皮肤烧伤深度检测的单边核磁共振浅层成像磁体系统

目前临床上烧伤深度判断缺少合适的检测设备和方法,主要由烧伤科医生主观判断,准确率不高。该文设计了一种用于烧伤深度判断的单边核磁共振浅层成像磁体系统,对该磁体系统的单边磁体结构、平面梯度线圈、射频线圈以及射频屏蔽进行了优化设计。采用该磁体系统对橡皮模型和猪皮烧伤模型进行成像实验,获得了两种模型的二维成像。结果表明由该磁体系统获得的图像与橡皮实验模型的形状相似,且图像中不同烧伤深度部分和正常对照部分的信号强度呈鲜明对比,初步证明该文设计的单边磁共振浅层成像磁体系统可用于皮肤烧伤深度的检测。     

超导磁体失超检测电路的设计

超导磁体的失超检测与保护是超导电力技术实用化的一个重要课题。为了实现对超导储能混合磁体的失超检测与保护,本论文根据混合磁体的特点及结构,在现有有源功率检测的基础上,提出了一种针对超导混合磁体的电压校正电路,通过对混合超导磁体上产生的失超信号进行隔离、放大、滤波以及比较等,实现失超信号的检测。设计并完成了失超检测系统硬件电路的制作与调试。通过搭建高温超导线圈的实验装置,在高温超导储能磁体上进行失超检测的实验研究,得出了电压矫正前后的线圈电压波形。给线圈分别以不同速度充电,研究充电速度对阀值电压、失超的起始点及临界电流的影响。实验线圈电压波形表明,该失超检测系统能及时、有效地检测超导混合磁体失超的发生。     

谐振式无线电能传输系统中检测电路的设计

针对磁耦合谐振式无线电能传输(WPT)系统中初、次级线圈之间位置和距离的改变,或者线圈区域内存在金属异物等情况,分析了系统参数的变化会造成谐振频率的偏移、损耗功率变大、发射线圈温度上升等影响,设计了初级设备的交变信号、发射电路电压、电流及发射线圈的温度等检测电路,并通过发送一组脉冲信号对负载进行判断,混入不同的硬币作为金属异物测试.实验结果表明,微控制器单元(MCU)能实时检测交变信号、发射电流、电压及发射线圈温度的变化,优化和改善WPT的工作参数,使系统工作在最优状态.     

一种结合快速L_1范数最小化算法的柱面匀场线圈目标场设计方法

目标场方法由Turner于1986年提出,到目前为止该方法已成为核磁共振成像系统中匀场、梯度线圈设计的一种主流方法。各种改进的目标场方法纷纷涌现。为解决线圈设计中的有限尺寸问题,Forbes和Crozier提出了一种新方法,该方法预先通过三角函数来约束线圈面上的电流密度分布,为克服方程求解过程中的病态问题,对电流密度表达式中的待定系数采用最小均方差(least square,LS)和L2范数相结合的方法来进行估计,成功得到了有限长匀场线圈的设计结果。该文采用最小均方差和L1范数结合的方法来对面电流密度分解表达式中的未知系数进行估计,并将该方法应用于核磁共振成像(magnetic resonance imaging,MRI)系统的匀场线圈设计中。系统主磁体长度为3 m,室温孔直径为1.6 m,目标区域(region of interest,ROI)直径为0.4 m。算例采用Matlab仿真平台,结果表明该文提出的LS-L1目标场方法相比于传统的LS-L2目标场方法磁场偏差更小、效果更好。     

Halbach磁体中柱面匀场线圈的设计方法

Halbach磁体场强高、体积小,是永磁台式核磁共振波谱仪的理想磁源。为开发高分辨永磁台式核磁共振波谱仪,需要使Halbach磁体的磁场达到高度均匀。引入在超导磁体中被广泛使用的鞍型线圈,设计了一种由鞍型线圈组成、能够对Halbach磁体的横向磁场进行补偿的柱面匀场线圈。仿真和实验结果证明了设计方法的正确性。     

