局部匀场在磁共振全身弥散成像中的应用

目的探讨局部匀场在磁共振全身弥散成像中的应用,提高全身弥散成像的质量。材料与方法对40例行全身弥散扫描成像的患者,在容易出现截断伪影的颈部和胸部交界处分别添加局部匀场和未添加局部匀场,扫描完成后经3D最大信号投影(maximum intensity projection,MIP)法重建,观察是否有截断伪影的出现。结果添加局部匀场后图像的截断伪影明显减少,添加匀场和未添加匀场图像质量之间的差异具有统计学意义(P0.01)。结论局部匀场在提高磁共振全身弥散成像质量,防止截断伪影的出现上具有重要的意义。     

口腔金属材料与影像学伪影的检测与比较

背景：磁共振成像是常用的临床影像检测方法,口腔金属材料在磁共振成像检查时会出现图像伪影,从而影响图像的质量以及诊断的正确性。目的：分析不同口腔金属材料在磁共振成像中产生的伪影强度以及影响伪影的因素。方法：分析9篇近10年关于口腔金属材料对磁共振成像影响的研究文献,从不同角度分析口腔常削金属材料钴铬合金、镍铬合金、钛合金、金铂合金、银铂合金以及纯钛等在磁共振影像中产生的伪影的大小,比较其影响伪影强度的因素。结果与结论：钴铬合金在磁共振图像中产生的伪影最人,并儿随着磁场强度的增强伪影增大,其中硬质钻铬合会产生的伪影强度大于软质钴铬合金产生的伪影强度,其次足镍铬合金产生的伪影,而金、银合金以及纯钛产生的伪影最小,并且磁场强度增强等外加干扰因素对伪影大小无明届影响。     

软、硬质钴铬合金及金合金对磁共振成像伪影的影响

背景:磁共振成像具有高对比度、无骨伪影、任意方位断层等优点,被广泛应用于头颈外科、神经外科和口腔颌面外科疾病的诊断.但某些金属材料在磁共振成像中形成的伪影严重影响了图像质量,这些材料的使用限制了磁共振成像在头颈部的应用和准确诊断.目的:观察硬质钴铬铸造合金、软质钴铬铸造合金、金合金3种金属材料在5种不同头颈部扫描序列中的伪影.方法:采用5种头颈常规及快速成像序列,分别对放置于圆柱型水模中心的3种金属试样进行轴位扫描,测量伪影,并进行统计分析.结果与结论:3种金属在磁共振成像相同成像序列中,金合金产生的伪影最小,硬质钴铬形成的伪影最大,各组间比较差异有显著性意义(P均<0.05);相同金属材料,自旋回波序列产生伪影最小,梯度回波序列产生伪影较大,平面回波序列产生伪影最大并使图像变形.结果提示,与非贵金属口腔修复材料相比,以贵金属作为口腔修复材料,在头颈部磁共振成像过程中产生的伪影最小,因此通过合理选择口腔金属修复材料及成像技术,可得到最佳磁共振成像.     

脑功能磁共振图像中Nyquist伪影的处理方法研究

Nyquist伪影是功能磁共振成像过程中产生的一种特有的伪影,使图像信噪比下降,并对图像的可视化及功能定位产生不利影响.尽可能地消除Nyquist伪影,无论对于改善图像的视觉效果,还是对后续的功能定位分析和可视化都具有重要意义.目前对Nyquist伪影还没有有效的处理方法.本文根据功能磁共振图像本身的信息,首次提出一种基于模糊聚类技术的Nyquist伪影的处理方法,能比较有效地消除Nyquist伪影.     

