图像重建的分块迭代算法

Iterative reconstruction in all sub-regions

图像重建重新投影(Reconstruction Re-projection)的迭代算法已在投影视野受限(Limited Field of View,LFOV)的情况下使用。视野受限情况下投影数据被截断。此处投影采用平行光束,扇形光束或锥形光束。该迭代算法对消除由于投影数据某处被截断所产生的截断伪影(Truncation Artifacts)非常有效。最近发现该算法不仅对消除截断伪影有效而且对消除条纹状伪影也有一些效果。条纹状伪影是在投影数据未被截断情况下由滤波反投影(Filterre dBackproiection,FBP)算法所产生的伪影。条纹状伪影与一个完整扫描所含的投影数及一个投影内的点数有关,该投影数越小伪影越严重。此发现引起了如下猜想,即该迭代算法或许也适用于投影数据未被截断的情况。本文对该迭代算法也做了进一步改进,使其更有利于减小条纹状伪影。主要的改进是把图像划分成较小的区域,以及剔除原迭代算法中用于数据被截断时的外插(Extrapolation)技术。本文投影数据采用平行光束,并通过仿真技术得到。迭代算法包含两次或者多次图像重建。第一次重建采用 FBP 算法。第二次图像重建时将第一次重建所得的图像切成小块;每一小块图像都被重新投影并再次重建;最终合并成一个完整图像。分块迭代算法的计算结果与 FBP 算法的以及传统的迭代法的结果做了比较,结果表明分块迭代算法的对消除伪影效果明显。

Iterative reconstruction re-projection algorithm was utilized to solve the limited field of view problems (LFOV)with cone-beam,fan-beam and cone-beam truncated projections.This algorithm is very effective at elimination of the truncation artifacts.Recently it has been found that this algorithm not only eliminates the truncation artifacts but also reduces strip artifacts,which exist in filtered back-projection(FBP)algorithm with non-truncated projections.The strip artifacts are related with the number of projections in a whole scan and the number of points inside one projection.The smaller the number is the severer the truncation artifacts are.This finding led the thought that perhaps this iterative algorithm is also suitable for non-truncated projections.This iterative algorithm was further improved in this paper in order to reduce the artifacts in case the projections are not truncated.The most important improvement was that the region of the object was divided to small sub-regions.The extrapolation process for truncated projections was taken away.The projections were made through simulation.The iterative algorithm constitutes with 2(or more)reconstructions.The 1^(st)reconstruction is normal FBP algorithm.In the 2^(ed)reconstruction the region of the object is divided to small sub-regions and the image in every sub-region is re-projected;the re-projections are reconstructed again and the sub-region image are put together.The sub-region iterative reconstruction algorithm was compared to the FBP algorithm and a tradition iterative algorithm.It was shown that the artifacts of the sub-region iterative algorithm can remarkably reduce the truncation artifacts compared to other algorithms.