Ohnesorge B, Flohr T, Klingenbeck-Regn K
Siemens Medical Engineering, Department CTC 2, D-91301 Forchheim, Germany.
Eur Radiol. 1999;9(3):563-9. doi: 10.1007/s003300050710.
X-ray photons which are scattered inside the object slice and reach the detector array increase the detected signal and produce image artifacts as "cupping" effects in large objects and dark bands between regions of high attenuation. The artifact amplitudes increase with scanned volume or slice width. Object scatter can be reduced in third generation computed tomography (CT) geometry by collimating the detector elements. However, a correction can still improve image quality. For fourth generation CT geometry, only poor anti-scatter collimation is possible and a numeric correction is necessary. This paper presents a correction algorithm which can be parameterized for third and fourth generation CT geometry. The method requires low computational effort and allows flexible application to different body regions by simple parameter adjustments. The object scatter intensity which is subtracted from the measured signal is calculated with convolution of the weighted and windowed projection data with a spatially invariant "scatter convolution function". The scatter convolution function is approximated for the desired scanner geometry from pencil beam simulations and measurements using coherent and incoherent differential scatter cross section data. Several examples of phantom and medical objects scanned with third and fourth generation CT systems are discussed. In third generation scanners, scatter artifacts are effectively corrected. For fourth generation geometry with poor anti-scatter collimation, object scatter artifacts are strongly reduced.
在物体切片内部散射并到达探测器阵列的X射线光子会增加检测到的信号，并产生图像伪影，如在大型物体中产生“杯状”效应以及在高衰减区域之间产生暗带。伪影幅度会随着扫描体积或切片宽度的增加而增大。在第三代计算机断层扫描（CT）几何结构中，可以通过准直探测器元件来减少物体散射。然而，进行校正仍可提高图像质量。对于第四代CT几何结构，只能进行效果不佳的反散射准直，因此需要进行数值校正。本文提出了一种校正算法，该算法可针对第三代和第四代CT几何结构进行参数化。该方法计算量小，通过简单的参数调整即可灵活应用于不同的身体部位。从测量信号中减去的物体散射强度是通过将加权和加窗投影数据与空间不变的“散射卷积函数”进行卷积来计算的。利用相干和非相干微分散射截面数据，通过铅笔束模拟和测量，针对所需的扫描仪几何结构对散射卷积函数进行了近似。文中讨论了使用第三代和第四代CT系统扫描的多个体模和医学物体的示例。在第三代扫描仪中，散射伪影得到了有效校正。对于反散射准直效果不佳的第四代几何结构，物体散射伪影大幅减少。