磁共振Propeller技术消除患者因运动及磁敏感效应造成伪影的作用

背景:运动伪影及磁敏感伪影严重影响MR成像质量,Propeller技术采用填充模式,以辐射状的叶片用螺旋的方式采集数据,在很大程度上解决了运动伪影及磁敏感伪影的问题.目的:探讨Propeller技术在头部伪影消除的应用价值.设计、时间及地点:对比观察实验,于2006-0712007-04在解放军成都军区昆明总医院完成.参试者:21例磁共振头颅日常检查中出现躁动或不能自控患者,5例固定义齿致磁敏感伪影明显者.方法:对21例磁共振头颅日常检查躁动或不能自控患者,常规T2FRFSE序列运动伪影明显,用Propeller技术进行T2WI扫描并进行对比;5例常规扩散加权图像序列磁敏感伪影明显者进行Propeller扩散加权图像扫描并进行对比.主要观察指标:Propeller技术应用后运动伪影与磁敏感伪影的改变情况.结果:21例应用Propeller技术进行T2WI扫描明显减少或消除了运动伪影,5例Propeller扩敞加权图像明显消除磁敏感效应引起的伪影.均生成较满意图像.结论:应用Propeller T2WI及扩散加权图像成像技术可以明显减少或消除患者因运动及磁敏感效应造成的伪影,显著提高图像质量,获得具有临床诊断价值的理想图像.     

磁共振血管MIP图像伪影形成机制、表现及解决办法探讨

目的:了解最大密度投影重建(MIP)图像伪影的形成机制,熟悉磁共振血管成像中MIP伪影的各种表现,探讨其解决办法。材料与方法:回顾性分析2003年12月~2004年6月磁共振血管检查患者共90例,其中头部3D TOF-MRA检查47例,颈部2D TOF-MRA检查20例,均同时进行了颈部3D CE-MRA检查;腹部3D CE-MRA检查25例。对各部位出现的MIP伪影进行分类、分析,探讨对不同表现形式的MIP伪影的解决办法。结果:①头部TOF-MRA的整体MIP图像上细小血管分支未显示共40处(后交通动脉32处,前交通动脉8处),通过对原始图像的观察或局部薄层重建使上述两处血管均得到显示。②20例颈部TOF-MRA检查中夸大血管狭窄程度共14处,均出现在血管狭窄程度在50%以上的血管,占病变总数的93.3%(14/15),CE-MRA对TOF-MRA夸大狭窄均做出了准确的矫正。颈部20例TOF-MRA中,颈外动脉锯齿状伪影36处,出现的几率为90%(36/40)。CE-MRA对此种伪影均做出了矫正。④MIP图像上在特定投影方向上血管分支不显示,通过改变投影方向或去掉高信号的背景组织此种伪影均得到解决。结论:磁共振血管检查MIP图像上出现伪影是常见现象,表现具有多样性;MIP图像伪影可造成错误的诊断。MIP图像的伪影可以通过不同的后处理方法或改变成像方法解决。在临床实际工作中不可单凭MIP图像做出最终诊断,需密切结合原始图像和不同处理方法的血管重建图像。     

多层螺旋CT评价脊柱金属置入物产生金属伪影及其影响因素

随着脊柱外科基础和临床研究的不断进展,椎弓根钉棒系统已经成为最常用的脊柱外科置入物.多层螺旋CT和磁共振成像可以产生切面图像.磁共振成像能够评估骨髓和软组织,对没有金属置入物的患者,术后磁共振成像检查仍是首选的成像模式.但对手术后带有金属置入物的患者,由于存在金属伪影,其成像质量大大降低.因此,多层螺旋CT就成为对置入材料、皮质骨及其解剖关系的重要评估手段.文章着重讨论在金属置入物存在的条件下,多层螺旋CT检查时金属伪影的产生机制、影响因素及伪影的消减技术问题.     

磁共振扫描序列与运动伪影的关系

磁共振成像检查时间较长,而患者保持身体基本不动的时间往往较短暂,因而图像上经常有运动伪影.通过适当安排扫描程序,可以减少运动伪影. 1 资料与方法 1.1 一般资料 收集我科2009年1月-2009年12月间的302例患者的膝关节MRI资料.患者年龄12～95岁,平均52岁.男 147 例,女 155 例.     

