磁共振成像中抑制伪影技术的研究进展

磁共振成像可对人体各部位多角度、多平面成像,具有高组织分辨力、空间分辨力、无硬性伪迹及无放射损伤等优点,在医学上得到了广泛的应用。然而,磁共振成像中所产生的伪影会严重影响MRI的质量及对病灶的精确定位。我们主要介绍了近年来抑制自旋回波平面成像技术中所产生的伪影及自主性运动伪影的研究进展。大量研究表明,怎样快速有效地抑制各种原因产生的伪影,仍然是研究人员所面临的一个非常棘手的问题。因此,还需不断寻求新的方法及思路来解决这一难题。     

抑制MR成像中刚体运动伪影的新方法

目的从成像空间出发,提出了一种新的纠正MRI刚性平移运动伪影的方法.方法利用梯度读出方向和相位编码方向的运动特性不同,分别采用不同的方法来消除刚性运动伪影.首先读出方向的运动,通过追踪频谱边缘非零区域和零区域的偏差进行估计,然后在频谱反方向移动相同的量来消除;利用改进的Snake算法,即梯度向量场方法提取目标区域的边界,然后利用相位迭代恢复算法消除残留的读出方向亚像素级伪影和相位编码方向的伪影.结果按国际通用方法生成SL头颅模板,通过对模板的仿真试验,证明修正后图像的信噪比大大提高,验证了方法的有效性和可靠性.结论本文提出的方法能够有效消除MR图像平移运动伪影,与传统的相位迭代恢复算法相比,对于较大运动伪影修正效果更好.     

基于感兴趣区提取的MRI运动伪影校正

为了抑制MRI成像过程中平移运动所造成的伪影,我们对平移运动伪影的后处理校正算法进行了研究,在采用蛇形算法和人工提取相结合提取兴趣区的基础上,利用基于频率域约束的相位恢复算法进行K空间相位校正。仿真和成像结果表明:该方法能准确地提取感兴趣区,有效地消除了MR图像中相位编码对应方向的平移运动伪影。     

MRI移动伪影的一种综合处理方法

磁共振成像过程中病人身体的移动会使图像有伪影,所以必须对图像进行纠正.采用改进的去伪算法,提出一个完全自动的去伪影方法.由于去伪之后的图像质量变差,本文提出一个基于小波和模糊集理论的增强算法,对去伪后图像进行处理.实验证明,所得到的图像基本上保留了图像的细节,图像的质量有明显地提高.     

MR图像Ghost伪影的校正

原始K空间奇偶回波单独重建图像数据时，根据二维多项式拟合参考扫描各点相位漂移估计值对图像进行相位校证，可减轻伪影的影响。但在噪声较严重的情况下，校正后的图像仍含有较严重的残余伪影。按相位编码方向对图像数据采用基于最小二乘法多项式拟合的方法可减轻噪声对图像的干扰，再利用二维抑制Ghost伪影算法能更有效地消除EPI成像过程中由于涡流引起的伪影。该算法的缺点是会引起图像信息强度的变化及造成图像高频信息的衰减。但由于MRI图像主要集中为低频信息，且Ghost伪影信息强度不超过图像信息最大强度20％时，该方法对图像信息的损失不会影响对病灶的识别。     

磁共振成像中伪影的研究

磁共振成像中所产生的伪影会严重影响图像的质量及对病灶的精确识别。对磁共振成像中各种伪影形成的原因及抑制方法进行阐述。大量研究表明,怎样快速有效地抑制各种原因产生的伪影,仍然是研究人员所面临的一个非常棘手的问题。因此,还需不断寻求新的方法及思路来解决这一难题,为医学观察和临床诊断提供可靠的依据。     

基于编码脉冲技术的医学超声组织谐波抑制

目的:为有效地抑制造影谐波成像中组织谐波的影响,探求一种基于编码脉冲技术的抑制方法。方法:提出一种基于编码脉冲技术的组织谐波抑制方法。该方法利用组织谐波的非线性满足一个多项式模型,而造影谐波中的微泡振动存在时间滞后,不满足此模型的特性,采取发射因子和接收因子对多项式模型贡献为零的性质,抑制了组织谐波的影响。结果:造影谐波成像中组织谐波得到了抑制。结论:通过理论分析和仿真实验,证实该方法可行。     

