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，…     

基于图像的回波平面成像ghost伪影消除方法

主要介绍了目前两种基于图像的EPI ghost伪影消除方法(Buonocore的方法和Lee的方法)，并进行了仿真实验。比较了2种方法的优缺点。通过比较，Buonocore的方法较好，而且，一种被Lee认为不合适的计算方法可以取得较好的效果。     

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.     

图像伪影的原因及识别意义

伪影(antifact)又称伪像(false image),它是指在重建图像过程中,所有不同类型的图像干扰和各种其它非随机干扰在图像上的表现,它对应的是受检体中根本不存在的组织或病灶的影像.它可能引起误诊,甚至导致医疗事故,所以,伪影的识别具有重要意义.     

CT图像锥形伪影的实验研究

目的：研究螺旋CT插值算法引起的图像伪影的规律。方法：采用自制的有机玻璃锥形体模在Siemens Plus4和GE Lightspeed16扫描仪上设置不同参数进行扫描。基于MATLAB7．0编程对图像进行分析，计算伪影的面积占原像面积的比重。结果：伪影的面积比随着螺距的增大而增大，随着体模半锥角的正切值和重建位置的半径的增大分别呈现增大和减小的趋势。临床照片中，锥形伪影突出表现在边界明显的部位。结论：锥形伪影与螺距和被扫描物属性存在确定关系。     

MR图像刚性平移运动伪影的自动逆向迭代修正

在MR扫描过程中患者的自主或非自主运动使获得的采集数据的相位发生偏移,因而重建图像含有运动伪影,严重地影响临床诊断。由于平移和旋转是最基本的刚性运动,故几乎所有刚性运动都可由他们复合而成。本研究针对MR图像刚性平移运动伪影提出了一个新的后处理方法:逆向迭代修正法(IIC)。通过逆向模拟患者的平移运动,逐次修正信号在K空间中已偏移的相位,然后根据最小熵约束准则,判断所模拟的逆向运动平移量是否等于患者的实际运动平移量,即修正到原来的位置。实验表明,应用本研究提出的逆向迭代修正算法,运动伪影能被修正到一个较为理想的状态。在修正后图像质量及计算耗时方面,优于其他算法。     

基于K空间对称性的Ghost伪影消除方法

Ghost伪影是回波平面成像（EPI）中一个很普遍的伪影，一般的方法是在扫描实际图像前利用参考扫描的方法对伪影进行消除。本研究提出了一个利用K空间原始数据的共轭对称性来消除ghost伪影的方法，且可以自动进行而不需要参考扫描。理想的K空间数据是共轭对称的。首先利用K空间的上述特性估计产生ghost伪影的K空间数据奇数和偶数行间的相位差，然后用迭代的方法对K空间数据进行校正，直到达到最好的ghost伪影消除效果。实验证明，所提出的方法可以有效的对ghost伪影进行消除。     

基于一种改进的相位校正算法消除MRI运动伪影

背景：近年来,MRI由于具有高的空间分辨率和软组织对比度,在临床上的运用越来越广泛。但是其成像时间较长,所以容易受到患者身体运动的影响,产生运动伪影。 目的：去除MRI图像成像时产生的伪影,改善图像质量。 方法：使用改进的相位矫正算法,并结合水平集算法去除图像伪影。去除伪影后使用模糊增强改善处理后图像的质量。 结果与结论：实验证明使用改进的相位矫正算法得到的图像比使用原始的相位矫正算法得到的图像效果更加理想。     

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。     

一种改进的基于互信息的3-D MRI图像配准方法

基于互信息的全局配准算法是近年来图像配准研究的热点之一.该方法具有精度高、鲁棒性强的特点.但目标函数由于插值算法的影响会陷入局部极值,引起误配准.本文实现了一种结合图像滤波预处理进行插值计算的方法,得到更加平滑的目标函数,有效的克服了"插值假像"问题.对标准数据库中的3-D MRI图像进行了刚性配准,实验结果表明该配准方法具有较高的精确度,具有很强的实用性.     

A Motion Compounding Technique for Speckle Reduction in Ultrasound Images

The quality of ultrasound images is usually influenced by speckle noise and the temporal decorrelation of the speckle patterns. To reduce the speckle noise, compounding techniques have been widely applied. Partially correlated images scanned on the same subject cross-section are combined to generate a compound image with improved image quality. However, the compounding technique might introduce image blurring if the transducer or the target moves too fast. This blurring effect becomes especially critical when assessing tissue deformation in clinical motion examinations. In this paper, an ultrasound motion compounding system is proposed to improve the quality of ultrasound motion sequences. The proposed motion compounding technique uses a hierarchical adaptive feature weighted motion estimation method to realign the frames before compounding. Each frame is first registered and warped to the reference frame before being compounded to reduce the speckle noise. Experimental results showed that the motion could be assessed accurately and better visualization could be achieved for the compound images, with improved signal-to-noise and contrast-to-noise ratios.     

Detecting Stripe Artifacts in Ultrasound Images

Brain perfusion diseases such as acute ischemic stroke are detectable through computed tomography (CT)-/magnetic resonance imaging (MRI)-based methods. An alternative approach makes use of ultrasound imaging. In this low-cost bedside method, noise and artifacts degrade the imaging process. Especially stripe artifacts show a similar signal behavior compared to acute stroke or brain perfusion diseases. This document describes how stripe artifacts can be detected and eliminated in ultrasound images obtained through harmonic imaging (HI). On the basis of this new method, both proper identification of areas with critically reduced brain tissue perfusion and classification between brain perfusion defects and ultrasound stripe artifacts are made possible.     

