【作者】 阎毅；

【导师】 孙怡；

【作者基本信息】 大连理工大学 ， 通信与信息系统， 2005， 硕士

【摘要】 随着工业CT在无损检测领域的广泛应用,如何能增进重建图像的质量和提高重建速度受到越来越多的关注。现有的反投影重建方法例如FDK方法虽然重建速度较快,但由于其锥角不能太大,所以重建的物体大小受到了一定的限制。之所以有这样的限制是因为单圆环的扫描轨迹不能满足由Smith提出的精确重建条件。本文引入了一种基于Grangeat理论的改进重建方法。由于这种方法在对Radon域阴影区进行插值后,采用滤波反投影方法进行重建,所以相对于Grangeat方法,运算复杂度较小,而相对于FDK方法,在大锥角的情况下重建图像质量会有所提高。在重建过程中,重建图像会受到与投影系统相关的因素的影响,导致图像质量下降。图像模糊问题是其中一个比较严重的问题,为了克服这个问题,很多工作者已经作了很有意义的工作,例如采用点扩散函数(Point-spread function)或者调制传递函数(ModulationTransfer Function)进行反卷积。在本文中采用调制传递函数对重建图像质量进行分析,其中调制传递函数可以通过对点扩散函数进行傅立叶变换得到。文中还通过计算机模拟先后列举了对于调制传递函数产生影响的因素。采用了数学模拟模型,分析调制传递函数相对于位置的变化趋势。除此之外,射线源焦点大小、成像时的放大倍数、探测器精度和重建图像的栅格大小也是影响调制传递函数的重要因素。通过对影响重建图像质量因素的分析,本文的最终目的是建立起一个包含多种因素的调制传递函数模型。最后采用FDK和Grangeat两种方法对物体进行重建,通过试验结果对比,对两种重建方法在重建质量及受上述因素影响的程度进行了分析。更多还原

【Abstract】 With the extensive application of industrial computed tomography in the field of non-destructive testing, how to improve the quality of the reconstructed image is receiving more and more concern. It is well known that in the existing cone-beam filtered backprojection reconstruction algorithms, such as FDK algorithm, the cone-angle is controlled within a narrow range. Thus the size of the tested work piece is limited. The reason of this limitation is that single circle orbit can not satisfy the sufficient and necessary conditions proposed by B.Smith. An improved cone-beam filtered back-projection reconstruction algorithm is introduced in this paper, which is based on the Grangeat’s algorithm. The aim of our algorithm is to improve the quality of the reconstructed image on large cone-angle situation. After filling the shadow zone by interpolation, filter backprojection algorithm is used to reconstruct the image. Comparing with Grangeat algorithm, the computational complexity of this algorithm can be reduced by using FFT. And comparing with FDK algorithm, the quality of the reconstructed image on big cone-angle situation can be improved.The qualities of initial reconstructed images also suffer from reduced resolution due to all kinds of factors which are relative to the cone-beam CT imaging system. The blurring is one of the most difficult problems caused by imaging system. To suppress the blurring in the reconstructed image, some research has been conducted on the deconvolution using a point spread function (PSF) or Modulation Transfer Function (MTF), MTF is used here instead of PSF. The MTF can be obtained by application of the two-dimensional (2-D) Fourier transform to the point-spread function (PSF).In this paper, the impact of various factors on MTF is subsequently illustrated with computer simulations. We use numerical phantom to calculate the MTF, and analyze its variation with position. The size of X-ray focus and magnification are also two important factors which influence MTF, so their effects on MTF are simulated. In addition, the grids of reconstruction and the sizes of pixel on the detector are also influence the MTF. Finally, we use 3D Feldkamp and Radon transform algorithm to reconstruct the images respectively. By comparing the results obtained from the two representative reconstruction algorithms, the quality of the reconstruction result can be evaluated by MTF.更多还原