this phase‐based conductivity imaging and magnitude‐based permittivity imaging. Using this approximation, three‐dimensional in vivo conductivity and permittivity maps of the human brain are obtained in 5 and 13 min, respectively, which can be considered a step toward clinical feasibility for EPT. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Hypometabolic cortical lesions in tuberous sclerosis with epilepsy: demonstration by positron emission tomography

Four patients with a well-established diagnosis of tuberous sclerosis and grand mal type epileptic seizures as their principal clinical symptom were examined by conventional surface electroencephalography (EEG), X-ray computed tomography, and positron emission tomography (PET) using the [18F]-2-fluoro-2-deoxyglucose method. The interictal EEG showed various abnormalities of poor localizing value, but no focal epileptic discharges. X-ray computed tomography demonstrated subependymal calcifications in all cases, although cortical lesions were found only twice. However, in the PET images of each patient one or two localized cortical foci with a metabolic rate for glucose more than 40% lower than in the respective contralateral region were clearly delineated. It may be assumed that those hypometabolic areas represent the epileptogenic cortical tubers, which are characteristic of the disease but usually cannot be detected in vivo by other methods.