Ohnesorge B, Flohr T, Schwarz K, Heiken J P, Bae K T
Siemens Medical Engineering, Division CT, Forchheim, Germany.
Med Phys. 2000 Jan;27(1):39-46. doi: 10.1118/1.598855.
The purpose of this paper is to develop a method of eliminating CT image artifacts generated by objects extending outside the scan field of view, such as obese or inadequately positioned patients. CT projection data are measured only within the scan field of view and thus are abruptly discontinuous at the projection boundaries if the scanned object extends outside the scan field of view. This data discontinuity causes an artifact that consists of a bright peripheral band that obscures objects near the boundary of the scan field of view. An adaptive mathematical extrapolation scheme with low computational expense was applied to reduce the data discontinuity prior to convolution in a filtered backprojection reconstruction. Despite extended projection length, the convolution length was not increased and thus the reconstruction time was not affected. Raw projection data from ten patients whose bodies extended beyond the scan field of view were reconstructed using a conventional method and our extended reconstruction method. Limitations of the algorithm are investigated and extensions for further improvement are discussed. The images reconstructed by conventional filtered backprojection demonstrated peripheral bright-band artifacts near the boundary of the scan field of view. Images reconstructed with our technique were free of such artifacts and clearly showed the anatomy at the periphery of the scan field of view with correct attenuation values. We conclude that bright-band artifacts generated by obese patients whose bodies extend beyond the scan field of view were eliminated with our reconstruction method, which reduces boundary data discontinuity. The algorithm can be generalized to objects with inhomogeneous peripheral density and to true "Region of Interest Reconstruction" from truncated projections.
本文的目的是开发一种消除由扫描视野外的物体（如肥胖或体位不当的患者）产生的CT图像伪影的方法。CT投影数据仅在扫描视野内测量，因此，如果扫描物体延伸到扫描视野之外，投影边界处的数据将突然不连续。这种数据不连续性会导致一种伪影，其表现为一条明亮的外周带，会遮挡扫描视野边界附近的物体。在滤波反投影重建的卷积之前，应用了一种计算成本低的自适应数学外推方案来减少数据不连续性。尽管投影长度增加了，但卷积长度并未增加，因此重建时间不受影响。使用传统方法和我们的扩展重建方法对十名身体延伸超出扫描视野的患者的原始投影数据进行了重建。研究了该算法的局限性，并讨论了进一步改进的扩展方法。通过传统滤波反投影重建的图像在扫描视野边界附近显示出外周亮带伪影。用我们的技术重建的图像没有此类伪影，并且清晰地显示了扫描视野周边的解剖结构，且具有正确的衰减值。我们得出结论，我们的重建方法消除了身体延伸超出扫描视野的肥胖患者产生的亮带伪影，该方法减少了边界数据的不连续性。该算法可以推广到外周密度不均匀的物体以及从截断投影进行真正的“感兴趣区域重建”。