基于图像域的Ghost伪影自动相位校正

回波平面成像过程中会产生Ghost伪影.抑制Ghost伪影的常用方法是利用参考扫描对实际扫描图像进行校正.本研究提出了一种无须参考扫描且基于图像域的自动相位校正方法.这种方法是对含伪影的图像作二维傅里叶变换,利用变换后的奇偶行数据分别重建图像以求取成像过程中奇偶回波之间的相位偏移.利用线性拟合或Marquardt-Levenber非线性拟合得到的无卷褶相位值对伪影图像进行校正,能有效抑制Ghost伪影.     

运动伪影对MR图像质量的影响

提高MR图像质量是医学影像工作者普遍关心的问题，但MR图像质量是一个受各种因素相互制约和关联的问题，即既要考虑提高图像信噪比和分辨率，又不能过分延长成像时间，所以是一个比较复杂的问题。而各种自主和不自主的运动引起的图像伪影又是影响图像质量的重要因素，故本文将着重讨论如何有效地控制运动伪影，以确保图像的质量。l运动伪影产生的原因运动伪影主要指身体活动，如人体的移动，组织器官的生理运动。由于心脏、呼吸及器官的运动等引起的伪影成为降低图像质量最常见的原因。MR时间较长，胸部及上腹部图像常形成很多运动伪影，…     

MRI脂肪抑制技术对膝关节隐匿性骨折的诊断价值

目的：探讨MRI脂肪抑制技术对膝关节隐匿性骨折的诊断价值。方法：回顾性分析118例高度怀疑膝关节隐匿性骨折患者的相关资料,患者均接受CT、MRI脂肪抑制序列检查,其后采用手术或者关节镜证实患者骨折病情,分析CT、MRI对膝关节隐匿性骨折的诊断准确性。结果：MRI对韧带损伤、半月板损伤以及关节腔积液等膝关节隐匿性骨折损伤类型检出率分别为25.42%、34.75%、27.14%,均显著高于CT的6.78%、15.25%、12.71%(P<0.05);MRI对骨折塌陷及水平错位检出率分别为61.02%、58.47%,均高于CT的40.68%、38.14%,其中MRI对1、2级骨折塌陷及水平错位检出率均高于CT,差异有统计学意义（P<0.05）;选择手术或者关节镜检查结果作为诊断膝关节隐匿性骨折的金标准,MRI诊断膝关节隐匿性骨折的灵敏度与准确度分别为95.60%、92.37%,均显著高于CT的78.02%、81.36%(P<0.05)。结论：相对于CT检查,MRI诊断膝关节隐匿性骨折的灵敏度与准确度较高,其在患者微小骨折塌陷及水平错位方面的检查优势明显。     

Integrated physiological flow simulator and pulse sequence monitoring system for MRI

A compact flow simulator for generating accurate pulsatile flow in a clinical magnetic resonance imaging (MRI) environment has been developed and integrated with a data acquisition (DAQ) system for recording scanner system activity and physiological waveforms. The flow simulator is relatively inexpensive, easy to construct and is controlled from a standard PC. The flow simulator is robust to repeated disassembly and reassembly (normalised cross-correlation 0.97) and generates accurate pulsatile flow (normalised cross-correlation 0.94). The DAQ system was used to monitor a standard MRI pulse sequence used in MR angiography and latent delays in the scanner gating subsystem.     

脉搏波在运动干预中参数指示作用的Meta分析

目的探究评价脉搏波在运动干预评价体系中作为参数的指示效用,为完善运动干预评价体系提供指导。方法通过网上检索中国知网(CNKI)、Web of Science、Latrobe University Library、PubMed等数据库,搜集运动干预中存在以脉搏波为参数进行指示的随机对照试验。由5名研究者依据Corhrane评价标准对纳入的文献进行严格质量评价和资料提取,对符合要求的文献采用RevMan 5.3软件进行Meta分析。具体分析指标包括 I^2、P 、CI、MD等。结果共纳入16篇文献,涉及312例对象。Meta分析的结果显示,运动干预对PWV[MD=0.72,95%CI(0.46,0.97), P <0.0001]、AIx [MD=1.97,95%CI(0.75,3.19)、 P =0.002]、PP[MD=2.16,95%CI(0.83,3.49), P =0.001]的影响具有统计学意义。结论脉搏波可以加强反映周身血管功能变化的检验效能,起到较好地完善运动干预评价体系的作用。     