功率天平磁体系统设计综述

在新国际单位制下,功率天平是复现质量单位千克最重要的实验装置之一。功率天平实验装置的核心部件是提供测量磁场的磁体系统,它在线圈位置处产生的磁通密度沿线圈导线积分形成磁几何因子,构建出电磁功率与机械功率之间的等价关系,继而可确定被测质量的量值。因此,磁体系统产生磁场的性能,对千克量值复现的准确性具有重要影响。文章梳理了功率天平磁体系统对千克复现准确性的影响,盘点了多种功率天平磁体系统设计的特点,分析和评估了不同磁体设计方案的性能。比较发现,以BIPM型磁体系统为代表的基于磁轭的径向磁体系统,在产生磁场的效率、磁屏蔽性能和设计对称性等方面均具有优势,是目前性能最优的功率天平磁体系统之一。     

单边核磁共振仪中射频线圈的优化

射频线圈是核磁共振系统中实施射频激励和接收核磁共振信号的核心结构,其性能直接决定了核磁共振信号的质量。针对单边核磁共振系统信噪比差的缺点,提出以空间均匀性和信噪比为优化参数的平面矩形螺旋线圈的优化方法。优化方法分为两步:1以射频磁场的均匀分布范围为优化目标,通过改变平面螺旋线圈的结构参数,运用毕奥-沙伐定律对RF线圈磁场进行计算,确定与目标区域匹配的线圈的基本结构。2以线圈的信噪比为优化目标,使用有限元仿真软件计算在步骤1中已确定的各种线圈的交流电阻,进而得到信噪比,从中选出信噪比最高的线圈。结果表明,最优的平面矩形螺旋线圈结构参数为:布线面积为17.2mm×17.2mm,线宽和线间距均为0.5mm,总匝数为10匝,均分在2mm厚电路板的双面,每个面上5匝;该射频线圈不仅能保证与主磁场B0均匀区域相匹配的激励区域,而且从信噪比测量实验可知,该线圈在目标区域内具有最好的信噪比。     

磁性粒子成像线型零磁场设计及性能分析

磁性粒子成像是一种具有高成像分辨率的示踪剂成像技术。针对现阶段成像系统的开放式扫描难题,利用高灵敏度的线型零磁场,实现高分辨率的二维扫描成像方法。设计一种具有开放式线圈结构的线型零磁场,利用梯度静磁场构造线型零磁场以确定示踪剂的位置,在均匀交变磁场实现线型零磁场的平移扫描。对该线圈结构进行详细的设计分析,首先确定实现高分辨率的磁性粒子成像线圈系统的电流驱动方式;接着对开放式线型零磁场的扫描方式进行有限元仿真计算分析;最后根据其驱动方式与平移扫描区域范围的关系,进行粒子质量分数模型的成像实验。实验结果表明,开放式线圈结构所构成的线型零磁场在1.316T/m的梯度磁场中,可以实现在成像区域为17mm×17mm内的磁性纳米粒子示踪剂的高分辨率成像,其分辨率可达亚毫米级,理论证明了开放式线型零磁场扫描方式用于磁性粒子成像的可行性。     

非铁磁材料的电磁超声接收过程数值模拟及实验研究

针对非铁磁金属板的缺陷检测问题,进行了电磁超声无损检测技术的数值模拟及实验研究。电磁超声换能器工作机理复杂,已有仿真大多是针对发射过程,并且被测体不含缺陷。该文研究建立了超声发射、电磁超声换能器接收的有限元模型,对被测体有无缺陷时线圈接收到的电压信号进行仿真分析。根据发射频率和超声导波的频散特性,设计并制作了回折形线圈,然后研究了线圈导体尺寸及线圈提离对电磁超声换能器性能的影响,并对有无缺陷的铝板进行了实验研究。结果表明:仿真中线圈接收到的电压信号能够反映缺陷的位置,实验结果与仿真结果相吻合。该工作对电磁超声被测试样有无缺陷时接收信号的分析和线圈设计有参考意义。     

一种永磁磁体外部特殊磁场区域的构建问题

单边开放式永磁型磁共振磁体结构的研究是一个全新的电工课题.本文基于鞍点理论,探索永磁磁体外部一侧构建均匀磁场区域的可行性问题,并根据可行性要求,通过对单边电流线圈结构的分析,给出一种单边永磁磁体结构方案,并应用一种数值法与半解析法相结合的混合算法分析这种特殊磁体结构的磁场.数值仿真结果表明,本文提出的永磁磁体结构能够较好地在磁体外部构建符合成像要求的薄片形磁场区域.鞍点理论在单边开放式MRI磁体结构设计中的成功应用,为单边永磁MRI磁体结构的分析提供了可靠的理论依据,而磁体外部特殊磁场区域的成功构建,为介入式磁共振系统的研究准备了条件.     