时空编码单扫描磁共振成像的基本原理及重建方法

单扫描磁共振成像技术凭借其良好的时间分辨率,近年来在扩散成像、实时动态三维成像、功能成像等生物医学领域得到广泛应用。在众多单扫描磁共振成像方法中,回波平面成像(EPI)是最常用的一种方法,但它存在一些局限性,特别是在高场条件下,其对磁场不均匀及化学位移效应十分敏感。基于时空编码的单扫描磁共振成像新方法,其对不均匀磁场及化学位移伪影有很好的鲁棒性,并延续了EPI良好的时间分辨率。作者主要阐述时空编码单扫描成像方法的基本原理、特性,时空编码图像的超分辨重建方法及其在生物医学领域的应用。     

一种消除MRI拉链伪影的脉冲序列

背景:磁共振成像会产生各种类型的伪影,经常出现的有运动伪影、卷褶伪影、金属伪影、部分容积效应伪影、化学位移伪影、相位伪影和拉链伪影等,但由自旋回波序列成像过程中形成拉链伪影的控制和校正方法还未见报道.目的:通过分析自旋回波序列成像过程中拉链伪影的产生机制,探索出一种新的消除拉链伪影的脉冲序列.方法:编写标准SE序列得到标准图,编写伪影序列得到直线干扰图像和拉链伪影图,编写改进序列得到改进后的除拉链伪影图.结果与结论:5°一瞬间梯度-120°回波序列可得到直线干扰图像,90°一瞬间梯度-120°回波序列可得到拉链伪影图,而改进后序列90°-120°一瞬间梯度回波可彻底消除拉链伪影.结果提示,改进后的脉冲序列显示的图形效果彻底消除了拉链伪影.     

Common artifacts encountered in thoracic magnetic resonance imaging: recognition, derivation, and solutions.

The article discusses the many types of thoracic magnetic resonance imaging artifacts, their derivations, and their current solutions. Magnetic resonance imaging artifacts of the thorax can be divided into two major categories. First, there are machine-related artifacts related to the machine's hardware: the intrinsic magnetic field, chemical shift, magnetic susceptibility, radio frequency leaks, metal artifacts, B1-homogeneity, gradient coils, truncation, aliasing, zipper artifacts, artifacts related to the surface coil profiles, pulse profiles, and crosstalk. Second, there are artifacts related to motion: voluntary patient motion, involuntary motion, and physiologic motions. Each one of these artifacts, and their solutions, will be discussed.     

磁敏感度相关脑区功能性MR成像参数的选择对EPI图像质量的影响

梯度平面回波成像（EPI）,不但可以实现对全脑图像的迅速采集,而且对血氧水平依赖（BOLD）相关的磁化率差异（即BOLD对比）具有高度的敏感性,因而被广泛应用于功能性磁共振成像研究。但是,T2加权EPI成像在与颅骨及含气腔隙邻近的特定脑区容易受到磁敏感度伪影的影响而造成信号的缺失。既要消除磁敏感度伪影的影响,又要保持对BOLD对比较高的敏感性,对此体素、回波时间（TE）以及层面选择方向的优化筛选在实际工作中至关重要。     

磁敏感性伪影的产生及抑制技术探讨

目的：正确认识MRI中的磁敏感性伪影，寻求减小或清除MSA的有效措施。材料与方法：回顾性分析我院86例有MSA且行各种解决措施的病人。结果：MSA可分为金属附属物伪影(40例)和自身磁敏感性差异伪影(46例)两类。可去掉的金属附属物伪影可消除，不能去掉的金属附属物(以检查安全可行为前提)和自身磁敏感差异伪影可尽量减少。结论：MRI中MSA可在一定程度上减少或消除。     

Noise analysis and MR pulse sequence optimization in MREIT using an injected current nonlinear encoding (ICNE) method