自适应脉搏氧系统设计与实现

介绍一种自适应脉搏氧测量系统,使用模拟开关切换反馈电阻阻值,粗调系统增益;通过前级反馈调节MCU的DA输出,改变恒流驱动大小,微调系统增益。在两种机制协调下,实现系统增益精确控制,实现信号的宽范围测量,满足在低灌注指数下对弱脉搏的信号采集,并实现脉率、灌注指数、血氧饱和度的精确计算。     

医用MRI系统快速获取梯度回波脉冲序列开发

目的:探讨在i_Open 0.36T医用磁共振成像(MRI)系统上开发快速获取梯度回波脉冲序列(RAGE)的可行性与实现方法。方法:pascal语言编辑源程序,可调参数控制激发层数,相位编码时序,激发脉冲相位循环,数据采集次数,各段梯度脉冲开关时序等,编排k空间数据记录方式,满足快速傅立叶变换重建需要。编译源程序,调试图像。结果:在256×256采集矩阵的情况下,单层面成像时间最小值4s,所获图像分辨率、信噪比等指标能满足临床诊断需要。结论:利用该方法设计快速获取梯度回波脉冲序列是可行的。     

Quantitative rotating frame relaxometry methods in MRI

Macromolecular degeneration and biochemical changes in tissue can be quantified using rotating frame relaxometry in MRI. It has been shown in several studies that the rotating frame longitudinal relaxation rate constant (R_1ρ) and the rotating frame transverse relaxation rate constant (R_2ρ) are sensitive biomarkers of phenomena at the cellular level. In this comprehensive review, existing MRI methods for probing the biophysical mechanisms that affect the rotating frame relaxation rates of the tissue (i.e. R_1ρ and R_2ρ) are presented. Long acquisition times and high radiofrequency (RF) energy deposition into tissue during the process of spin‐locking in rotating frame relaxometry are the major barriers to the establishment of these relaxation contrasts at high magnetic fields. Therefore, clinical applications of R_1ρ and R_2ρ MRI using on‐ or off‐resonance RF excitation methods remain challenging. Accordingly, this review describes the theoretical and experimental approaches to the design of hard RF pulse cluster‐ and adiabatic RF pulse‐based excitation schemes for accurate and precise measurements of R_1ρ and R_2ρ. The merits and drawbacks of different MRI acquisition strategies for quantitative relaxation rate measurement in the rotating frame regime are reviewed. In addition, this review summarizes current clinical applications of rotating frame MRI sequences. Copyright © 2016 John Wiley & Sons, Ltd.     

基于容积脉搏波血流动力学检测的血流动力学参数的临床验证

目的 改进脉搏波无创检测仪的检测方式,考虑到桡动脉检测的操作复杂,因而选择操作简便的指脉检测.方法 通过大量的临床数据分析,确定指脉检测方式与桡动脉检测方式之间的转换关系,计算基于容积脉搏波的部分血流动力学参数,并和超声心动检测结果进行对比.结果 用指端容积脉搏血流方法同步检测的血流参数和超声心动检测血流参数的结果对比,差异没有统计学意义.结论 所得结果表明,在应用指端容积脉搏血流方法检测心输出量及其他血流参数是可行的.     