Imaging gray brain matter with a double-inversion pulse sequence to suppress CSF and white matter signals

The design of a double-inversion recovery (DIR) sequence, to selectively image gray brain matter, is described. A suitable choice of inversion times allows the cerebrospinal fluid (CSF) and white matter to be suppressed, to image the cortex alone. Consistently good results were achieved in a group of normal volunteers using the same inversion times throughout. The DIR sequence was found give clear delineation of the complex folds of the cerebral cortex.     

Effects of riluzole on the evolution of focal cerebral ischemia: a magnetic resonance imaging study

The aim of this study was to investigate the effects of riluzole on the lesion induced by a permanent middle cerebral artery occlusion (MCAO) in rats. Riluzole at 4 or 8 mg/kg i.v. significantly reduced the cortical ischemic brain damage. With the most effective dose of 8 mg/kg, the time evolution of the lesion was assessed by T₂-weighted magnetic resonance imaging (MRI) repeated on the same animals after MCAO. MRI obtained at 24, 48, and 72 hours after MCAO showed a progressive increase of the ischemic lesion, except in the cortex of the riluzole-treated rats (8 mg/kg i.v.). Furthermore, there was no difference between lesion volumes as measured by MRI or by histology. This study indicates that MRI may be a valuable method to quantifyin vivo the neuroprotective profile of a drug.     

Functional brain imaging using EPI at 3 T

Functional magnetic resonance imaging (fMRI) is becoming an important tool in the mapping of brain activation. However there are two main concerns that need to be answered before functional imaging can be considered truly useful as a neurophysiological tool. The first is that the detected activation may be derived from large veins and, thus, be spatially separate from the underlying brain activity. The second is the incomplete understanding of the brain transfer function and its relation to brain activity, blood flow, and metabolism. This work contains initial results that will help address these points. Models of the brain vasculature predict that signal changes on SE (spin-echo) images are expected to be much smaller in magnitude but very accurate in localizing true areas of activation than on GE (gradient-echo) images which are susceptable to large veins. By comparing activation from SE and GE EPI at 3 T, we have shown that the regions of activation are spatially very similar, suggesting that GE activation is closely linked to the underlying brain activity. We have identified an experimental impulse response of the brain following 8-s visual stimulation. This impulse response can be used to successfully predict the frequency response obtained experimentally and its shape suggests a resonance phenomenon. This suggests the brain transfer function can be modeled from linear response theory corresponding to the inherent feedback control mechanisms of the brain homeostasis. Continuation of this early work will help to identify the links between fMRI signal change and underlying brain physiology.     

MR T2-weighted image of subacute cerebral infarct can be isointense to the surrounding brain: MR fogging in cerebral infarct

Cerebral infarcts initially showing as markedly hyperintense on magnetic resonance (MR)T ₂-weighted images decreased in intensity and became nearly isointense to normal brain tissue in subsequent MR studies. This MR fogging was observed in 7 (23%) out of 31 cases of cortical infarct and 4 (20%) out of 20 cases of perforator infarct in the second to sixth weeks of the disease. In all fogging cases, significant contrast enhancement (CE) was seen in the fogging area after intravenous administration of MR contrast agent. The CE study is recommended in MR of cerebral infarct during the subacute and early chronic stage.     

基于互信息算法的选择性神经网络集成在大脑胶质瘤诊断中的应用

本文利用基于互信息算法的选择性神经网络集成技术从大脑胶质瘤病例中获取在大脑胶质瘤良恶性程度的诊断知识,并与利用基于遗传算法的选择性神经网络集成算法,支持向量机,单个神经网络和提取规则技术的结果作了比较。结果显示,利用基于互信息算法的选择性神经网络集成技术得到的结果是优于其他结果的。     

