Glover G H, Pelc N J
Med Phys. 1980 May-Jun;7(3):238-48. doi: 10.1118/1.594678.
Dark streaks connecting the petrous bones are often observed in cranial transverse section CT reconstructions. These artifacts are usually only slightly diminished by two-pass beam hardening corrections. However, it is found that by narrowing the slice thickness the artifacts are substantially reduced. In this paper, it is shown that axial partial volume effects can account for the presence of the artifacts. These axial partial volume effects occur when, at any point in the slice, the object has axial variations in attenuation. In such cases the logarithm of the integrated intensity measured by the detector is not a linear function of the integrated attenuation (even for monochromatic beams). This nonlinearity causes inconsistencies in the data set which in turn can cause streaks in the image. We have studied the partial volume effect using computer simulation. Algorithms are presented whose purpose is to correct for these effects by estimating the axial variation using neighboring slices. These correction algorithms are successful in computer simulation cases but failed with clinical data. It is concluded that no practical correction method is viable unless overlap scanning is employed. However, thin-slice scanning for sections where these artifacts are common is perhaps a more preferable solution.
在颅脑横断面CT重建图像中，经常可以观察到连接岩骨的黑色条纹。这些伪影通常仅通过两次束硬化校正略有减少。然而，研究发现，通过缩小切片厚度，伪影可大幅减少。本文表明，轴向部分容积效应可解释这些伪影的存在。当切片中的任何一点处物体的衰减存在轴向变化时，就会发生这些轴向部分容积效应。在这种情况下，探测器测量的积分强度的对数不是积分衰减的线性函数（即使对于单色束也是如此）。这种非线性会导致数据集中出现不一致，进而在图像中产生条纹。我们使用计算机模拟研究了部分容积效应。提出了一些算法，其目的是通过使用相邻切片估计轴向变化来校正这些效应。这些校正算法在计算机模拟情况下是成功的，但在临床数据中却失败了。得出的结论是，除非采用重叠扫描，否则没有可行的实用校正方法。然而，对于这些伪影常见的部位进行薄层扫描可能是更可取的解决方案。