Compressed sensing with signal averaging for improved sensitivity and motion artifact reduction in fluorine‐19 MRI

Fluorine‐19 (¹⁹F) MRI of injected perfluorocarbon emulsions (PFCs) allows for the non‐invasive quantification of inflammation and cell tracking, but suffers from a low signal‐to‐noise ratio and extended scan time. To address this limitation, we tested the hypotheses that a ¹⁹F MRI pulse sequence that combines a specific undersampling regime with signal averaging has both increased sensitivity and robustness against motion artifacts compared with a non‐averaged fully sampled pulse sequence, when both datasets are reconstructed with compressed sensing. As a proof of principle, numerical simulations and phantom experiments were performed on selected variable ranges to characterize the point spread function of undersampling patterns, as well as the vulnerability to noise of undersampling and reconstruction parameters with paired numbers of x signal averages and acceleration factor x (NAx ‐AFx ). The numerical simulations demonstrated that a probability density function that uses 25% of the samples to fully sample the k‐space central area allowed for an optimal balance between limited blurring and artifact incoherence. At all investigated noise levels, the Dice similarity coefficient (DSC) strongly depended on the regularization parameters and acceleration factor. In phantoms, the motion robustness of an NA8‐AF8 undersampling pattern versus NA1‐AF1 was evaluated with simulated and real motion patterns. Differences were assessed with the DSC, which was consistently higher for the NA8‐AF8 compared with the NA1‐AF1 strategy, for both simulated and real cyclic motion patterns (P < 0.001). Both strategies were validated in vivo in mice (n = 2) injected with perfluoropolyether. Here, the images displayed a sharper delineation of the liver with the NA8‐AF8 strategy than with the NA1‐AF1 strategy. In conclusion, we validated the hypotheses that in ¹⁹F MRI the combination of undersampling and averaging improves both the sensitivity and the robustness against motion artifacts.     

3D-patient-specific geometry of the muscles involved in knee motion from selected MRI images

Patient-specific muscle geometry is not only an interesting clinical tool to evaluate different pathologies and treatments, but also provides an essential input data to more realistic musculoskeletal models. The protocol set up in our study provided the 3D-patient-specific geometry of the 13 main muscles involved in the knee joint motion from a few selected magnetic resonance images (MRIs). The contours of the muscles were identified on five to seven MRI axial slices. A parametric-specific object was then constructed for each muscle and deformed to fit those contours. The 13 muscles were obtained within 1 h, with less than 5% volume error and 5 mm point-surface error (2RMS). From this geometry, muscle volumes and volumic fractions of asymptomatic and anterior cruciate ligament deficient subjects could easily be computed and compared to previous studies. This protocol provides an interesting precision/time trade-off to obtain patient-specific muscular geometry.

A pulse artifact removal method considering artifact variations in the simultaneous recording of EEG and fMRI

A simultaneous recording of electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) can provide high spatiotemporal information of brain activity. However, a proper analysis of the EEG signals is often hindered by various artifacts. In particular, pulse artifact (PA) induced from the heartbeat of a subject interferes with reliable measurements of the EEG signal. A new PA removal method that takes into account the delay variation between the heartbeat and PA and the window size variation in PA is presented in order to improve the detection and suppression of PA in EEG signals. A PA is classified into either a normal PA or a deformed PA. Only normal PAs are averaged to generate a PA template that is used to remove PAs from the measured EEG signals. The performance of the proposed method was evaluated by simulated data and real EEG measurements from epilepsy patients. The results are compared with those from conventional methods.     

Monitoring of the effect of intervertebral disc nucleus pulposus ablation by MRI

In order to investigate intervertebral disc (IVD) degeneration and repair, a quantitative non‐invasive tool is needed. Various MRI methods including qCPMG, which yields dipolar echo relaxation time (T_DE), magnetization transfer contrast (MTC), and ¹H and ²H double quantum filtered (DQF) MRI were used in the present work to monitor changes in rat IVD after ablation of the nucleus pulposus (NP), serving as a model of severe IVD degeneration. In the intact IVD, a clear distinction between the annulus fibrosus (AF) and the NP is obtained on T₂ and T_DE weighted images as well as on MTC maps, reflecting the high concentration of ordered collagen fibers in the AF. After ablation of the NP, the distinction between the compartments is lost. T₂ and T_DE relaxation times are short throughout the disc and MTC is high. ¹H and ²H DQF signal, which in intact discs is obtained only for the AF, is now observable throughout the tissue. These results indicate that after ablation, there is an ingression of collagen fibers from the AF into the area that was previously occupied by the NP, as was confirmed by histology. Copyright © 2010 John Wiley & Sons, Ltd.