Development of a Voxel-Matching Technique for Substantial Reduction of Subtraction Artifacts in Temporal Subtraction Images Obtained from Thoracic MDCT

A temporal subtraction image, which is obtained by subtraction of a previous image from a current one, can be used for enhancing interval changes (such as formation of new lesions and changes in existing abnormalities) on medical images by removing most of the normal structures. However, subtraction artifacts are commonly included in temporal subtraction images obtained from thoracic computed tomography and thus tend to reduce its effectiveness in the detection of pulmonary nodules. In this study, we developed a new method for substantially removing the artifacts on temporal subtraction images of lungs obtained from multiple-detector computed tomography (MDCT) by using a voxel-matching technique. Our new method was examined on 20 clinical cases with MDCT images. With this technique, the voxel value in a warped (or nonwarped) previous image is replaced by a voxel value within a kernel, such as a small cube centered at a given location, which would be closest (identical or nearly equal) to the voxel value in the corresponding location in the current image. With the voxel-matching technique, the correspondence not only between the structures but also between the voxel values in the current and the previous images is determined. To evaluate the usefulness of the voxel-matching technique for removal of subtraction artifacts, the magnitude of artifacts remaining in the temporal subtraction images was examined by use of the full width at half maximum and the sum of a histogram of voxel values, which may indicate the average contrast and the total amount, respectively, of subtraction artifacts. With our new method, subtraction artifacts due to normal structures such as blood vessels were substantially removed on temporal subtraction images. This computerized method can enhance lung nodules on chest MDCT images without disturbing misregistration artifacts.     

Estimation of Motion in Angiographic Images

The effect of organ and tissue motion on X-ray image quality in angiography is discussed. It is shown that estimation of such motion may benefit the diagnostic value of images in several ways. The block-matching method is described. This method provides information on motion in the form of an offset vector field. The modifications of the block-matching algorithm described in this work include static region segmentation based on noise estimation and application of an additional flicker coefficient that improves the confidence of the method in case of changes in illumination.     

Motion Artefact Reduction in Dual-Energy Radiography

Biomedical Engineering - Dual-energy radiography is a relatively simple but powerful tool of X-ray diagnosis. It increases the diagnostic value of radiographic examination by generating separate...     

A Study of Grid Artifacts Formation and Elimination in Computed Radiographic Images

Computed radiography (CR) has many advantages such as filmless operations, efficiency, and convenience. Furthermore, it is easier to integrate with the picture archiving and communication systems. Another important advantage is that CR images generally have a wider dynamic range than conventional screen film. Unfortunately, grid artifacts and moiré pattern artifacts may be present in CR images. These artifacts become a more serious problem when viewing CR images on a computer monitor when a clinic grade monitor is not available. Images produced using a grid with higher frequency or a Potter–Bucky grid (i.e., a moving grid, Bucky for short) can reduce occurrence but cannot guarantee elimination of these artifacts [CR & PACS (2000); Detrick F (2001), pp 7–8]. In this paper, the formation of the artifacts is studied. We show that the grid artifacts occur in a narrow band of frequency in the frequency domain. The frequency can be determined, accurately located, and thus removed from the frequency domain. When comparing the results obtained from the proposed method against the results obtained using previous computer methods, we show that our method can achieve better image quality.     

消除呼吸阻抗测量时运动干扰方法中关键参数的选取初探

本文对该方法中关键参数的选取进行实验研究。得出了该参数的变化趋势，为正确应用该方法得到更好的处理结果奠定了基础。     

An Ocular Artifacts Removal Method Based on Canonical Correlation Analysis and Two-Channel EEG Recordings

In order to more effectively apply an artifact removal method in an online brain-computer interface(BCI) system, a new method based on canonical correlation analysis(CCA) and two-channel electroencephalography(EEG) recordings to quickly remove ocular artifacts (OA) is proposed in this paper. Considering both the formation of EEG signals contaminated by OA and the spread of OA, vertical electrooculography (VEOG) was appropriately introduced in CCA, and the blind source separation (BSS) method based on CCA was used in a new way during the OA removal process. Both experimental and comparison with ICA and SOBI results show that the new method with simple calculation and fast processing speed can effectively separate and remove OA using only two-channel EEG recordings, with retaining useful EEG signals. Hence, this method used in an online BCI system will be more effective.     

Influences of Motion Artifacts on Three-Dimensional Reconstruction Volume and Conformal Radiotherapy Planning

Objective ：To investigate the influences of motion artifacts on three-dimensional （3D） reconstruction volume and conformal radiotherapy planning. Methods： A phantom which can mimic the clip motion of lung tumor along the cranial-caudal direction is constructed by step motor, small ball of polyethylene and potato. Ten different scan protocols were set and CT data of the phantom were acquired by using a commercial GE LightSpeedl6 CT scanner. The 3D reconstruction of the CT data was implemented by adopting volume-rendering technology of GE AdvantageSim 6.0 system. The reconstructed volumes of each target in different scan protocols were measured through 3D measuring tools. Thus, relative deviations of the reconstruction volumes between moving targets and static ones were determined. The three-dimensional conformal radiation therapy （3D- CRT） plans and conformal fields were created and compared for a static/moving target with the WiMRT treatment planning system （TPS）. Results：For a static target, there was no obvious difference among the 3D reconstruction volumes when the CT data were acquired with different pitches and slices. The appearance of 3D reconstruction volume and 3D conformal field of a moving target was quite different from that of static one. The maximum relative deviation is nearly 90% for a moving target scanned with different scan protocols. The relative deviations are variable among the different targets, about from -39.8% to 89.5% for a smaller target and from - 18.4% to 20.5% for a larger one. Conclusion ：The motion artifacts have great effects on 3 D-CRT planning and reconstruction volume, which will greatly induce distorted conformal radiation fields and false DVHs for a moving target.