基于视觉通路的脑电信号频谱能量分布研究

目前关于光诱发脑电信号的频谱研究主要采用实验测试方法,因缺乏理论模型而导致结论多样化。针对这一不足,结合计算模型和电生理相关知识,建立视觉通路模型,仿真不同频率光刺激下视觉通路皮层锥体细胞产生的电信号,并分析对应的频谱能量分布,最后采集相同实验环境下、不同受试者的脑电信号,与仿真结果进行比较,以此验证模型的有效性。研究结果表明：低频光刺激信号（2、5、10 Hz ）更易诱发脑电信号节律同步化的响应,频谱能量主要集中在基波和低次谐波处,而由高频光刺激（30 Hz）引起响应的频谱能量主要集中在低频。     

Transient global brain ischemia in the rat: spatial distribution, extension, and evolution of lesions evaluated by magnetic resonance imaging

A newly-developed model of transient global ischemia in the rat was evaluated by magnetic resonance imaging (MRI) in terms of localization of brain lesions, their extent and severity, and temporal evolution. Such a model, consisting of bilateral occlusion of common carotid arteries for 10 minutes and mild hypoxia (15% O₂) for 20 minutes induces delayed neuronal degeneration, necrosis, and gliosis (detected histologically and immunohistochemically). Ischemia was assessed by full suppression of spontaneous electroencephalographic activity. A “hybrid” T₂-/diffusion-weighted MR sequence enhancing more effectively the contrast between injured and intact tissues as compared to T₂-weighted MRI was used at 24, 48, 72, and 96 hours and at 7 days postischemia. Twenty hypoxic-ischemic rats showed a considerable variability in brain damage. In 8, there were no MRI-detectable lesions at any interval. In the other 12 rats, the severity and extension of neuronal damage varied markedly, but the lesions were always localized (monolaterally in 8 and bilaterally in 4 rats) in the occipital, temporal, or parietal cerebral cortex. Mainly, they were of intermediate severity or were severe (as assessed by MRI hyperintensity) and were accompanied by usually less severe lesions in the thalamus and/or caudate putamen. The hippocampus was affected moderately or severely in 4 of 12 rats. In most cases, there was at 48 hours a considerable growth in severity and/or extension of lesions, which usually remained stable at later intervals. In conclusion, MRI allowed us to follow brain lesions during the first week in this relatively simple and noninvasive model of transient global ischemia.     

Fast 1 H spectroscopic imaging using steady state free precession and spectral–spatial RF pulses

Recently, new methods for fast ¹H spectroscopic imaging based on the condition of steady state free precession (SSFP) were introduced to achieve a high signal-to-noise ratio at short minimum measurement times. In this work, a major improvement is presented to overcome a crucial drawback in some of the former sequences: the lack of spatial selectivity. Good spectral selectivity at very high sampling efficiency can be achieved by using spectral–spatial RF pulses, and combined with localised shimming. Results are shown from both phantom experiments and in vivo studies on the rat brain acquired at 4.7 T.Parts of this work were presented at ESMRMB 2004.     

集成学习在脑机接口分类算法中的研究

提出了一种基于独立分量分析的支持向量机集成学习算法,用于脑机接口中P300字符识别.首先由P300信号分解出独立分量,基于Bagging算法送入支持向量机基分类器进行集成学习,通过平均的方法获得对应类别概率进行分类决策.数据来源于P300字符拼写实验,不同导联和不同序列的分类结果表明,该分类算法学习效率和分类精度高,全...     

An Inductive Link with Integrated Receiving Coil—Coupling Coefficient and Link Efficiency

Inductive powering can free microelectromechanical systems (MEMS) devices from tethering to a power supply, thus expanding their scope of applications. We investigate inductive powering for MEMS using a microfabricated coil as a receiver operating at and below 1 MHz. The microcoil is designed to enclose the MEMS functions to achieve maximum coupling, and is built into a silicon substrate by our inlaid electroplating process, so that the microcoil has a small resistance despite its long trace. Non-negligible parasitic effects from the silicon substrate affect the characterization of an inductive link and its optimization. Taking into account the parasitic effects of Si microcoils, the coupling coefficient of the link is determined by a coil-model scheme that is different from conventional strategies. Furthermore, an equivalent circuit is developed for our link and used to analyze link operation over the frequency range of 4 kHz to 4 MHz. With measured link parameters, link efficiency is calculated with our equivalent circuit, and the results agree better with experiments than do conventional models. The equivalent circuit also indicates that microcoils with high quality factor at low frequencies, such as our inlaid electroplated coils, improve link performance, while parasitic capacitance has little effect.