Magnetic resonance current density imaging (MRCDI) and magnetic resonance electrical impedance tomography (MREIT) visualize an internal distribution of current density and/or conductivity by injecting current into an electrically conductive object such as the human body using an MRI scanner. MREIT measures the induced magnetic flux density which appears in the phase part of the acquired MR image data. Recently, the injected current nonlinear encoding (ICNE) method in MREIT extended the duration of the current injection until the end of a reading gradient to maximize the signal intensity of the magnetic flux density. In this paper, we investigate the signal-to-noise ratio (SNR) of the magnetic flux density measured by the ICNE method in the presence of a zero-mean Gaussian random noise in measured k-space MR data. Based on the analysis of the noise standard deviation s(B(z)) of the magnetic flux density, we determine an optimal combination between the current injection pulse width T(c) and data acquisition time T(s) which minimize the noise level of the measured magnetic flux density for a given echo time T(E). On one hand, theoretically, the proposed ICNE MR pulse sequence using the optimal data acquisition time T(s)* reduces the noise level of the measured magnetic flux density by about 42.3% compared with the optimal data acquisition time of the conventional MREIT pulse sequence. On the other hand, practically, the prolonged T(s)* may result in undesirable artifacts including blurring, chemical shift and phase error along the phase encoding direction. We observe that the noise level is a function of the data acquisition time T(s) and the rate of change in the noise level is slow near T(s)=T(s)*. Numerical phantom experiments show that a compromised T(s) between the ordinary data acquisition time and the optimal T(s)* reduces a relatively large amount of undesirable artifacts and almost maintains the optimized noise level of the measured magnetic flux density.     

Image artifacts in fast magnetic resonance imaging.

Image artifacts are unwanted, spurious signal intensities that interfere with clinical diagnosis. This article gives an overview of image artifacts in magnetic resonance imaging. We discuss the causes of these artifacts, provide clinical examples, and offer solutions to avoid them.     

Image reconstruction of compressed sensing MRI using graph-based redundant wavelet transform

Compressed sensing magnetic resonance imaging has shown great capacity for accelerating magnetic resonance imaging if an image can be sparsely represented. How the image is sparsified seriously affects its reconstruction quality. In the present study, a graph-based redundant wavelet transform is introduced to sparsely represent magnetic resonance images in iterative image reconstructions. With this transform, image patches is viewed as vertices and their differences as edges, and the shortest path on the graph minimizes the total difference of all image patches. Using the l₁ norm regularized formulation of the problem solved by an alternating-direction minimization with continuation algorithm, the experimental results demonstrate that the proposed method outperforms several state-of-the-art reconstruction methods in removing artifacts and achieves fewer reconstruction errors on the tested datasets.     

Advances in musculoskeletal magnetic resonance imaging

The technology of musculoskeletal magnetic resonance imaging is advancing at a dramatic rate. Magnetic resonance imaging is now done at medium and higher field strengths with more specialized surface coils and with more variable pulse sequences and postprocessing techniques than ever before. These numerable technical advances are advantageous because they lead to an increased signal-to-noise ratio and increased variety of soft tissue contrast options. However, at the same time, they potentially produce more imaging artifacts when compared with past techniques. Substantial technical advances have considerable clinical challenges in musculoskeletal radiology such as postoperative patient imaging, cartilage mapping, and molecular imaging. In this review, we consider technical advances in hardware and software of musculoskeletal magnetic resonance imaging along with their clinical applications.     

MRI运动伪影校正方法与实现

MRI过程中,患者的轻微运动会造成重建图像中含有运动伪影,降低成像质量。作者对基于最小熵约束的MRI运动伪影校正方法进行改进,自动缩小每次迭代k空间线的数目,同时改变原来单一步长的校正模式,在迭代过程中自动减小步长,节约了校正时间,改善了校正效果。基于Matlab图形用户界面,实现了MRI运动伪影校正算法,对仿真数据和实验数据的处理进行了显示,为进一步开发新的伪影校正方法提供了参考。