CT（Computed Tomography）技术不仅能清晰精确地显示物体内部细节的结构关系及缺陷状况，还能定量算出细节的辐射吸收数据，得出物质组成，因此已广泛应用于医学病情诊断和工业的缺陷识别及属性测量等领域。CT图像环形伪影是由探测器像元响应不一致等因素造成，使CT图像的质量下降，给图像噪声处理以及图像分割等后续处理造成困扰，降低了图像的识别能力和测量精度。在临床上由于伪影与病理组织图像表现相似可能导致误诊。因此最大程度的减少环形伪影是十分必要的。 本论文根据环形伪影在正弦图和重建后图像中的特点提出了两种校正方法：基于弦图分集差分的环形伪影校正法和基于极坐标改进二维低通滤波的环形伪影校正法。 基于弦图分集差分的环形伪影校正法是一种图像重建前的处理方法。该方法在提取伪影信息时，将CT图像的投影正弦图依次进行所有行的像素各自按列叠加求和后采用空域微分处理，避免了传统阈值判断伪影方法中阈值难以选择的问题。且对伪影数据的校正都是按列进行，可避免传统插值法中校正伪影数据受相邻数据干扰。最重要的是该方法可对初步处理后的图像进行分子集再次处理，能够有效去除残留的弧形伪影数据，使重建的CT图像完全去除残留伪影。 基于极坐标改进二维低通滤波的环形伪影校正法是图像重建后的处理方法。该方法为了解决常用的二维低通滤波法在过滤伪影信息时，会将原图像中的高频信息一同过滤，设计了一种带狭缝的二维低通滤波器，通过坐标转换，对极坐标图像的频域信息进行滤波，在去除线性伪影的同时能尽可能地保证原始图像细节信息。再经过傅立叶逆变换和坐标逆变换可得到去除环形伪影的校正图像。 利用上述两种校正方法分别对实验室仿真CT图像和真实CT图像进行了校正实验，结果表明两种方法都能有效去除环形伪影。基于弦图的方法是在图像重建前进行数据处理，所以更好地保证了图像的分辨率。基于极坐标的方法在得不到投影数据的情况下可有效去除重建图像中的环形伪影。

CT(Computed Tomography) technology can not only show the internal details of the object structure clearly, but also give details of material quantitatively. It has been used in industrial imperfection detection, medical diagnosis, and the measurement of properties. Ring artifacts are caused by bad pixel channel. It makes CT image difficult to identify and impedes post processing. It could lead to wrongly diagnosis because pathology image has similar characteristics with ring artifact. Therefore, removing such artifact is indispensable. According to ring artifact characteristics in projection sinogram image and reconstruction image by studying the causes, two kinds of ring artifact correction methods are proposed in the paper: ring artifact correction based on projection sinogram with subset differential and ring artifact correction based on polar coordinates with improved two dimension low pass filter. Ring artifact correction based on projection sinogram with subset differential is a pre-processing method. It uses spatial differential after sum up pixels of all lines separately when extracts artifact information, it avoids choosing threshold value with traditional method. Since artifact datum is processed separately, it can avoid Adjacent data interference in interpolation method. The most important thing is, This method can process the preliminary processed image by separating the data to subsets to remove the remain arc artifact data accurately. Ring artifact correction based on polar coordinates with improved two dimension low pass filter is a post-processing method. It can protect the detail information in original image when filtering the artifact information with common two dimension low pass filter by designing a two dimension low pass filter with a slit. Through coordinates transform, using the designed filter to process the artifact image in frequency domain. It can protect the original detail information and remove the artifact information as much as possible. After Fourier inverse transformation and coordinate inverse transformation, the artifacts can be removed. Using the above two correction methods for laboratory simulation and real CT images in experiments, from the results, we can clearly see that two kinds of methods can effectively remove the ring artifacts of CT images. The method based on projection sinogram processes the artifact data before reconstruction, so it can promise a better resolution of processed CT image. The method based on polar coordinates can remove the artifact information well when the projection data can not be obtained.